Foreword

This document, started in early 1995, is considered the single most comprehensive technical reference to Game Boy available to the public.

You are reading a new version of it, maintained in the Markdown format and enjoying renewed community attention, correcting and updating it with recent findings. To learn more about the legacy and the mission of this initiative, check History.

Contributing

This project is open source, released under the CC0 license. Everyone is welcome to help, provide feedback and propose additions or improvements. The git repository is hosted at github.com/gbdev/pandocs, where you can learn more about how you can contribute, find detailed contribution guidelines and procedures, file Issues and send Pull Requests.

There is a Discord chat dedicated to the gbdev community.

Finally, you can also contact us and send patches via email: pandocs (at) gbdev.io.

Using this document

In the top navigation bar, you will find a series of icons.

By clicking on the icon you will toggle an interactive table of contents to navigate the document. You can also use → and ← keys on your keyboard to go to the following and previous page.

The lets you choose among 5 different themes and color schemes to please your reading experience.

You can search anywhere by pressing s on your keyboard or clicking the icon.

The icon allows you to suggest an edit on the current page by directly opening the source file in the git repository.

This document version was produced from git commit 18adfb8 (2024-11-09 22:28:16 +0100).

Acknowledgements

The maintenance and expansion of this project wouldn’t be possible without the continued commitment and support of:

• The gbdev community, for providing precious feedback, content, support and invaluable knowledge.
• Contributors of code and content on the Pan Docs project.
• DigitalOcean, for sponsoring this initiative and lifting us from any hosting and infrastructural cost in the last few years.
• Backers, for allowing us to push the community projects further and spread open culture while staying independent and free.

Authors

Antonio Niño Díaz, Antonio Vivace, Beannaich, Cory Sandlin, Eldred “ISSOtm” Habert, Elizafox, Furrtek, Gekkio, Jeff Frohwein, John Harrison, Lior “LIJI32” Halphon, Mantidactyle, Marat Fayzullin, Martin “nocash” Korth, Pan of ATX, Pascal Felber, Paul Robson, T4g1, TechFalcon, endrift, exezin, jrra, kOOPa, mattcurrie, nitro2k01, pinobatch, Pat Fagan, Alvaro Burnett.

Special thanks

FrankenGraphics, zeta0134.

History

Pan Docs - also known as GAMEBOY.TXT or GBSPEC.TXT - is an old document dating back to early 1995, originally written by Pan of Anthrox. It has been one of the most important references for Game Boy hackers, emulators and homebrew developers during the last 25 years.

ADDRESS1.PCX, one of the diagrams attached to the first version, released January 28th, 1995

After its release (1995-2008), it received a number of revisions, corrections and updates, maintaining its TXT format. This folder provides a historical archive of those versions.

In 2008, a wikified version (using Martin Korth’s 2001 revision as a baseline) has been published. The document was split into different articles and it continued being maintained and updated in that form.

In 2020, after the discussion in this RFC we migrated the last updated version to plain Markdown and made github.com/gbdev/pandocs the new home of this resource, where it can receive new public discussions and contributions, maintain its legacy and historical relevance, while making use of modern tools and workflows to be visualized and distributed.

From 2020 to May 2021 we used VuePress to render the markdown files as web pages.

Since May 2021, we rely on mdBook.

We are releasing everything (content, sources, code, figures) under the CC0 license (Public Domain).

Specifications

Memory Map

The Game Boy has a 16-bit address bus, which is used to address ROM, RAM, and I/O.

I/O Ranges

The Game Boy uses the following I/O ranges:

VRAM memory map

VRAM is, by itself, normal RAM, and may be used as such; however, the PPU interprets it in specific ways.

Bank 1 does not exist except on CGB, where it can be switched to (only in CGB Mode) using the VBK register.

Each bank first contains 384 tiles, of 16 bytes each. These tiles are commonly thought of as grouped in three “blocks” of 128 tiles each; see this detailed explanation for more details.

After the tiles, each bank contains two maps, 32×32 (= 1024) bytes each. The two banks are however different here: bank 0 contains tile maps, while bank 1 contains the corresponding attribute maps.

Here is a visualisation of how VRAM is laid out; hover over elements to see some details.

Jump Vectors in first ROM bank

The following addresses are supposed to be used as jump vectors:

• RST instructions: 0000, 0008, 0010, 0018, 0020, 0028, 0030, 0038
• Interrupts: 0040, 0048, 0050, 0058, 0060

However, this memory area (0000-00FF) may be used for any other purpose in case that your program doesn’t use any (or only some) rst instructions or interrupts. rst is a 1-byte instruction that works similarly to the 3-byte call instruction, except that the destination address is restricted. Since it is 1-byte sized, it is also slightly faster.

Cartridge Header in first ROM bank

The memory area at 0100-014F contains the cartridge header. This area contains information about the program, its entry point, checksums, information about the used MBC chip, the ROM and RAM sizes, etc. Most of the bytes in this area are required to be specified correctly.

External Memory and Hardware

The areas from 0000-7FFF and A000-BFFF address external hardware on the cartridge, which is essentially an expansion board. Typically this is a ROM and SRAM or, more often, a Memory Bank Controller (MBC). The RAM area can be read from and written to normally; writes to the ROM area control the MBC. Some MBCs allow mapping of other hardware into the RAM area in this way.

Cartridge RAM is often battery buffered to hold saved game positions, high score tables, and other information when the Game Boy is turned off. For specific information read the chapter about Memory Bank Controllers.

Echo RAM

The range E000-FDFF is mapped to WRAM, but only the lower 13 bits of the address lines are connected, with the upper bits on the upper bank set internally in the memory controller by a bank swap register. This causes the address to effectively wrap around. All reads and writes to this range have the same effect as reads and writes to C000-DDFF.

Nintendo prohibits developers from using this memory range. The behavior is confirmed on all official hardware. Some emulators (such as VisualBoyAdvance <1.8) don’t emulate Echo RAM. In some flash cartridges, echo RAM interferes with SRAM normally at A000-BFFF. Software can check if Echo RAM is properly emulated by writing to RAM (avoid values 00 and FF) and checking if said value is mirrored in Echo RAM and not cart SRAM.

FEA0–FEFF range

Nintendo indicates use of this area is prohibited. This area returns $FF when OAM is blocked, and otherwise the behavior depends on the hardware revision.

• On DMG, MGB, SGB, and SGB2, reads during OAM block trigger OAM corruption. Reads otherwise return $00.

• On CGB revisions 0-D, this area is a unique RAM area, but is masked with a revision-specific value.

• On CGB revision E, AGB, AGS, and GBP, it returns the high nibble of the lower address byte twice, e.g. FFAx returns $AA, FFBx returns $BB, and so forth.

Hardware Registers

Graphics Overview

The Game Boy outputs graphics to a 160×144 pixel LCD, using a quite complex mechanism to facilitate rendering.

Tiles

Similarly to other retro systems, pixels are not manipulated individually, as this would be expensive CPU-wise. Instead, pixels are grouped in 8×8 squares, called tiles (or sometimes “patterns” or “characters”), often considered as the base unit in Game Boy graphics.

A tile does not encode color information. Instead, a tile assigns a color ID to each of its pixels, ranging from 0 to 3. For this reason, Game Boy graphics are also called 2bpp (2 bits per pixel). When a tile is used in the Background or Window, these color IDs are associated with a palette. When a tile is used in an object, the IDs 1 to 3 are associated with a palette, but ID 0 means transparent.

Palettes

A palette consists of an array of colors, 4 in the Game Boy’s case. Palettes are stored differently in monochrome and color versions of the console.

Modifying palettes enables graphical effects such as quickly flashing some graphics (damage, invulnerability, thunderstorm, etc.), fading the screen, “palette swaps”, and more.

Layers

The Game Boy has three “layers”, from back to front: the Background, the Window, and the Objects. Some features and behaviors break this abstraction, but it works for the most part.

Background

The background is composed of a tilemap. A tilemap is a large grid of tiles. However, tiles aren’t directly written to tilemaps, they merely contain references to the tiles. This makes reusing tiles very cheap, both in CPU time and in required memory space, and it is the main mechanism that helps work around the paltry 8 KiB of video RAM.

The background can be made to scroll as a whole, writing to two hardware registers. This makes scrolling very cheap.

Window

The window is sort of a second background layer on top of the background. It is fairly limited: it has no transparency, it’s always a rectangle and only the position of the top-left pixel can be controlled.

Possible usage include a fixed status bar in an otherwise scrolling game (e.g. Super Mario Land 2).

Objects

The background layer is useful for elements scrolling as a whole, but it’s impractical for objects that need to move separately, such as the player.

The objects layer is designed to fill this gap: objects are made of 1 or 2 stacked tiles (8×8 or 8×16 pixels) and can be displayed anywhere on the screen.

To summarise:

• Tile, an 8×8-pixel chunk of graphics.
• Object, an entry in object attribute memory, composed of 1 or 2 tiles. Can be moved independently of the background.

VRAM Tile Data

Tile data is stored in VRAM in the memory area at $8000-$97FF; with each tile taking 16 bytes, this area defines data for 384 tiles. In CGB Mode, this is doubled (768 tiles) because of the two VRAM banks.

Each tile (or character) has 8×8 pixels and has a color depth of 2 bits per pixel, allowing each pixel to use one of 4 colors or gray shades. Tiles can be displayed as part of the Background/Window maps, and/or as objects (movable sprites). Color 0 has a special meaning in objects - it’s transparent, allowing the background or other objects behind it to show through.

There are three “blocks” of 128 tiles each:

Tiles are always indexed using an 8-bit integer, but the addressing method may differ:

• The “$8000 method” uses $8000 as its base pointer and uses an unsigned addressing, meaning that tiles 0-127 are in block 0, and tiles 128-255 are in block 1.
• The “$8800 method” uses $9000 as its base pointer and uses a signed addressing, meaning that tiles 0-127 are in block 2, and tiles -128 to -1 are in block 1; or, to put it differently, “$8800 addressing” takes tiles 0-127 from block 2 and tiles 128-255 from block 1.

(You can notice that block 1 is shared by both addressing methods)

Objects always use “$8000 addressing”, but the BG and Window can use either mode, controlled by LCDC bit 4.

Data format

Each tile occupies 16 bytes, where each line is represented by 2 bytes:

For each line, the first byte specifies the least significant bit of the color ID of each pixel, and the second byte specifies the most significant bit. In both bytes, bit 7 represents the leftmost pixel, and bit 0 the rightmost. For example, the tile data $3C $7E $42 $42 $42 $42 $42 $42 $7E $5E $7E $0A $7C $56 $38 $7C appears as follows:

For the first row, the values $3C $7E are 00111100 and 01111110 in binary. The leftmost bits are 0 and 0, thus the color ID is binary 00, or 0. The next bits are 0 and 1, thus the color ID is binary 10, or 2 (remember to flip the order of the bits!). The full eight-pixel row evaluates to 0 2 3 3 3 3 2 0.

A tool for viewing tiles can be found here.

So, each pixel has a color ID of 0 to 3. The color numbers are translated into real colors (or gray shades) depending on the current palettes, except that when the tile is used in an OBJ the color ID 0 means transparent. The palettes are defined through registers BGP, OBP0 and OBP1, and BCPS/BGPI, BCPD/BGPD, OCPS/OBPI and OCPD/OBPD (CGB Mode).

VRAM Tile Maps

The Game Boy contains two 32×32 tile maps in VRAM at the memory areas $9800-$9BFF and $9C00-$9FFF. Any of these maps can be used to display the Background or the Window.

Tile Indexes

Each tile map contains the 1-byte indexes of the tiles to be displayed.

Tiles are obtained from the Tile Data Table using either of the two addressing modes (described in VRAM Tile Data), which can be selected via the LCDC register.

Since one tile has 8×8 pixels, each map holds a 256×256 pixels picture. Only 160×144 of those pixels are displayed on the LCD at any given time.

BG Map Attributes (CGB Mode only)

In CGB Mode, an additional map of 32×32 bytes is stored in VRAM Bank 1 (each byte defines attributes for the corresponding tile-number map entry in VRAM Bank 0, that is, 1:9800 defines the attributes for the tile at 0:9800):

• Priority: 0 = No; 1 = Colors 1–3 of the corresponding BG/Window tile are drawn over OBJ, regardless of OBJ priority
• Y flip: 0 = Normal; 1 = Tile is drawn vertically mirrored
• X flip: 0 = Normal; 1 = Tile is drawn horizontally mirrored
• Bank: 0 = Fetch tile from VRAM bank 0; 1 = Fetch tile from VRAM bank 1
• Color palette: Which of BGP0–7 to use

Bit 4 is ignored by the hardware, but can be written to and read from normally.

Note that, for example, if the byte at 0:9800 is $2A, the attribute at 1:9800 doesn’t define properties for ALL tiles $2A on-screen, but only the one at 0:9800!

BG-to-OBJ Priority in CGB Mode

In CGB Mode, the priority between the BG (and window) layer and the OBJ layer is declared in three different places:

• BG Map Attribute bit 7
• LCDC bit 0
• OAM Attributes bit 7

We can infer the following rules from the table below:

• If the BG color index is 0, the OBJ will always have priority;
• Otherwise, if LCDC bit 0 is clear, the OBJ will always have priority;
• Otherwise, if both the BG Attributes and the OAM Attributes have bit 7 clear, the OBJ will have priority;
• Otherwise, BG will have priority.

The following table shows the relations between the 3 flags:

This test ROM can be used to observe the above.

Background (BG)

The SCY and SCX registers can be used to scroll the Background, specifying the origin of the visible 160×144 pixel area within the total 256×256 pixel Background map. The visible area of the Background wraps around the Background map (that is, when part of the visible area goes beyond the map edge, it starts displaying the opposite side of the map).

In Non-CGB mode, the Background (and the Window) can be disabled using LCDC bit 0.

Window

Besides the Background, there is also a Window overlaying it. The content of the Window is not scrollable; it is always displayed starting at the top left tile of its tile map. The only way to adjust the Window is by modifying its position on the screen, which is done via the WX and WY registers. The screen coordinates of the top left corner of the Window are (WX-7,WY). The tiles for the Window are stored in the Tile Data Table. Both the Background and the Window share the same Tile Data Table.

Whether the Window is displayed can be toggled using LCDC bit 5. But in Non-CGB mode this bit is only functional as long as LCDC bit 0 is set. Enabling the Window makes Mode 3 slightly longer on scanlines where it’s visible. (See WX and WY for the definition of “Window visibility”.)

Object Attribute Memory (OAM)

The Game Boy PPU can display up to 40 movable objects (or sprites), each 8×8 or 8×16 pixels. Because of a limitation of hardware, only ten objects can be displayed per scanline. Object tiles have the same format as BG tiles, but they are taken from tile blocks 0 and 1 located at $8000-8FFF and have unsigned numbering.

Object attributes reside in the object attribute memory (OAM) at $FE00-FE9F. (This corresponds to the sprite attribute table on a TMS9918 VDP.) Each of the 40 entries consists of four bytes with the following meanings:

Byte 0 — Y Position

Y = Object’s vertical position on the screen + 16. So for example:

• Y=0 hides an object,
• Y=2 hides an 8×8 object but displays the last two rows of an 8×16 object,
• Y=16 displays an object at the top of the screen,
• Y=144 displays an 8×16 object aligned with the bottom of the screen,
• Y=152 displays an 8×8 object aligned with the bottom of the screen,
• Y=154 displays the first six rows of an object at the bottom of the screen,
• Y=160 hides an object.

Byte 1 — X Position

X = Object’s horizontal position on the screen + 8. This works similarly to the examples above, except that the width of an object is always 8. An off-screen value (X=0 or X>=168) hides the object, but the object still contributes to the limit of ten objects per scanline. This can cause objects later in OAM not to be drawn on that line. A better way to hide an object is to set its Y-coordinate off-screen.

Byte 2 — Tile Index

In 8×8 mode (LCDC bit 2 = 0), this byte specifies the object’s only tile index ($00-$FF). This unsigned value selects a tile from the memory area at $8000-$8FFF. In CGB Mode this could be either in VRAM bank 0 or 1, depending on bit 3 of the following byte. In 8×16 mode (LCDC bit 2 = 1), the memory area at $8000-$8FFF is still interpreted as a series of 8×8 tiles, where every 2 tiles form an object. In this mode, this byte specifies the index of the first (top) tile of the object. This is enforced by the hardware: the least significant bit of the tile index is ignored; that is, the top 8×8 tile is “NN & $FE”, and the bottom 8×8 tile is “NN | $01”.

Byte 3 — Attributes/Flags

• Priority: 0 = No, 1 = BG and Window colors 1–3 are drawn over this OBJ
• Y flip: 0 = Normal, 1 = Entire OBJ is vertically mirrored
• X flip: 0 = Normal, 1 = Entire OBJ is horizontally mirrored
• DMG palette [Non CGB Mode only]: 0 = OBP0, 1 = OBP1
• Bank [CGB Mode Only]: 0 = Fetch tile from VRAM bank 0, 1 = Fetch tile from VRAM bank 1
• CGB palette [CGB Mode Only]: Which of OBP0–7 to use

Writing data to OAM

The recommended method is to write the data to a buffer in normal RAM (typically WRAM) first, then to copy that buffer to OAM using the DMA transfer functionality.

While it is also possible to write data directly to the OAM area by accessing it normally, this only works during the HBlank and VBlank periods.

Object Priority and Conflicts

There are two kinds of “priorities” as far as objects are concerned. The first one defines which objects are ignored when there are more than 10 on a given scanline. The second one decides which object is displayed on top when some overlap (the Game Boy being a 2D console, there is no Z coordinate).

Selection priority

During each scanline’s OAM scan, the PPU compares LY (using LCDC bit 2 to determine their size) to each object’s Y position to select up to 10 objects to be drawn on that line. The PPU scans OAM sequentially (from $FE00 to $FE9F), selecting the first (up to) 10 suitably-positioned objects.

Since the PPU only checks the Y coordinate to select objects, even off-screen objects count towards the 10-objects-per-scanline limit. Merely setting an object’s X coordinate to X = 0 or X ≥ 168 (160 + 8) will hide it, but it will still count towards the limit, possibly causing another object later in OAM not to be drawn. To keep off-screen objects from affecting on-screen ones, make sure to set their Y coordinate to Y = 0 or Y ≥ 160 (144 + 16). (Y ≤ 8 also works if object size is set to 8×8.)

Drawing priority

When opaque pixels from two different objects overlap, which pixel ends up being displayed is determined by another kind of priority: the pixel belonging to the higher-priority object wins. However, this priority is determined differently when in CGB mode.

• In Non-CGB mode, the smaller the X coordinate, the higher the priority. When X coordinates are identical, the object located first in OAM has higher priority.
• In CGB mode, only the object’s location in OAM determines its priority. The earlier the object, the higher its priority.

OAM DMA Transfer

FF46 — DMA: OAM DMA source address & start

Writing to this register starts a DMA transfer from ROM or RAM to OAM (Object Attribute Memory). The written value specifies the transfer source address divided by $100, that is, source and destination are:

Source:      $XX00-$XX9F   ;XX = $00 to $DF
Destination: $FE00-$FE9F

The transfer takes 160 M-cycles: 640 dots (1.4 lines) in normal speed, or 320 dots (0.7 lines) in CGB Double Speed Mode. This is much faster than a CPU-driven copy.

OAM DMA bus conflicts

On DMG, during OAM DMA, the CPU can access only HRAM (memory at $FF80-$FFFE). For this reason, the programmer must copy a short procedure (see below) into HRAM, and use this procedure to start the transfer from inside HRAM, and wait until the transfer has finished.

On CGB, the cartridge and WRAM are on separate buses. This means that the CPU can access ROM or cartridge SRAM during OAM DMA from WRAM, or WRAM during OAM DMA from ROM or SRAM. However, because a call writes a return address to the stack, and the stack and variables are usually in WRAM, it’s still recommended to busy-wait in HRAM for DMA to finish even on CGB.

While an OAM DMA is in progress, the PPU cannot read OAM properly either. Thus, most programs execute DMA during Mode 1, inside or immediately after their VBlank handler. But it is also possible to execute it during display redraw (Modes 2 and 3), allowing to display more than 40 objects on the screen (that is, for example 40 objects in the top half, and other 40 objects in the bottom half of the screen), at the cost of a couple lines that lack objects. If the transfer is started during Mode 3, graphical glitches may happen.

The details:

• If OAM DMA is active during OAM scan (mode 2), most PPU revisions read each object as being off-screen and thus hidden on that line.
• If OAM DMA is active during rendering (mode 3), the PPU reads whatever 16-bit word the DMA unit is writing to OAM when the object is fetched. This causes an incorrect tile number and attributes for objects already determined to be in range.

Best practices

This 10-byte routine starts a transfer and waits for it to finish. Many games copy a routine like it into HRAM and call it during Mode 1.

run_dma:
    ld a, HIGH(start address)
    ldh [$FF46], a  ; start DMA transfer (starts right after instruction)
    ld a, 40        ; delay for a total of 4×40 = 160 M-cycles
.wait
    dec a           ; 1 M-cycle
    jr nz, .wait    ; 3 M-cycles
    ret

If HRAM is tight, this more compact procedure saves 5 bytes of HRAM at the cost of a few M-cycles spent jumping to the tail in HRAM.

run_dma:          ; This part must be in ROM.
    ld a, HIGH(start address)
    ld bc, $2846  ; B: wait time; C: LOW($FF46)
    jp run_dma_tail


run_dma_tail:     ; This part must be in HRAM.
    ldh [c], a
.wait
    dec b
    jr nz, .wait
    ret z         ; Conditional `ret` is 1 M-cycle slower, which avoids
                  ; reading from the stack on the last M-cycle of DMA.

If starting a mid-frame transfer, wait for Mode 0 first so that the transfer cleanly overlaps Mode 2 on the next two lines, making objects invisible on those lines.

LCD Control

FF40 — LCDC: LCD control

LCDC is the main LCD Control register. Its bits toggle what elements are displayed on the screen, and how.

• LCD & PPU enable: 0 = Off; 1 = On
• Window tile map area: 0 = 9800–9BFF; 1 = 9C00–9FFF
• Window enable: 0 = Off; 1 = On
• BG & Window tile data area: 0 = 8800–97FF; 1 = 8000–8FFF
• BG tile map area: 0 = 9800–9BFF; 1 = 9C00–9FFF
• OBJ size: 0 = 8×8; 1 = 8×16
• OBJ enable: 0 = Off; 1 = On
• BG & Window enable / priority [Different meaning in CGB Mode]: 0 = Off; 1 = On

LCDC.7 — LCD enable

This bit controls whether the LCD is on and the PPU is active. Setting it to 0 turns both off, which grants immediate and full access to VRAM, OAM, etc.

When the display is disabled the screen is blank, which on DMG is displayed as a white “whiter” than color #0.

On SGB, the screen doesn’t turn white, it appears that the previous picture sticks to the screen. (TODO: research this more.)

When re-enabling the LCD, the PPU will immediately start drawing again, but the screen will stay blank during the first frame.

LCDC.6 — Window tile map area

This bit controls which background map the Window uses for rendering. When it’s clear (0), the $9800 tilemap is used, otherwise it’s the $9C00 one.

LCDC.5 — Window enable

This bit controls whether the window shall be displayed or not. This bit is overridden on DMG by bit 0 if that bit is clear.

Changing the value of this register mid-frame triggers a more complex behaviour: see further below.

Note that on CGB models, setting this bit to 0 then back to 1 mid-frame may cause the second write to be ignored. (TODO: test this.)

LCDC.4 — BG and Window tile data area

This bit controls which addressing mode the BG and Window use to pick tiles.

Objects (sprites) aren’t affected by this, and will always use the $8000 addressing mode.

LCDC.3 — BG tile map area

This bit works similarly to LCDC bit 6: if the bit is clear (0), the BG uses tilemap $9800, otherwise tilemap $9C00.

LCDC.2 — OBJ size

This bit controls the size of all objects (1 tile or 2 stacked vertically).

Be cautious when changing object size mid-frame. Changing from 8×8 to 8×16 pixels mid-frame within 8 scanlines of the bottom of an object causes the object’s second tile to be visible for the rest of those 8 lines. If the size is changed during mode 2 or 3, remnants of objects in range could “leak” into the other tile and cause artifacts.

LCDC.1 — OBJ enable

This bit toggles whether objects are displayed or not.

This can be toggled mid-frame, for example to avoid objects being displayed on top of a status bar or text box.

(Note: toggling mid-scanline might have funky results on DMG? Investigation needed.)

LCDC.0 — BG and Window enable/priority

LCDC.0 has different meanings depending on Game Boy type and Mode:

Non-CGB Mode (DMG, SGB and CGB in compatibility mode): BG and Window display

When Bit 0 is cleared, both background and window become blank (white), and the Window Display Bit is ignored in that case. Only objects may still be displayed (if enabled in Bit 1).

CGB Mode: BG and Window master priority

When Bit 0 is cleared, the background and window lose their priority - the objects will be always displayed on top of background and window, independently of the priority flags in OAM and BG Map attributes.

When Bit 0 is set, pixel priority is resolved as described here.

Using LCDC

LCDC is a powerful tool: each bit controls a lot of behavior, and can be modified at any time during the frame.

One of the important aspects of LCDC is that unlike VRAM, the PPU never locks it. It’s thus possible to modify it mid-scanline!

Faux-layer textbox/status bar

A problem often seen in 8-bit games is objects rendering on top of the textbox/status bar. It’s possible to prevent this using LCDC if the textbox/status bar is “alone” on its scanlines:

• Set LCDC.1 to 1 for gameplay scanlines
• Set LCDC.1 to 0 for textbox/status bar scanlines

Usually, these bars are either at the top or bottom of the screen, so the bit can be set by the VBlank and/or STAT handlers. Hiding objects behind a right-side window is more challenging.

LCD Status Registers

FF44 — LY: LCD Y coordinate [read-only]

LY indicates the current horizontal line, which might be about to be drawn, being drawn, or just been drawn. LY can hold any value from 0 to 153, with values from 144 to 153 indicating the VBlank period.

FF45 — LYC: LY compare

The Game Boy constantly compares the value of the LYC and LY registers. When both values are identical, the “LYC=LY” flag in the STAT register is set, and (if enabled) a STAT interrupt is requested.

FF41 — STAT: LCD status

• LYC int select (Read/Write): If set, selects the LYC == LY condition for the STAT interrupt.
• Mode 2 int select (Read/Write): If set, selects the Mode 2 condition for the STAT interrupt.
• Mode 1 int select (Read/Write): If set, selects the Mode 1 condition for the STAT interrupt.
• Mode 0 int select (Read/Write): If set, selects the Mode 0 condition for the STAT interrupt.
• LYC == LY (Read-only): Set when LY contains the same value as LYC; it is constantly updated.
• PPU mode (Read-only): Indicates the PPU’s current status.

Spurious STAT interrupts

A hardware quirk in the monochrome Game Boy makes the LCD interrupt sometimes trigger when writing to STAT (including writing $00) during OAM scan, HBlank, VBlank, or LY=LYC. It behaves as if $FF were written for one M-cycle, and then the written value were written the next M-cycle. Because the GBC in DMG mode does not have this quirk, two games that depend on this quirk (Ocean’s Road Rash and Vic Tokai’s Xerd no Densetsu) will not run on a GBC.

LCD Position and Scrolling

These registers can be accessed even during Mode 3, but modifications may not take effect immediately (see further below).

FF42–FF43 — SCY, SCX: Background viewport Y position, X position

These two registers specify the top-left coordinates of the visible 160×144 pixel area within the 256×256 pixels BG map. Values in the range 0–255 may be used.

The PPU calculates the bottom-right coordinates of the viewport with those formulas: bottom := (SCY + 143) % 256 and right := (SCX + 159) % 256. As suggested by the modulo operations, in case the values are larger than 255 they will “wrap around” towards the top-left corner of the tilemap.

FF4A–FF4B — WY, WX: Window Y position, X position plus 7

These two registers specify the on-screen coordinates of the Window’s top-left pixel.

The Window is visible (if enabled) when both coordinates are in the ranges WX=0..166, WY=0..143 respectively. Values WX=7, WY=0 place the Window at the top left of the screen, completely covering the background.

Mid-frame behavior

Scrolling

The scroll registers are re-read on each tile fetch, except for the low 3 bits of SCX, which are only read at the beginning of the scanline (for the initial shifting of pixels).

All models before the CGB-D read the Y coordinate once for each bitplane (so a very precisely timed SCY write allows “desyncing” them), but CGB-D and later use the same Y coordinate for both no matter what.

Window

While the Window should work as just mentioned, writing to WX, WY etc. mid-frame shows a more articulated behavior.

For the window to be displayed on a scanline, the following conditions must be met:

• WY condition was triggered: i.e. at some point in this frame the value of WY was equal to LY (checked at the start of Mode 2 only)
• WX condition was triggered: i.e. the current X coordinate being rendered + 7 was equal to WX
• Window enable bit in LCDC is set

If the WY condition has already been triggered and at the start of a row the window enable bit was set, then resetting that bit before the WX condition gets triggered on that row yields a nice window glitch pixel where the window would have been activated.

Palettes

LCD Monochrome Palettes

FF47 — BGP (Non-CGB Mode only): BG palette data

This register assigns gray shades to the color IDs of the BG and Window tiles.

Each of the two-bit values map to a color thusly:

In CGB Mode the color palettes are taken from CGB palette memory instead.

FF48–FF49 — OBP0, OBP1 (Non-CGB Mode only): OBJ palette 0, 1 data

These registers assigns gray shades to the color indexes of the OBJs that use the corresponding palette. They work exactly like BGP, except that the lower two bits are ignored because color index 0 is transparent for OBJs.

LCD Color Palettes (CGB only)

The CGB has a small amount of RAM used to store its color palettes. Unlike most of the hardware interface, palette RAM (or CRAM for Color RAM) is not accessed directly, but instead through the following registers:

FF68 — BCPS/BGPI (CGB Mode only): Background color palette specification / Background palette index

This register is used to address a byte in the CGB’s background palette RAM. Since there are 8 palettes, 8 palettes × 4 colors/palette × 2 bytes/color = 64 bytes can be addressed.

First comes BGP0 color number 0, then BGP0 color number 1, BGP0 color number 2, BGP0 color number 3, BGP1 color number 0, and so on. Thus, address $03 allows accessing the second (upper) byte of BGP0 color #1 via BCPD, which contains the color’s blue and upper green bits.

• Auto-increment: 0 = Disabled; 1 = Increment “Address” field after writing to BCPD / OCPD (even during Mode 3, despite the write itself failing), reads never cause an increment
• Address: Specifies which byte of BG Palette Memory can be accessed through BCPD

Unlike BCPD, this register can be accessed outside VBlank and HBlank.

FF69 — BCPD/BGPD (CGB Mode only): Background color palette data / Background palette data

This register allows to read/write data to the CGBs background palette memory, addressed through BCPS/BGPI. Each color is stored as little-endian RGB555:

Much like VRAM, data in palette memory cannot be read or written during the time when the PPU is reading from it, that is, Mode 3.

FF6A–FF6B — OCPS/OBPI, OCPD/OBPD (CGB Mode only): OBJ color palette specification / OBJ palette index, OBJ color palette data / OBJ palette data

These registers function exactly like BCPS and BCPD respectively; the 64 bytes of OBJ palette memory are entirely separate from Background palette memory, but function the same.

Note that while 4 colors are stored per OBJ palette, color #0 is never used, as it’s always transparent. It’s thus fine to write garbage values, or even leave color #0 uninitialized.

RGB Translation by CGBs

When developing graphics on PCs, note that the RGB values will have different appearance on CGB displays as on VGA/HDMI monitors calibrated to sRGB color. Because the GBC is not lit, the highest intensity will produce light gray rather than white. The intensities are not linear; the values $10-$1F will all appear very bright, while medium and darker colors are ranged at $00-0F.

The CGB display’s pigments aren’t perfectly saturated. This means the colors mix quite oddly: increasing the intensity of only one R/G/B color will also influence the other two R/G/B colors. For example, a color setting of $03EF (Blue=$00, Green=$1F, Red=$0F) will appear as Neon Green on VGA displays, but on the CGB it’ll produce a decently washed out Yellow. See the image above.

RGB Translation by GBAs

Even though GBA is described to be compatible to CGB games, most CGB games are completely unplayable on older GBAs because most colors are invisible (black). Of course, colors such like Black and White will appear the same on both CGB and GBA, but medium intensities are arranged completely different. Intensities in range $00–07 are invisible/black (unless eventually under best sunlight circumstances, and when gazing at the screen under obscure viewing angles), unfortunately, these intensities are regularly used by most existing CGB games for medium and darker colors.

This problem with low brightness levels does not affect later GBA SP units and Game Boy Player. Thus ideally, the player should have control of this brightness correction.

Rendering overview

Terminology

The entire frame is not drawn atomically; instead, the image is drawn by the PPU (Pixel-Processing Unit) progressively, directly to the screen. A frame consists of 154 scanlines; during the first 144, the screen is drawn top to bottom, left to right.

The main implication of this rendering process is the existence of raster effects: modifying some rendering parameters in the middle of rendering. The most famous raster effect is modifying the scrolling registers between scanlines to create a “wavy” effect.

A “dot” = one 2²² Hz (≅ 4.194 MHz) time unit. Dots remain the same regardless of whether the CPU is in Double Speed mode, so there are 4 dots per Single Speed M-cycle, and 2 per Double Speed M-cycle.

PPU modes

While the PPU is accessing some video-related memory, that memory is inaccessible to the CPU (writes are ignored, and reads return garbage values, usually $FF).

Mode 3 length

During Mode 3, by default the PPU outputs one pixel to the screen per dot, from left to right; the screen is 160 pixels wide, so the minimum Mode 3 length is 160 + 12¹ = 172 dots.

Unlike most game consoles, the Game Boy does not always output pixels steadily²: some features cause the rendering process to stall for a couple dots. Any extra time spent stalling lengthens Mode 3; but since scanlines last for a fixed number of dots, Mode 0 is therefore shortened by that same amount of time.

Three things can cause Mode 3 “penalties”:

• Background scrolling: At the very beginning of Mode 3, rendering is paused for SCX % 8 dots while the same number of pixels are discarded from the leftmost tile.
• Window: After the last non-window pixel is emitted, a 6-dot penalty is incurred while the BG fetcher is being set up for the window.
• Objects: Each object drawn during the scanline (even partially) incurs a 6- to 11-dot penalty (see below).

On DMG and GBC in DMG mode, mid-scanline writes to BGP allow observing this behavior precisely, as any delay shifts the write’s effect to the left by that many dots.

OBJ penalty algorithm

Only the OBJ’s leftmost pixel matters here, transparent or not; it is designated as “The Pixel” in the following.

1. Determine the tile (background or window) that The Pixel is within. (This is affected by horizontal scrolling and/or the window!)
2. If that tile has not been considered by a previous OBJ yet³:
   1. Count how many of that tile’s pixels are strictly to the right of The Pixel.
   2. Subtract 2.
   3. Incur this many dots of penalty, or zero if negative (from waiting for the BG fetch to finish).
3. Incur a flat, 6-dot penalty (from fetching the OBJ’s tile).

Exception: an OBJ with an OAM X position of 0 (thus, completely off the left side of the screen) always incurs a 11-dot penalty, regardless of SCX.

Pixel FIFO

Introduction

FIFO stands for First In, First Out. The first pixel to be pushed to the FIFO is the first pixel to be popped off. In theory that sounds great, in practice there are a lot of intricacies.

There are two pixel FIFOs. One for background pixels and one for object (sprite) pixels. These two FIFOs are not shared. They are independent of each other. The two FIFOs are mixed only when popping items. Objects take priority unless they’re transparent (color 0) which will be explained in detail later. Each FIFO can hold up to 16 pixels. The FIFO and Pixel Fetcher work together to ensure that the FIFO always contains at least 8 pixels at any given time, as 8 pixels are required for the Pixel Rendering operation to take place. Each FIFO is manipulated only during mode 3 (pixel transfer).

Each pixel in the FIFO has four properties:

• Color: a value between 0 and 3
• Palette: on CGB a value between 0 and 7 and on DMG this only applies to objects
• Sprite Priority: on CGB this is the OAM index for the object and on DMG this doesn’t exist
• Background Priority: holds the value of the OBJ-to-BG Priority bit

FIFO Pixel Fetcher

The fetcher fetches a row of 8 background or window pixels and queues them up to be mixed with object pixels. The pixel fetcher has 5 steps. The first four steps take 2 dots each and the fifth step is attempted every dot until it succeeds. The order of the steps are as follows:

• Get tile
• Get tile data low
• Get tile data high
• Sleep
• Push

Get Tile

This step determines which background/window tile to fetch pixels from. By default the tilemap used is the one at $9800 but certain conditions can change that.

When LCDC.3 is enabled and the X coordinate of the current scanline is not inside the window then tilemap $9C00 is used.

When LCDC.6 is enabled and the X coordinate of the current scanline is inside the window then tilemap $9C00 is used.

The fetcher keeps track of which X and Y coordinate of the tile it’s on:

If the current tile is a window tile, the X coordinate for the window tile is used, otherwise the following formula is used to calculate the X coordinate: ((SCX / 8) + fetcher’s X coordinate) & $1F. Because of this formula, fetcherX can be between 0 and 31.

If the current tile is a window tile, the Y coordinate for the window tile is used, otherwise the following formula is used to calculate the Y coordinate: (currentScanline + SCY) & 255. Because of this formula, fetcherY can be between 0 and 255.

The fetcher’s X and Y coordinate can then be used to get the tile from VRAM. However, if the PPU’s access to VRAM is blocked then the value for the tile is read as $FF.

CGB can access both tile index and the attributes in the same clock dot.

Get Tile Data Low

Check LCDC.4 for which tilemap to use. At this step CGB also needs to check which VRAM bank to use and check if the tile is flipped vertically. Once the tilemap, VRAM and vertical flip is calculated the tile data is retrieved from VRAM. However, if the PPU’s access to VRAM is blocked then the tile data is read as $FF.

The tile data retrieved in this step will be used in the push steps.

Get Tile Data High

Same as Get Tile Data Low except the tile address is incremented by 1.

The tile data retrieved in this step will be used in the push steps.

This also pushes a row of background/window pixels to the FIFO. This extra push is not part of the 8 steps, meaning there’s 3 total chances to push pixels to the background FIFO every time the complete fetcher steps are performed.

Push

Pushes a row of background/window pixels to the FIFO. Since tiles are 8 pixels wide, a “row” of pixels is 8 pixels from the tile to be rendered based on the X and Y coordinates calculated in the previous steps.

Pixels are only pushed to the background FIFO if it’s empty.

This is where the tile data retrieved in the two Tile Data steps will come in handy. Depending on if the tile is flipped horizontally the pixels will be pushed to the background FIFO differently. If the tile is flipped horizontally the pixels will be pushed LSB first. Otherwise they will be pushed MSB first.

Sleep

Do nothing.

VRAM Access

At various times during PPU operation read access to VRAM is blocked and the value read is $FF:

• LCD turning off
• At scanline 0 on CGB when not in double speed mode
• When switching from mode 3 to mode 0
• On CGB when searching OAM and index 37 is reached

At various times during PPU operation read access to VRAM is restored:

• At scanline 0 on DMG and CGB when in double speed mode
• On DMG when searching OAM and index 37 is reached
• After switching from mode 2 (oam search) to mode 3 (pixel transfer)

NOTE: These conditions are checked only when entering STOP mode and the PPU’s access to VRAM is always restored upon leaving STOP mode.

Mode 3 Operation

As stated before the pixel FIFO only operates during mode 3 (pixel transfer). At the beginning of mode 3 both the background and OAM FIFOs are cleared.

The Window

When rendering the window the background FIFO is cleared and the fetcher is reset to step 1. When WX is 0 and the SCX & 7 > 0 mode 3 is shortened by 1 dot.

When the window has already started rendering there is a bug that occurs when WX is changed mid-scanline. When the value of WX changes after the window has started rendering and the new value of WX is reached again, a pixel with color value of 0 and the lowest priority is pushed onto the background FIFO.

Sprites

The following is performed for each object on the current scanline if LCDC.1 is enabled (this condition is ignored on CGB) and the X coordinate of the current scanline has an object on it. If those conditions are not met then object fetching is canceled.

At this point the fetcher is advanced one step until it’s at step 5 or until the background FIFO is not empty. Advancing the fetcher one step here lengthens mode 3 by 1 dot. This process may be canceled after the fetcher has advanced a step.

When SCX & 7 > 0 and there is an object at X coordinate 0 of the current scanline then mode 3 is lengthened. The amount of dots this lengthens mode 3 by is whatever the lower 3 bits of SCX are. After this penalty is applied object fetching may be canceled. Note that the timing of the penalty is not confirmed. It may happen before or after waiting for the fetcher. More research needs to be done.

After checking for objects at X coordinate 0 the fetcher is advanced two steps. The first advancement lengthens mode 3 by 1 dot and the second advancement lengthens mode 3 by 3 dots. After each fetcher advancement there is a chance for an object fetch cancel to occur.

The lower address for the row of pixels of the target object tile is now retrieved and lengthens mode 3 by 1 dot. Once the address is retrieved this is the last chance for object fetch cancel to occur. Exiting object fetch lengthens mode 3 by 1 dot. The upper address for the target object tile is now retrieved and does not shorten mode 3.

At this point VRAM Access is checked for the lower and upper addresses for the target object. Before any mixing is done, if the OAM FIFO doesn’t have at least 8 pixels in it then transparent pixels with the lowest priority are pushed onto the OAM FIFO. Once this is done each pixel of the target object row is checked. On CGB, horizontal flip is checked here. If the target object pixel is not white and the pixel in the OAM FIFO is white, or if the pixel in the OAM FIFO has higher priority than the target object’s pixel, then the pixel in the OAM FIFO is replaced with the target object’s properties.

Now it’s time to render a pixel! The same process described in Object Fetch Canceling is performed: a pixel is rendered and the fetcher is advanced one step. This advancement lengthens mode 3 by 1 dot if the X coordinate of the current scanline is not 160. If the X coordinate is 160 the PPU stops processing objects (because they won’t be visible).

Everything in this section is repeated for every object on the current scanline unless it was decided that fetching should be canceled or the X coordinate is 160.

Pixel Rendering

This is where the background FIFO and OAM FIFO are mixed. There are conditions where either a background pixel or an object pixel will have display priority.

If there are pixels in the background and OAM FIFOs then a pixel is popped off each. If the OAM pixel is not transparent and LCDC.1 is enabled then the OAM pixel’s background priority property is used if it’s the same or higher priority as the background pixel’s background priority.

Pixels won’t be pushed to the LCD if there is nothing in the background FIFO or the current pixel is pixel 160 or greater.

If LCDC.0 is disabled then the background is disabled on DMG and the background pixel won’t have priority on CGB. When the background pixel is disabled the pixel color value will be 0, otherwise the color value will be whatever color pixel was popped off the background FIFO. When the pixel popped off the background FIFO has a color value other than 0 and it has priority then the object pixel will be discarded.

At this point, on DMG, the color of the pixel is retrieved from the BGP register and pushed to the LCD. On CGB when palette access is blocked a black pixel is pushed to the LCD.

When an object pixel has priority, the color value is retrieved from the popped pixel from the OAM FIFO. On DMG the color for the pixel is retrieved from either the OBP1 or OBP0 register depending on the pixel’s palette property. If the palette property is 1 then OBP1 is used, otherwise OBP0 is used. The pixel is then pushed to the LCD. On CGB when palette access is blocked, a black pixel is pushed to the LCD.

The pixel is then finally pushed to the LCD.

CGB Palette Access

At various times during PPU operation read access to the CGB palette is blocked and a black pixel pushed to the LCD when rendering pixels:

• LCD turning off
• First HBlank of the frame
• When searching OAM and index 37 is reached
• After switching from mode 2 (oam search) to mode 3 (pixel transfer)
• When entering HBlank (mode 0) and not in double speed mode, blocked 2 dots later no matter what

At various times during PPU operation read access to the CGB palette is restored and pixels are pushed to the LCD normally when rendering pixels:

• At the end of mode 2 (oam search)
• For only 2 dots when entering HBlank (mode 0) and in double speed mode

Object Fetch Canceling

Object fetching may be canceled if LCDC.1 is disabled while the PPU is fetching an object from OAM. This canceling lengthens mode 3 by the amount of dots the previous instruction took plus the residual dots left for the PPU to process. When OAM fetching is canceled, a pixel is rendered, and the fetcher is advanced one step. This advancement lengthens mode 3 by 1 dot if the current pixel is not 160. If the current pixel is 160 the PPU stops processing objects because they won’t be visible.

Audio Overview

Game Boy audio is sometimes called “8-bit”. This does not refer to the bit depth of the sound generated, but rather that it has sound capabilities typical of 8-bit consoles. Like much of its contemporary hardware, the Game Boy produces sound generated by simple digital circuits.

Architecture

The Game Boy has four sound generation units, called channels 1 through 4, notated “CH1”, “CH2”, etc. Unlike some other sound chips, such as the C64’s SID or the Atari 5200’s POKEY, each sound channel is specialized in a way largely different from the other channels.

Each channel generates an electronic signal; these signals are then mixed into two new channels (for stereo: one for the left ear, one for the right ear), which are then individually amplified, and then output either to the headphone jack, or the speaker¹.

Channels 1 and 2, the “pulse channels”, produce pulse width modulated waves with 4 fixed pulse width settings. Channel 3, the “wave” channel, produces arbitrary user-supplied waves. Channel 4 is the “noise” channel, producing a pseudo-random wave.

The VIN channel is an analog signal received directly from the cartridge, allowing external hardware to supply a fifth sound channel. No licensed games used this feature, and it was omitted from the Game Boy Advance.

Common concepts

APU

The Game Boy’s sound chip is called the APU.

The APU runs off the same master clock as the rest of the Game Boy, which is to say, it is fully synced with the CPU and PPU. This also means that the APU runs about 2.4% faster on the SGB1, increasing frequencies by as much and thus sounding slightly higher-pitched. The SGB2 rectifies this issue.

All interfaces to the APU use durations instead of frequencies, which may be confusing as signal theory and music are more typically based on the latter. Thus, durations will be expressed from their frequencies: for example, a “256 Hz tick” means “1 ∕ 256th of a second”.

The length of APU ticks is not affected by CGB double speed, so the APU works just the same regardless of CPU speed.

Triggering

Triggering a channel causes it to turn on if it wasn’t², and to start playing its wave from the beginning³. Most changes to a channel’s parameters take effect immediately, but some require re-triggering the channel.

Volume & envelope

The volume can be controlled in two ways: there is a “master volume” control⁴ (which has separate settings for the left and right outputs), and each channel’s volume can be individually set as well (CH3’s less precisely than the others).

Additionally, an envelope can be configured for CH1, CH2 and CH4, which allows automatically adjusting the volume over time. The parameters that can be controlled are the initial volume, the envelope’s direction (but not its slope), and its duration. Internally, all envelopes are ticked at 64 Hz, and every 1–7 of those ticks, the volume will be increased or decreased.

Length timer

All channels can be individually set to automatically shut themselves down after a certain amount of time.

If the functionality is enabled, a channel’s length timer ticks up⁵ at 256 Hz (tied to DIV-APU) from the value it’s initially set at. When the length timer reaches 64 (256 for wave channel), the channel is turned off.

Frequency

Music notes and audio waves are typically manipulated in terms of frequency⁶, i.e. how often the signal repeats per second. However, as explained above, the Game Boy APU primarily works with durations; thus, periods will be used instead of frequency.⁷

Audio Registers

Audio registers are named following a NRxy scheme, where x is the channel number (or 5 for “global” registers), and y is the register’s ID within the channel. Since many registers share common properties, a notation is often used where e.g. NRx2 is used to designate NR12, NR22, NR32, and NR42 at the same time, for simplicity.

As a rule of thumb, for any x in 1, 2, 3, 4:

• NRx0 is some channel-specific feature (if present),
• NRx1 controls the length timer,
• NRx2 controls the volume and envelope,
• NRx3 controls the period (maybe only partially),
• NRx4 has the channel’s trigger and length timer enable bits, as well as any leftover bits of period;

…but there are some exceptions.

One of the pitfalls of the NRxy naming convention is that the register’s purpose is not immediately clear from its name, so some alternative names have been proposed, such as AUDENA for NR52.

Global control registers

FF26 — NR52: Audio master control

• Audio on/off (Read/Write): This controls whether the APU is powered on at all (akin to LCDC bit 7). Turning the APU off drains less power (around 16%), but clears all APU registers and makes them read-only until turned back on, except NR52¹.

• CHn on? (Read-only): Each of these four bits allows checking whether channels are active². Writing to those does not enable or disable the channels, despite many emulators behaving as if.

  A channel is turned on by triggering it (i.e. setting bit 7 of NRx4)³. A channel is turned off when any of the following occurs:

  • The channel’s length timer is enabled in NRx4 and expires, or
  • For CH1 only: when the period sweep overflows⁴, or
  • The channel’s DAC is turned off. The envelope reaching a volume of 0 does NOT turn the channel off!

FF25 — NR51: Sound panning

Each channel can be panned hard left, center, hard right, or ignored entirely.

Setting a bit to 1 enables the channel to go into the selected output.

Note that selecting or de-selecting a channel whose DAC is enabled will cause an audio pop.

FF24 — NR50: Master volume & VIN panning

• VIN left/right: These work exactly like the bits in NR51. They should be set at 0 if external sound hardware is not being used.

• Left/right volume: These specify the master volume, i.e. how much each output should be scaled.

  A value of 0 is treated as a volume of 1 (very quiet), and a value of 7 is treated as a volume of 8 (no volume reduction). Importantly, the amplifier never mutes a non-silent input.

Sound Channel 1 — Pulse with period sweep

FF10 — NR10: Channel 1 sweep

This register controls CH1’s period sweep functionality.

• Pace: This dictates how often sweep “iterations” happen, in units of 128 Hz ticks⁵ (7.8 ms). Note that the value written to this field is not re-read by the hardware until a sweep iteration completes, or the channel is (re)triggered.

  However, if 0 is written to this field, then iterations are instantly disabled (but see below), and it will be reloaded as soon as it’s set to something else.

• Direction: 0 = Addition (period increases); 1 = Subtraction (period decreases)

• Individual step: On each iteration, the new period $L_{t+1}$ is computed from the current one $L_t$ as follows:

  $$L_{t+1}=L_t\pm \frac{L_t}{2^{\mathrm{step}}}$$

On each sweep iteration, the period in NR13 and NR14 is modified and written back.

In addition mode, if the period value would overflow (i.e. $L_{t+1}$ is strictly more than $7FF), the channel is turned off instead. This occurs even if sweep iterations are disabled by the pace being 0.

Note that if the period ever becomes 0, the period sweep will never be able to change it. For the same reason, the period sweep cannot underflow the period (which would turn the channel off).

FF11 — NR11: Channel 1 length timer & duty cycle

This register controls both the channel’s length timer and duty cycle (the ratio of the time spent low vs. high). The selected duty cycle also alters the phase, although the effect is hardly noticeable except in combination with other channels.

• Duty cycle (Read/Write): Controls the output waveform as follows:

  Value (binary)	Duty cycle	Waveform
  00	12.5 %
  01	25 %
  10	50 %
  11	75 %

  It’s worth noting that there is no audible difference between the 25 % and 75 % duty cycle settings.

• Initial length timer (Write-only): The higher this field is, the shorter the time before the channel is cut.

FF12 — NR12: Channel 1 volume & envelope

This register controls the digital amplitude of the “high” part of the pulse, and the sweep applied to that setting.

• Initial volume: How loud the channel initially is. Note that these bits are readable, but are not updated by the envelope functionality!
• Env dir: The envelope’s direction; 0 = decrease volume over time, 1 = increase volume over time.
• Sweep pace: The envelope ticks at 64 Hz, and the channel’s envelope will be increased / decreased (depending on bit 3) every Sweep pace of those ticks. A setting of 0 disables the envelope.

Setting bits 3-7 of this register all to 0 (initial volume = 0, envelope = decreasing) turns the DAC off (and thus, the channel as well), which may cause an audio pop.

FF13 — NR13: Channel 1 period low [write-only]

This register stores the low 8 bits of the channel’s 11-bit “period value”. The upper 3 bits are stored in the low 3 bits of NR14.

The period divider of pulse and wave channels is an up counter. Each time it is clocked, its value increases by 1; when it overflows (being clocked when it’s already 2047, or $7FF), its value is set from the contents of NR13 and NR14. This means it treats the value in the period as a negative number in 11-bit two’s complement. The higher the period value in the register, the lower the period, and the higher the frequency. For example:

• Period value $500 means -$300, or 1 sample per 768 input cycles
• Period value $740 means -$C0, or 1 sample per 192 input cycles

The pulse channels’ period dividers are clocked at 1048576 Hz, once per four dots, and their waveform is 8 samples long. This makes their sample rate equal to $\frac{1048576}{2048-\mathrm{period\_value}}$ Hz. with a resulting tone frequency equal to $\frac{131072}{2048-\mathrm{period\_value}}$ Hz.

• Period value $500 means -$300, or 1 sample per 768 input cycles
  or (1048576 ÷ 768) = 1365.3 Hz sample rate
  or (1048576 ÷ 768 ÷ 8) = 170.67 Hz tone frequency
• Period value $740 means -$C0, or 1 sample per 192 input cycles
  or (1048576 ÷ 192) = 5461.3 Hz sample rate
  or (1048576 ÷ 192 ÷ 8) = 682.67 Hz tone frequency

Period value $740 produces a higher frequency than $500. Even though the period value $740 is not four times $500, $740 produces a frequency that is four times that of $500, or two octaves higher, because ($800 - $740) or 192 is one-quarter of ($800 - $500) or 768.

FF14 — NR14: Channel 1 period high & control

• Trigger (Write-only): Writing any value to NR14 with this bit set triggers the channel.
• Length enable (Read/Write): Takes effect immediately upon writing to this register.
• Period (Write-only): The upper 3 bits of the period value; the lower 8 bits are stored in NR13.

Sound Channel 2 — Pulse

This sound channel works exactly like channel 1, except that it lacks a period sweep (and thus an equivalent to NR10). Please refer to the corresponding CH1 register:

• NR21 ($FF16) → NR11
• NR22 ($FF17) → NR12
• NR23 ($FF18) → NR13
• NR24 ($FF19) → NR14

Sound Channel 3 — Wave output

While other channels only offer limited control over the waveform they generate, this channel allows outputting any wave. It’s thus sometimes called a “voluntary wave” channel.

While the “length” of the wave is fixed at 32 “samples”, 4-bit each, the speed at which it is read can be customized. It’s possible to “shorten” the wave by either feeding it a repeating pattern, or doubling each sample and doubling the read rate. It’s also possible to artificially “increase” the wave’s length by loading a new wave as soon as the whole buffer has been read; this is sometimes used for full-on sample playback.

FF1A — NR30: Channel 3 DAC enable

This register controls CH3’s DAC. Like other channels, turning the DAC off immediately turns the channel off as well.

The DAC is often turned off just before writing to wave RAM to avoid issues with accessing it; see further below for more info.

Turning the DAC off may cause an audio pop.

FF1B — NR31: Channel 3 length timer [write-only]

This register controls the channel’s length timer.

The higher the length timer, the shorter the time before the channel is cut.

FF1C — NR32: Channel 3 output level

This channel lacks the envelope functionality that the other three channels have, and has a much coarser volume control.

• Output level: Controls the channel’s volume as follows:

  Bits 6-5 (binary)	Output level
  00	Mute (No sound)
  01	100% volume (use samples read from Wave RAM as-is)
  10	50% volume (shift samples read from Wave RAM right once)
  11	25% volume (shift samples read from Wave RAM right twice)

FF1D — NR33: Channel 3 period low [write-only]

This register stores the low 8 bits of the channel’s 11-bit “period value”. The upper 3 bits are stored in the low 3 bits of NR34.

The wave channel’s period divider is clocked at 2097152 Hz, once per two dots, and its waveform is 32 samples long. This makes their sample rate equal to $\frac{2097152}{2048-\mathrm{period\_value}}$ Hz. with a resulting tone frequency equal to $\frac{65536}{2048-\mathrm{period\_value}}$ Hz.

• Period value $500 means -$300, or 1 sample per 768 input cycles
  or (2097152 ÷ 768) = 2730.7 Hz sample rate
  or (2097152 ÷ 768 ÷ 32) = 85.333 Hz tone frequency
• Period value $740 means -$C0, or 1 sample per 192 input cycles
  or (2097152 ÷ 192) = 10923 Hz sample rate
  or (2097152 ÷ 192 ÷ 32) = 341.33 Hz tone frequency

Given the same period value, the tone frequency of the wave channel is generally half that of a pulse channel, or one octave lower.

FF1E — NR34: Channel 3 period high & control

• Trigger (Write-only): Writing any value to NR34 with this bit set triggers the channel.

  RETRIGGERING CAUTION

  On monochrome consoles only, retriggering CH3 while it’s about to read a byte from wave RAM causes wave RAM to be corrupted in a generally unpredictable manner.

  PLAYBACK DELAY

  Triggering the wave channel does not immediately start playing wave RAM; instead, the last sample ever read (which is reset to 0 when the APU is off) is output until the channel next reads a sample. (Source)

• Length enable (Read/Write): Takes effect immediately upon writing to this register.

• Period (Write-only): The upper 3 bits of the period value; the lower 8 bits are stored in NR33.

FF30–FF3F — Wave pattern RAM

Wave RAM is 16 bytes long; each byte holds two “samples”, each 4 bits.

As CH3 plays, it reads wave RAM left to right, upper nibble first. That is, $FF30’s upper nibble, $FF30’s lower nibble, $FF31’s upper nibble, and so on.

Accessing wave RAM while CH3 is active (i.e. playing) causes accesses to misbehave:

• On AGB, reads return $FF, and writes are ignored. (Source)
• On monochrome consoles, wave RAM can only be accessed on the same cycle that CH3 does. Otherwise, reads return $FF, and writes are ignored.
• On other consoles, the byte accessed will be the one CH3 is currently reading⁶; that is, if CH3 is currently reading one of the first two samples, the CPU will really access $FF30, regardless of the address being used. (Source)

Wave RAM can be accessed normally even if the DAC is on, as long as the channel is not active. (Source) This is especially relevant on GBA, whose mixer behaves as if DACs are always enabled.

Sound Channel 4 — Noise

This channel is used to output white noise⁷, which is done by randomly switching the amplitude between two levels fairly fast.

The frequency can be adjusted in order to make the noise appear “harder” (lower frequency) or “softer” (higher frequency).

The random function that switches the output level can also be manipulated. Certain settings can cause the wave to be more regular, sounding closer to a pulse than noise.

FF20 — NR41: Channel 4 length timer [write-only]

This register controls the channel’s length timer.

The higher the length timer, the shorter the time before the channel is cut.

FF21 — NR42: Channel 4 volume & envelope

This register functions exactly like NR12, so please refer to its documentation.

FF22 — NR43: Channel 4 frequency & randomness

This register allows controlling the way the amplitude is randomly switched.

• Clock shift: See the frequency formula below.

• LFSR width: 0 = 15-bit, 1 = 7-bit (more regular output; some frequencies sound more like pulse than noise).

  LFSR lockup

  Switching from 15- to 7-bit mode when the LFSR is in a certain state can “lock it up”, which essentially silences CH4; this can be avoided by retriggering CH4, which resets the LFSR.

• Clock divider: See the frequency formula below. Note that divider = 0 is treated as divider = 0.5 instead.

The frequency at which the LFSR is clocked is $\frac{262144}{\mathrm{divider}\times 2^{\mathrm{shift}}}$ Hz.

If the bit shifted out is a 0, the channel emits a 0; otherwise, it emits the volume selected in NR42.

FF23 — NR44: Channel 4 control

• Trigger (Write-only): Writing any value to NR14 with this bit set triggers the channel.
• Length enable (Read/Write): Takes effect immediately upon writing to this register.

Audio Details

tl;dr:

The PPU is a bunch of state machines, and the APU is a bunch of counters.

Each of the four “conceptual” channels is composed of a “generation” circuit (designated “channel” in the above diagram), and a DAC. The digital value produced by the generator, which ranges between $0 and $F (0 and 15), is linearly translated by the DAC into an analog¹ value between -1 and 1 (the unit is arbitrary).

The four analog channel outputs are then fed into the mixer², which selectively adds them (depending on NR51) into two analog outputs (Left and Right). Thus, the analog range of those outputs is 4× that of each channel, -4 to 4.

Then, both of these two get their respective volume scaled, once from NR50, and once from the volume knob (if the console has one). Note that the former step never mutes a non-silent input, but the latter can.

Each of the two analog outputs then goes through a high-pass filter (HPF). For short, a HPF constantly tries to “pull” the signal towards analog 0 (neutral); the reason for that is explained further below.

PCM registers

These two registers, only present in the Game Boy Color and later models, allow reading the output of the generation circuits directly; this is very useful for testing “internal” APU behavior. These registers are not documented in any known Nintendo manual.

FF76 — PCM12 (CGB Mode only): Digital outputs 1 & 2 [read-only]

The low nibble is a copy of sound channel 1’s digital output, the high nibble a copy of sound channel 2’s.

FF77 — PCM34 (CGB Mode only): Digital outputs 3 & 4 [read-only]

Same, but with channels 3 and 4.

Finer technical explanation

DIV-APU

A “DIV-APU” counter is increased every time DIV’s bit 4 (5 in double-speed mode) goes from 1 to 0, therefore at a frequency of 512 Hz (regardless of whether double-speed is active). Thus, the counter can be made to increase faster by writing to DIV while its relevant bit is set (which clears DIV, and triggers the falling edge).

The following events occur every N DIV-APU ticks:

Mixer

A high-pass filter (HPF) removes constant biases over time. The HPFs therefore remove the DC offset created by inactive channels with an enabled DAC, and off-center waveforms.

The HPF is more aggressive on GBA than on GBC, which itself is more aggressive than on DMG. (The more “aggressive” a HPF, the faster it pulls the signal towards “analog 0”; this tends to also distort waveforms.)

DACs

Channel x’s DAC is enabled if and only if [NRx2] & $F8 != 0; the exception is CH3, whose DAC is directly controlled by bit 7 of NR30 instead. Note that the envelope functionality changes the volume, but not the value stored in NRx2, and thus doesn’t disable the DACs.

If a DAC is enabled, the digital range $0 to $F is linearly translated to the analog range -1 to 1, in arbitrary units. Importantly, the slope is negative: “digital 0” maps to “analog 1”, not “analog -1”.

If a DAC is disabled, it fades to an analog value of 0, which corresponds to “digital 7.5”. The nature of this fade is not entirely deterministic and varies between models.

NR52’s low 4 bits report whether the channels are turned on, not their DACs.

Channels

A channel is activated by a write to NRx4’s MSB, unless its DAC is off, which forces it to be disabled as well. The opposite is not true, however: a disabled channel outputs 0, which an enabled DAC will dutifully convert into “analog 1”.

A channel can be deactivated in one of the following ways:

• Turning off its DAC
• Its length timer expiring
• (CH1 only) Frequency sweep overflowing the frequency

Pulse channels (CH1, CH2)

Each pulse channel has an internal “duty step” counter, which is used to index into the selected waveform (each background stripe corresponds to one “duty step”)⁴. The “duty step” increments at the channel’s sample rate, which is 8 times the channel’s frequency).

The “duty step” counter cannot be reset, except by turning the APU off, which sets both back to 0. Retriggering a pulse channel causes its “duty step timer” to reset, thus retriggering a pulse channel often enough will cause its “duty step” to never advance.

When first starting up a pulse channel, it will always output a (digital) zero.

Wave channel (CH3)

CH3 has an internal “sample index” counter. The “sample index” increments at the channel’s sample rate, which is 32 times the channel’s frequency Each time it increments, the corresponding “sample” (nibble) is read from wave RAM. (This means that sample #0 is skipped when first starting up CH3.)

CH3 does not emit samples directly, but stores every sample read into a buffer, and emits that continuously; (re)triggering the channel does not clear nor refresh this buffer, so the last sample ever read will be emitted again. This buffer is cleared when turning the APU on, so CH3 will emit a “digital 0” when first powered on.

CH3 output level control does not, in fact, alter the output level. It shifts the digital value CH3 is outputting, not the analog value. This only matters when changing the setting mid-playback: the digital values being shifted bias them towards 0, which biases the analog output towards “1”; the HPF will smooth this over time, but not instantly.

Noise channel (CH4)

CH4 revolves around a LFSR, pictured above. The LFSR has 16 bits: 15 bits for its current state and 1 bit to temporarily store the next bit to shift in.

When CH4 is ticked (at the frequency specified via NR43):

1. The result of $\overline{{\mathrm{LFSR}}_0\odot {\mathrm{LFSR}}_1}$ (1 if bit 0 and bit 1 are identical, 0 otherwise) is written to bit 15.
2. If “short mode” was selected in NR43, then bit 15 is copied to bit 7 as well.
3. Finally, the entire LFSR is shifted right, and bit 0 selects between 0 and the chosen volume.

The LFSR is set to 0 when (re)triggering the channel.

Game Boy Advance audio

The APU was reworked pretty heavily for the GBA, which introduces some slightly different behavior:

• Instead of mixing being done by analog circuitry, it’s instead done digitally; then, sound is converted to an analog signal and an offset is added (see SOUNDBIAS in GBATEK for more details).
• This also means that the GBA APU has no DACs. Instead, they are emulated digitally such that a disabled “DAC” behaves like an enabled DAC receiving 0 as its input.
• Additionally, CH3’s DAC has its output inverted. In particular, this causes the channel to emit a loud spike when disabled; therefore, it’s a good idea to “disconnect” the channel using NR51 before accessing wave RAM.

None of the additional features (more wave RAM, digital FIFOs, etc.) are available to CGB programs.

Joypad Input

FF00 — P1/JOYP: Joypad

The eight Game Boy action/direction buttons are arranged as a 2×4 matrix. Select either action or direction buttons by writing to this register, then read out the bits 0-3.

• Select buttons: If this bit is 0, then buttons (SsBA) can be read from the lower nibble.

• Select d-pad: If this bit is 0, then directional keys can be read from the lower nibble.

• The lower nibble is Read-only. Note that, rather unconventionally for the Game Boy, a button being pressed is seen as the corresponding bit being 0, not 1.

  If neither buttons nor d-pad is selected ($30 was written), then the low nibble reads $F (all buttons released).

Usage in SGB software

Beside for normal joypad input, SGB games misuse the joypad register to output SGB command packets to the SNES, also, SGB programs may read out gamepad states from up to four different joypads which can be connected to the SNES. See SGB description for details.

Serial Data Transfer (Link Cable)

Communication between two Game Boy systems happens one byte at a time. One Game Boy generates a clock signal internally and thus controls when the exchange happens. In SPI terms, the Game Boy generating the clock is called the “master” while the other one (the “slave” Game Boy) receives it. If it hasn’t gotten around to loading up the next data byte at the time the transfer begins, the last one will go out again. Alternately, if it’s ready to send the next byte but the last one hasn’t gone out yet, it has no choice but to wait.

FF01 — SB: Serial transfer data

Before a transfer, it holds the next byte that will go out.

During a transfer, it has a blend of the outgoing and incoming bytes. Each cycle, the leftmost bit is shifted out (and over the wire) and the incoming bit is shifted in from the other side:

FF02 — SC: Serial transfer control

• Transfer enable (Read/Write): If 1, a transfer is either requested or in progress.
• Clock speed [CGB Mode only] (Read/Write): If set to 1, enable high speed serial clock (~256 kHz in single-speed mode)
• Clock select (Read/Write): 0 = External clock (“slave”), 1 = Internal clock (“master”).

The master Game Boy will load up a data byte in SB and then set SC to $81 (Transfer requested, use internal clock). It will be notified that the transfer is complete in two ways: SC’s Bit 7 will be cleared (that is, SC will be set up $01), and also a Serial interrupt will be requested.

The other Game Boy will load up a data byte and has to set SC’s Bit 7 (that is, SC=$80) to enable the serial port. The externally clocked Game Boy will have a serial interrupt requested at the end of the transfer, and SC’s Bit 7 will be cleared.

Internal Clock

In Non-CGB Mode the Game Boy supplies an internal clock of 8192Hz only (allowing to transfer about 1 KByte per second minus overhead for delays). In CGB Mode four internal clock rates are available, depending on Bit 1 of the SC register, and on whether the CGB Double Speed Mode is used:

External Clock

The external clock is typically supplied by another Game Boy, but might be supplied by another computer (for example if connected to a PC’s parallel port), in that case the external clock may have any speed. Even the old/monochrome Game Boy is reported to recognize external clocks of up to 500 kHz. And there is no limitation in the other direction: even when suppling an external clock speed of “1 bit per month,” the Game Boy will eagerly wait for the next bit to be transferred. It isn’t required that the clock pulses are sent at a regular interval either.

Timeouts

When using external clock then the transfer will not complete until the last bit is received. In case that the second Game Boy isn’t supplying a clock signal, if it gets turned off, or if there is no second Game Boy connected at all) then transfer will never complete. For this reason the transfer procedure should use a timeout counter, and abort the communication if no response has been received during the timeout interval.

Disconnects

On a disconnected link cable, the input bit on a master will start to read 1. This means a master will start to receive $FF bytes.

If a disconnection happens during transmission, the input will be pulled up to 1 over a 20uSec period. (TODO: Only measured on a CGB rev E) This means if the slave was sending a 0 bit at the time of the disconnect, you will read 0 bits for up to 20 μs. Which on a CGB at the highest speed can be more then a byte.

Delays and Synchronization

The master Game Boy should always execute a small delay after each transfer, in order to ensure that the other Game Boy has enough time to prepare itself for the next transfer. That is, the Game Boy with external clock must have set its transfer start bit before the Game Boy with internal clock starts the transfer. Alternately, the two Game Boy systems could switch between internal and external clock for each transferred byte to ensure synchronization.

Transfer is initiated when the master Game Boy sets its Transfer Start Flag. This bit is automatically set to 0 (on both) at the end of transfer. Reading this bit can be used to determine if the transfer is still active.

Timer and Divider Registers

FF04 — DIV: Divider register

This register is incremented at a rate of 16384Hz (~16779Hz on SGB). Writing any value to this register resets it to $00. Additionally, this register is reset when executing the stop instruction, and only begins ticking again once stop mode ends. This also occurs during a speed switch. (TODO: how is it affected by the wait after a speed switch?)

Note: The divider is affected by CGB double speed mode, and will increment at 32768Hz in double speed.

FF05 — TIMA: Timer counter

This timer is incremented at the clock frequency specified by the TAC register ($FF07). When the value overflows (exceeds $FF) it is reset to the value specified in TMA (FF06) and an interrupt is requested, as described below.

FF06 — TMA: Timer modulo

When TIMA overflows, it is reset to the value in this register and an interrupt is requested. Example of use: if TMA is set to $FF, an interrupt is requested at the clock frequency selected in TAC (because every increment is an overflow). However, if TMA is set to $FE, an interrupt is only requested every two increments, which effectively divides the selected clock by two. Setting TMA to $FD would divide the clock by three, and so on.

If a TMA write is executed on the same M-cycle as the content of TMA is transferred to TIMA due to a timer overflow, the old value is transferred to TIMA.

FF07 — TAC: Timer control

• Enable: Controls whether TIMA is incremented. Note that DIV is always counting, regardless of this bit.

• Clock select: Controls the frequency at which TIMA is incremented, as follows:

  Clock select	Increment every	Frequency (Hz)
  		DMG, SGB2, CGB in single-speed mode	SGB1	CGB in double-speed mode
  00	256 M-cycles	4096	~4194	8192
  01	4 M-cycles	262144	~268400	524288
  10	16 M-cycles	65536	~67110	131072
  11	64 M-cycles	16384	~16780	32768

Note that writing to this register may increase TIMA once!

Timer obscure behaviour

Timer Global Circuit

Relation between Timer and Divider register

This is a schematic of the circuit involving TAC and DIV:

Notice how the bits themselves are connected to the multiplexer and then to the falling-edge detector; this causes a few odd behaviors:

• Resetting the entire system counter (by writing to DIV) can reset the bit currently selected by the multiplexer, thus sending a “Timer tick” and/or “DIV-APU event” pulse early.
• Changing which bit of the system counter is selected (by changing the “Clock select” bits of TAC) from a bit currently set to another that is currently unset, will send a “Timer tick” pulse. (For example: if the system counter is equal to $3FF0 and TAC to $FC, writing $05 or $06 to TAC will instantly send a “Timer tick”, but $04 or $07 won’t.)
• On monochrome consoles, disabling the timer if the currently selected bit is set, will send a “Timer tick” once. This does not happen on Color models.
• On Color models, a write to TAC that fulfills the previous bullet’s conditions and turns the timer on (it was disabled before) may or may not send a “Timer tick”. The exact behaviour varies between individual consoles.

Timer overflow behavior

When TIMA overflows, the value from TMA is copied, and the timer flag is set in IF, but one M-cycle later. This means that TIMA is equal to $00 for the M-cycle after it overflows.

This only happens when TIMA overflows from incrementing, it cannot be made to happen by manually writing to TIMA.

Here is an example; SYS represents the lower 8 bits of the system counter, and TAC is $FD (timer enabled, bit 1 of SYS selected as source):

Here are some unexpected behaviors:

1. Writing to TIMA during cycle A acts as if the overflow didn’t happen! TMA will not be copied to TIMA (the value written will therefore stay), and bit 2 of IF will not be set. Writing to DIV, TAC, or other registers won’t prevent the IF flag from being set or TIMA from being reloaded.
2. Writing to TIMA during cycle B will be ignored; TIMA will be equal to TMA at the end of the cycle anyway.
3. Writing to TMA during cycle B will have the same value copied to TIMA as well, on the same cycle.

Here is how TIMA and TMA interact:

Interrupts

IME: Interrupt master enable flag [write only]

IME is a flag internal to the CPU that controls whether any interrupt handlers are called, regardless of the contents of IE. IME cannot be read in any way, and is modified by these instructions/events only:

• ei: Enables interrupt handling (that is, IME := 1)
• di: Disables interrupt handling (that is, IME := 0)
• reti: Enables interrupts and returns (same as ei immediately followed by ret)
• When an interrupt handler is executed: Disables interrupts before calling the interrupt handler

IME is unset (interrupts are disabled) when the game starts running.

The effect of ei is delayed by one instruction. This means that ei followed immediately by di does not allow any interrupts between them. This interacts with the halt bug in an interesting way.

FFFF — IE: Interrupt enable

• VBlank (Read/Write): Controls whether the VBlank interrupt handler may be called (see IF below).
• LCD (Read/Write): Controls whether the LCD interrupt handler may be called (see IF below).
• Timer (Read/Write): Controls whether the Timer interrupt handler may be called (see IF below).
• Serial (Read/Write): Controls whether the Serial interrupt handler may be called (see IF below).
• Joypad (Read/Write): Controls whether the Joypad interrupt handler may be called (see IF below).

FF0F — IF: Interrupt flag

• VBlank (Read/Write): Controls whether the VBlank interrupt handler is being requested.
• LCD (Read/Write): Controls whether the LCD interrupt handler is being requested.
• Timer (Read/Write): Controls whether the Timer interrupt handler is being requested.
• Serial (Read/Write): Controls whether the Serial interrupt handler is being requested.
• Joypad (Read/Write): Controls whether the Joypad interrupt handler is being requested.

When an interrupt request signal (some internal wire going from the PPU/APU/… to the CPU) changes from low to high, the corresponding bit in the IF register becomes set. For example, bit 0 becomes set when the PPU enters the VBlank period.

Any set bits in the IF register are only requesting an interrupt. The actual execution of the interrupt handler happens only if both the IME flag and the corresponding bit in the IE register are set; otherwise the interrupt “waits” until both IME and IE allow it to be serviced.

Since the CPU automatically sets and clears the bits in the IF register, it is usually not necessary to write to the IF register. However, the user may still do that in order to manually request (or discard) interrupts. Just like real interrupts, a manually requested interrupt isn’t serviced unless/until IME and IE allow it.

Interrupt Handling

1. The IF bit corresponding to this interrupt and the IME flag are reset by the CPU. The former “acknowledges” the interrupt, while the latter prevents any further interrupts from being handled until the program re-enables them, typically by using the reti instruction.
2. The corresponding interrupt handler (see the IE and IF register descriptions above) is called by the CPU. This is a regular call, exactly like what would be performed by a call <address> instruction (the current PC is pushed onto the stack and then set to the address of the interrupt handler).

The following interrupt service routine is executed when control is being transferred to an interrupt handler:

1. Two wait states are executed (2 M-cycles pass while nothing happens; presumably the CPU is executing nops during this time).
2. The current value of the PC register is pushed onto the stack, consuming 2 more M-cycles.
3. The PC register is set to the address of the handler (one of: $40, $48, $50, $58, $60). This consumes one last M-cycle.

The entire process lasts 5 M-cycles.

Interrupt Priorities

In the following circumstances it is possible that more than one bit in the IF register is set, requesting more than one interrupt at once:

1. More than one interrupt request signal changed from low to high at the same time.
2. Several interrupts have been requested while IME/IE didn’t allow them to be serviced.
3. The user has written a value with several bits set (for example binary 00011111) to the IF register.

If IME and IE allow the servicing of more than one of the requested interrupts, the interrupt with the highest priority is serviced first. The priorities follow the order of the bits in the IE and IF registers: Bit 0 (VBlank) has the highest priority, and Bit 4 (Joypad) has the lowest priority.

Nested Interrupts

The CPU automatically disables all the other interrupts by setting IME=0 when it services an interrupt. Usually IME remains zero until the interrupt handler returns (and sets IME=1 by means of the reti instruction). However, if you want to allow the servicing of other interrupts (of any priority) during the execution of an interrupt handler, you may do so by using the ei instruction in the handler.

Interrupt Sources

INT $40 — VBlank interrupt

This interrupt is requested every time the Game Boy enters VBlank (Mode 1).

The VBlank interrupt occurs ca. 59.7 times a second on a handheld Game Boy (DMG or CGB) or Game Boy Player and ca. 61.1 times a second on a Super Game Boy (SGB). This interrupt occurs at the beginning of the VBlank period (LY=144). During this period video hardware is not using VRAM so it may be freely accessed. This period lasts approximately 1.1 milliseconds.

INT $48 — STAT interrupt

There are various sources which can trigger this interrupt to occur as described in STAT register ($FF41).

The various STAT interrupt sources (modes 0-2 and LYC=LY) have their state (inactive=low and active=high) logically ORed into a shared “STAT interrupt line” if their respective enable bit is turned on.

A STAT interrupt will be triggered by a rising edge (transition from low to high) on the STAT interrupt line.

Using the STAT interrupt

One very popular use is to indicate to the user when the video hardware is about to redraw a given LCD line. This can be useful for dynamically controlling the SCX/SCY registers ($FF43/$FF42) to perform special video effects.

Example application: set LYC to WY, enable LY=LYC interrupt, and have the handler disable objects. This can be used if you use the window for a text box (at the bottom of the screen), and you want objects (sprites) to be hidden by the text box.

INT $50 — Timer interrupt

The timer interrupt is requested every time that the timer overflows (that is, when TIMA exceeds $FF).

INT $58 — Serial interrupt

The serial interrupt is requested upon completion of a serial data transfer. In other words, eight serial clock cycles after starting a transfer (by setting SC bit 7), the incoming data will be in SB and the interrupt will be requested.

INT $60 — Joypad interrupt

The Joypad interrupt is requested when any of P1 bits 0-3 change from High to Low. This happens when a button is pressed (provided that the action/direction buttons are enabled by bit 5/4, respectively), however, due to switch bounce, one or more High to Low transitions are usually produced when pressing a button.

Using the joypad interrupt

This interrupt is useful to identify button presses if we have only selected either action (bit 5) or direction (bit 4), but not both. If both are selected and, for example, a bit is already held Low by an action button, pressing the corresponding direction button would make no difference. The only meaningful purpose of the Joypad interrupt would be to terminate the STOP (low power) standby state. GBA SP, because of the different buttons used, seems to not be affected by switch bounce.

halt

halt is an instruction that pauses the CPU (during which less power is consumed) when executed. The CPU wakes up as soon as an interrupt is pending, that is, when the bitwise AND of IE and IF is non-zero.

Most commonly, IME is set. In this case, the CPU simply wakes up, and before executing the instruction after the halt, the interrupt handler is called normally.

If IME is not set, there are two distinct cases, depending on whether an interrupt is pending as the halt instruction is first executed.

• If no interrupt is pending, halt executes as normal, and the CPU resumes regular execution as soon as an interrupt becomes pending. However, since IME=0, the interrupt is not handled.
• If an interrupt is pending, halt immediately exits, as expected, however the “halt bug”, explained below, is triggered.

halt bug

When a halt instruction is executed with IME = 0 and [IE] & [IF] != 0, the halt instruction ends immediately, but pc fails to be normally incremented.

Under most circumstances, this causes the byte after the halt to be read a second time (and this behaviour can repeat if said byte executes another halt instruction). But, if the halt is immediately followed by a jump to elsewhere, then the behaviour will be slightly different; this is possible in only one of two ways:

• The halt comes immediately after a ei instruction (whose effect is typically delayed by one instruction, hence IME still being zero for the halt): the interrupt is serviced and the handler called, but the interrupt returns to the halt, which is executed again, and thus waits for another interrupt. (Source)
• The halt is immediately followed by a rst instruction: the rst instruction’s return address will point at the rst itself, instead of the byte after it. Notably, a ret would return to the rst an execute it again.

If the bugged halt is preceded by a ei and followed by a rst, the former “wins”.

CGB Registers

This chapter describes only Game Boy Color (GBC or CGB) registers that didn’t fit into normal categories — most CGB registers are described in the chapter about Video Display (Color Palettes, VRAM Bank, VRAM DMA Transfers, and changed meaning of Bit 0 of LCDC Control register). Also, a changed bit is noted in the chapter about the Serial/Link port.

Unlocking CGB functions

When using any CGB registers (including those in the Video/Link chapters), you must first unlock CGB features by changing byte 0143 in the cartridge header. Typically, use a value of $80 for games which support both CGB and monochrome Game Boy systems, and $C0 for games which work on CGBs only. Otherwise, the CGB will operate in monochrome “Non CGB” compatibility mode.

Detecting CGB (and GBA) functions

CGB hardware can be detected by examining the CPU accumulator (A-register) directly after startup. A value of $11 indicates CGB (or GBA) hardware, if so, CGB functions can be used (if unlocked, see above). When A=$11, you may also examine Bit 0 of the CPUs B-Register to separate between CGB (bit cleared) and GBA (bit set), by that detection it is possible to use “repaired” color palette data matching for GBA displays.

Documented registers

LCD VRAM DMA Transfers

FF51–FF52 — HDMA1, HDMA2 (CGB Mode only): VRAM DMA source (high, low) [write-only]

These two registers specify the address at which the transfer will read data from. Normally, this should be either in ROM, SRAM or WRAM, thus either in range 0000-7FF0 or A000-DFF0. [Note: this has yet to be tested on Echo RAM, OAM, FEXX, IO and HRAM]. Trying to specify a source address in VRAM will cause garbage to be copied.

The four lower bits of this address will be ignored and treated as 0.

FF53–FF54 — HDMA3, HDMA4 (CGB Mode only): VRAM DMA destination (high, low) [write-only]

These two registers specify the address within 8000-9FF0 to which the data will be copied. Only bits 12-4 are respected; others are ignored. The four lower bits of this address will be ignored and treated as 0.

FF55 — HDMA5 (CGB Mode only): VRAM DMA length/mode/start

These registers are used to initiate a DMA transfer from ROM or RAM to VRAM. The Source Start Address may be located at 0000-7FF0 or A000-DFF0, the lower four bits of the address are ignored (treated as zero). The Destination Start Address may be located at 8000-9FF0, the lower four bits of the address are ignored (treated as zero), the upper 3 bits are ignored either (destination is always in VRAM).

Writing to this register starts the transfer, the lower 7 bits of which specify the Transfer Length (divided by $10, minus 1), that is, lengths of $10-$800 bytes can be defined by the values $00-$7F. The upper bit indicates the Transfer Mode:

Bit 7 = 0 — General-Purpose DMA

When using this transfer method, all data is transferred at once. The execution of the program is halted until the transfer has completed. Note that the General Purpose DMA blindly attempts to copy the data, even if the LCD controller is currently accessing VRAM. So General Purpose DMA should be used only if the Display is disabled, or during VBlank, or (for rather short blocks) during HBlank. The execution of the program continues when the transfer has been completed, and FF55 then contains a value of $FF.

Bit 7 = 1 — HBlank DMA

The HBlank DMA transfers $10 bytes of data during each HBlank, that is, at LY=0-143, no data is transferred during VBlank (LY=144-153), but the transfer will then continue at LY=00. The execution of the program is halted during the separate transfers, but the program execution continues during the “spaces” between each data block. Note that the program should not change the Destination VRAM bank (FF4F), or the Source ROM/RAM bank (in case data is transferred from bankable memory) until the transfer has completed! (The transfer should be paused as described below while the banks are switched).

Upon halting the CPU (using the halt instruction), the transfer will also be halted and will be resumed only when the CPU resumes execution (test rom exhibiting this behaviour).

Reading from Register FF55 returns the remaining length (divided by $10, minus 1), a value of $FF indicates that the transfer has completed. It is also possible to terminate an active HBlank transfer by writing zero to Bit 7 of FF55. In that case reading from FF55 will return how many $10 “blocks” remained (minus 1) in the lower 7 bits, but Bit 7 will be read as “1”. Stopping the transfer doesn’t set HDMA1-4 to $FF.

Confirming if the DMA Transfer is Active

Reading Bit 7 of FF55 can be used to confirm if the DMA transfer is active (1=Not Active, 0=Active). This works under any circumstances - after completion of General Purpose, or HBlank Transfer, and after manually terminating a HBlank Transfer.

Transfer Timings

In both Normal Speed and Double Speed Mode it takes about 8 μs to transfer a block of $10 bytes. That is, 8 M-cycles in Normal Speed Mode [1], and 16 “fast” M-cycles in Double Speed Mode [2]. Older MBC controllers (like MBC1-3) and slower ROMs are not guaranteed to support General Purpose or HBlank DMA, that’s because there are always 2 bytes transferred per microsecond (even if the itself program runs it Normal Speed Mode).

This allows for a transfer of 2280 bytes during VBlank, which is up to 142.5 tiles.

VRAM Banks

The CGB has twice the VRAM of the DMG, but it is banked and either bank has a different purpose.

FF4F — VBK (CGB Mode only): VRAM bank

This register can be written to change VRAM banks. Only bit 0 matters, all other bits are ignored.

VRAM bank 1

VRAM bank 1 is split like VRAM bank 0 ; 8000-97FF also stores tiles (just like in bank 0), which can be accessed the same way as (and at the same time as) bank 0 tiles. 9800-9FFF contains the attributes for the corresponding Tile Maps.

Reading from this register will return the number of the currently loaded VRAM bank in bit 0, and all other bits will be set to 1.

FF4D — KEY1 (CGB Mode only): Prepare speed switch

• Current speed (Read-only): 0 = Single-speed mode, 1 = Double-speed mode
• Switch armed (Read/Write): 0 = No, 1 = Armed

This register is used to prepare the Game Boy to switch between CGB Double Speed Mode and Normal Speed Mode. The actual speed switch is performed by executing a stop instruction after Bit 0 has been set. After that, Bit 0 will be cleared automatically, and the Game Boy will operate at the “other” speed. The recommended speed switching procedure in pseudocode would be:

IF KEY1_BIT7 != DESIRED_SPEED THEN
   IE = $00       ; (FFFF) = $00
   JOYP = $30     ; (FF00) = $30
   KEY1 = $01     ; (FF4D) = $01
   STOP
ENDIF

The CGB is operating in Normal Speed Mode when it is first turned on. Note that using the Double Speed Mode increases the power consumption; therefore, it would be recommended to use Single Speed whenever possible.

In Double Speed Mode the following will operate twice as fast as normal:

• The CPU (2.10 MHz, so 1 M-cycle = approx. 0.5 µs)
• Timer and Divider Registers
• Serial Port (Link Cable)
• DMA Transfer to OAM

And the following will keep operating as usual:

• LCD Video Controller
• HDMA Transfer to VRAM
• All Sound Timings and Frequencies

The CPU stops for 2050 M-cycles (= 8200 T-cycles) after the stop instruction is executed. During this time, the CPU is in a strange state. DIV does not tick, so some audio events are not processed. Additionally, VRAM/OAM/… locking is “frozen”, yielding different results depending on the PPU mode it’s started in:

• HBlank / VBlank (Mode 0 / Mode 1): The PPU cannot access any video memory, and produces black pixels
• OAM scan (Mode 2): The PPU can access VRAM just fine, but not OAM, leading to rendering background, but not objects (sprites)
• Rendering (Mode 3): The PPU can access everything correctly, and so rendering is not affected

TODO: confirm whether interrupts can occur (just the joypad one?) during the pause, and consequences if so

FF56 — RP (CGB Mode only): Infrared communications port

This register allows to input and output data through the CGBs built-in Infrared Port. When reading data, bit 6 and 7 must be set (and obviously Bit 0 must be cleared — if you don’t want to receive your own Game Boy’s IR signal). After sending or receiving data you should reset the register to $00 to reduce battery power consumption again.

• Read enable (Read/Write): 0 = Disable (bit 1 reads 1), 3 = Enable
• Receiving (Read-only): 0 = Receiving IR signal, 1 = Normal
• Emitting (Read/Write): 0 = LED off, 1 = LED on

Note that the receiver will adapt itself to the normal level of IR pollution in the air, so if you would send a LED ON signal for a longer period, then the receiver would treat that as normal (=OFF) after a while. For example, a Philips TV Remote Control sends a series of 32 LED ON/OFF pulses (length 10us ON, 17.5us OFF each) instead of a permanent 880us LED ON signal. Even though being generally CGB compatible, the GBA does not include an infra-red port.

FF6C — OPRI (CGB Mode only): Object priority mode

This register serves as a flag for which object priority mode to use. While the DMG prioritizes objects by x-coordinate, the CGB prioritizes them by location in OAM. This flag is set by the CGB bios after checking the game’s CGB compatibility.

OPRI has an effect if a PGB value (0xX8, 0xXC) is written to KEY0 but STOP hasn’t been executed yet, and the write takes effect instantly.

• Priority mode (Read/Write): 0 = CGB-style priority, 1 = DMG-style priority

FF70 — SVBK (CGB Mode only): WRAM bank

In CGB Mode, 32 KiB of internal RAM are available. This memory is divided into 8 banks of 4 KiB each. Bank 0 is always available in memory at C000–CFFF, banks 1–7 can be selected into the address space at D000–DFFF.

• WRAM bank (Read/Write): Writing a value will map the corresponding bank to D000–DFFF, except 0, which maps bank 1 instead.

Undocumented registers

These are undocumented CGB Registers. The purpose of these registers is unknown (if any). It isn’t recommended to use them in your software, but you could, for example, use them to check if you are running on an emulator or on DMG hardware.

FF72–FF73 — Bits 0–7 (CGB Mode only)

Both of these registers are fully read/write. Their initial value is $00.

FF74 — Bits 0–7 (CGB Mode only)

In CGB mode, this register is fully readable and writable. Its initial value is $00.

Otherwise, this register is read-only, and locked at value $FF.

FF75 — Bits 4–6 (CGB Mode only)

Only bits 4, 5 and 6 of this register are read/write enabled. Their initial value is 0.

GBC Infrared Communication

This section was originally compiled by Shonumi in the “Dan Docs”. Upstream source can be found here.

The Game Boy Color came with an infrared port on the very top of the handheld. Previously, where IR communications had to be done with special cartridges (like the HuC-1 variants), the Game Boy itself now had the hardware built-in. Unfortunately, the feature was never popular outside of a few games and accessories. The IR port essentially sends out signals and is also capable of receiving them, allowing for fast, wireless, line-of-sight transmission.

• GBC comes with one IR port. Capable of sending and receiving an IR signal (two separate diodes).
• Turning on the IR light does drain battery, hence not recommended leaving it on when not in use
• IR port can communicate with non-GBC devices, e.g. anything that sends an IR signal (TV remotes, Wiimotes, household lamps, etc)

Communication Types

While a number of games may use similar formats for their IR communications, there is no “standard” protocol that all games use. IR communication is entirely determined by the game’s code, hence it can vary wildly depending on needs. However, all communications fall into one of several general categories as described below:

• 1-Player Init: These only require one GBC to initiate IR transfers. Both GBCs typically wait for an infrared signal. When one player presses a button, the GBC starts sending pulses. This setup is not unlike how 2-Player Serial I/O is handled (with master and slave Game Boys). Examples include Super Mario Bros. DX score exchange and the GBC-to-GBC Mystery Gifts in Pokemon Gold/Silver/Crystal. Most IR compatible games fall into this group.
• 2-Player Init: Transfers require both GBCs to initiate at roughly the same time. Examples include Pokemon Pinball score exchange, Pokemon TCG’s “Card Pop”, and trading/fighting Charaboms in the Bomberman games.
• Active Object Init: Transfers require the GBC to interact with another non-GBC device capable of both sending and receiving infrared signals. These objects are designed to work specifically with GBCs and send pulses in much the same manner as a GBC would. Examples include Mystery Gifts via the Pokemon Pikachu 2 and trading Points via the Pocket Sakura.
• Inactive Object Init: Transfers require the GBC to interact with another non-GBC device capable of sending infrared signals but not necessarily receiving them. These objects may not be designed to work specifically with GBCs (notable exception is the Full Changer). Communication is input-only for these cases. Examples include Zok Zok Heroes, Chee Chai Alien, the Bomberman Max games’ special stages, and Mission Impossible’s TV remote feature.

Communication Protocol

Again, there is no set or established infrared protocol that games must follow. Many games vary in their approach. For example, the 2nd Generation Pokemon games use the GBC’s hardware timers, while others have hardcoded values that count cycles to check timing. The simplest form is a bare-bones communication protocol, i.e. something like a binary Morse code where a “0” is a long ON-OFF pulse and “1” is a short ON-OFF pulse or vice versa. Properly done, data could have been short, compact, and easily converted into bytes in RAM. Sakura Taisen GB seems to follow this model in its communications with the Pocket Sakura. Not all games do this, however, and appear to be doing who knows that, opting instead for customized and specialized communications unique to each title. To illustrate this idea, it would be possible to use a table of given lengths of IR ON-OFF pulses so that individual bytes could be sent all at once instead of in a binary, bit-by-bit manner. A number of games try to send a few pulses when pressing input like the A button and wait for another GBC to echo that in response, but after the handshake, most of the IR pulses are impossible to understand without disassembling the code.

One thing to note is that 4 games in particular do share somewhat similar IR protocols, at least regarding the initial handshake between 2 GBCs. They are Pokemon TCG 1 & 2 and Bomberman Max Red & Blue, all from the “2-Player Init” category above. Typically, IR capable GBC games will continually wait for an IR signal on both sides, i.e. the “1-Player Init” category. When one player presses certain input, that GBC takes the initiative and sends out a few IR pulses. That is to say, for most IR games, it only takes just one player to start the entire process.

The handshake for the 4 games previously mentioned, however, requires both players to input at almost the same time. One has to be slightly faster or slower than the other. Each side continually sends a few IR pulses, then reads the sensor to see if anything was received. If so, the GBCs begin to sync. The idea is that one side should be sending while the other is checking, and then the handshake completes. This is why one side needs to be faster or slower to input; if they are sending IR signals at the same time, they don’t see anything when reading the sensor. As a result, both GBCs cannot input at exactly the same time. Practically speaking, this is unlikely to happen under normal circumstances, since most humans can’t synchronize their actions down to a handful of microseconds, so the handshake will normally succeed.

RP Register

The following is just theory. This handshake is possibly an artifact of the HuC-1. Consider that the Japanese version of Pokemon TCG 1 used the HuC-1 for its IR communications, and the developers may have borrowed the “best practices” used by other HuC-1/“GB KISS” games. When bringing Pokemon TCG 1 overseas, the IR handling code was likely minimally adapted to use the GBC’s IR port, with the underlying protocol remaining unchanged in most regards. Pokemon TCG 2 ditched the HuC-1 in favor of the GBC IR port, so the IR code from non-Japanese versions of Pokemon TCG 1 was copy+pasted. The Bomberman games were made by Hudson Soft, literally the same people who created the HuC-1 in the first place. They too probably used the same protocol that had worked forever in their “GB KISS” games, so they used the same handshake method as before, just on the GBC IR port now. More research into the HuC-1 itself and the games needs to be done to confirm any of this.

On the GBC, the MMIO register located at $FF56 controls infrared communication. Simply known as “RP” (Radiation Port? Reception Port? Red Port???), it is responsible for sending and receiving IR signals. Below is a diagram of the 8-bit register:

Turning on the IR light is as simple as writing to Bit 0 of RP. Reading is a bit more complicated. Bits 6 and 7 must both be set ($C0), to read Bit 1, otherwise Bit 1 returns 1, acting as if no signal is detected, except in edge cases detailed below in “Obscure Behavior”. With signal reading enabled, Bit 1 will determine the status of any incoming IR signals. Like other Game Boy MMIO registers, unused bits read high (set to 1).

Signal Fade

The IR sensor in the GBC adapts to the current level of IR light. That is to say, if the GBC receives a sustained IR signal beyond a certain amount of time, eventually the sensor treats this as a new “normal” level of IR light, and Bit 1 of RP goes back to 1. This is called the signal “fade” because it may appear as if the signal disappears.

Signal fade time is dependent on length and has an inverse relationship with the distance between a GBC and the IR light. The closer a GBC is to the IR source, the longer the fade time. The farther away a GBC is to the IR source, the shorter the fade time. One possible explanation for everything is that the IR signal is weaker on the receiving end, so the signal is prone to get “lost” to surrounding noise. The GBC IR sensor is probably good at sending IR signals (evidenced by the Mission Impossible cheat to turn a GBC into a TV remote) but not so good at picking up signals (evidenced by Chee Chai Aliens plastic add-on to enhance IR reception).

At about 3.0 to 3.5 inches (7.62 to 8.89 cm) signal fade time appears to be around 3ms. Optimal distance seems to be 2.5 to 4.0 inches (6.35 to 10.16 cm) to maintain a fade time close to 3ms and avoid potential miscommunication. One oddity of note is that putting two GBCs very close together (physically touching) produced unusually short fade times, far shorter than 3ms. There may be some sort of interference at that range.

Obscure Behavior

The RP register has one very strange quirk. Disabling Bits 6 and 7 and then subsequently re-enabling them causes Bit 1 to go to zero under certain conditions. In other words, the IR sensor will act as if it is detecting a signal if reading the signal is disabled then enabled. It seems this behavior happens in the presence of any light; covering up the sensor during the read signal disable/enable causes the sensor to act normally. It’s possible that the sensor resets itself (to its lowest level of detection???) and immediately detects any infrared sources, even from ambient/environmental light. The presence of any noise may temporarily trick the sensor into “seeing” IR light. By abusing this behavior, the GBC has some rudimentary ability to gauge the type of nearby lighting:

Writing $00 to RP, followed by $C0 will trigger these results listed above. One very important thing to note is that when enabling Bits 6 and 7 (writing $C0), it does take some time for the sensor to register legitimate IR light coming into the sensor. I.e. if you want to use this method to detect what kind of light a GBC is looking at, the software needs to loop for a bit until Bit 1 of RP changes. Generally a few hundred cycles in double-speed mode will suffice. If Bit 1 of RP remains 1 after the loop, it’s safe to assume the lighting is either ambient or dark. This delay doesn’t seem to happen when Bits 6 and 7 are never disabled (which is what most official GBC software does). Games typically write either $C0 or $C1 to RP, with a small handful setting it to $00 initially when setting up other MMIO registers (Pokemon G/S/C does this).

The downside to this method is that when detecting a bright IR source, the sensor quickly adjusts to this new level, and the next attempt at writing $00 followed by $C0 to RP will result in readings of dark or ambient (typically dark though). Essentially the bright result only appears briefly when transitioning from lower levels of light, then it “disappears” thanks to the short time it takes for IR signal fade. Designing a game mechanic (darkness and light) around this quirk is still possible, although it would require careful thought and planning to properly work around the observed limitations.

One suggested method: once the Bright setting is detected, switch to writing only $C0 to RP so that the IR sensor works normally. If IR light stops being detected, switch to alternating $00 and $C0 writes as described above to determine Dark or Ambient settings. Whether it’s practical or not to do this in a game remains theoretical at this point.

SGB Description

General Description

The Super Game Boy (SGB) is an adapter cartridge that allows to play Game Boy games on a SNES (Super Nintendo Entertainment System) gaming console. In detail, you plug the Game Boy cartridge into the SGB cartridge, then plug the SGB cartridge into the SNES, and then connect the SNES to your TV Set. In result, games can be played and viewed on the TV Set, and are controlled by using the SNES joypad(s).

More Technical Description

The SGB cartridge contains a Game Boy system on chip, with its normal CPU and video and sound controller. It also has a bridge circuit, the ICD2, to translate joypad input, video signal, and control packets between the SNES and GB as well as a system software ROM that runs on the SNES. The system software forwards button presses from the controllers and sends the video signal to the TV set, giving both the player and the game a small amount of control over the appearance.

Normal Monochrome Games

Any Game Boy games which have been designed for monochrome handheld Game Boy systems will work with the SGB hardware as well. The SGB will apply a four color palette to these games by replacing the normal four shades of gray. The 160×144 pixel game screen is displayed in the middle of the 256×224 pixel SNES screen (the unused area is filled by a screen border bitmap). The user may access built-in menus, allowing to change color palette data, to select between several pre-defined borders, etc.

Games that have been designed to support SGB functions may also access the following additional features:

Colorized Game Screen

There’s limited ability to colorize the game screen by assigning custom color palettes to each 20×18 display characters, however, this works mainly for static display data such like title screens or status bars, the 20×18 color attribute map cannot be scrolled, and it is not possible to assign separate colors to moveable foreground objects (sprites), so that animated screen regions will be typically restricted to using a single palette of four colors only.

SNES Foreground Sprites

Up to 24 additional 16-color objects of 8×8 or 16×16 pixels, can be displayed. When replacing (or just overlaying) the normal Game Boy objects by SNES objects it’d be thus possible to display objects with other colors than normal background area. This method doesn’t appear to be very popular, even though it appears to be quite easy to implement, however, the bottommost character line of the game screen will be masked out because this area is used to transfer object positions to the SNES. (Later versions of the SGB system software remove much of this object enhancement.)

The SGB Border

The possibly most popular and most impressive feature is to replace the default SGB screen border by a custom bitmap which is stored in the game cartridge. A border can be much more colorful than the game screen, as it uses 4 bits per pixel (16 color per tile) instead of 2 bpp (4 colors/tile).

Multiple Joypads

Up to four joypads can be conected to the SNES, and SGB software may read-out each of these joypads separately, allowing up to four players to play the same game simultaneously. Unlike for multiplayer handheld games, this requires only one game cartridge and only one SGB/SNES, and no link cables are required, the downside is that all players must share the same display screen.

Sound Functions

Alongside normal Game Boy sound, a number of digital sound effects is pre-defined in the SNES BIOS, these effects may be accessed quite easily. Programmers whom are familiar with SNES sounds may also access the SNES sound chip, or use the SNES “Kankichi” sequencer engine directly in order to produce other sound effects or music.

Taking Control of the SNES CPU

Finally, it is possible to write program code or data into SNES memory, and to execute such program code by using the SNES CPU.

SGB System Clock

Because the SGB is synchronized to the SNES CPU, the Game Boy system clock is directly chained to the SNES system clock. In result, the Game Boy CPU, video controller, timers, and sound frequencies will be all operated approx 2.4% faster than handheld systems. Basically, this should be no problem, and the game will just run a little bit faster. However sensitive musicians may notice that sound frequencies are a bit too high, particularly in programs that use GB sound alongside Kankichi. Programs that support SGB functions may avoid this effect by reducing frequencies of Game Boy sounds when having detected SGB hardware. Also, “PAL version” SNES models which use a 50Hz display refresh rate (rather than 60Hz) result in respectively slower Game Boy timings.

• NTSC SGB: 21.477 MHz master clock, 4.2955 MHz GB clock, 2.41% fast
• PAL SGB: 21.281 MHz master clock, 4.2563 MHz GB clock, 1.48% fast
• NTSC SGB2: Separate 20.972 MHz crystal, correct speed

Unlocking and Detecting SGB Functions

Cartridge Header

SGB games are required to have a cartridge header with Nintendo logo and proper checksum just as normal Game Boy games. Also, two special entries must be set in order to unlock SGB functions:

• SGB flag: Must be set to $03 for SGB games
• Old licensee code: Must be set to $33 for SGB games

When these entries aren’t set, the game will still work just like all “monochrome” Game Boy games, but it cannot access any of the special SGB functions.

Detecting SGB hardware

SGB hardware can be detected by examining the initial value of the C register directly after startup: a value of $14 indicates SGB or SGB2 hardware. It is also possible to separate between SGB and SGB2 by examining the initial value of the A register directly after startup. Note that the DMG and MGB share initial A register values with the SGB and SGB2 respectively.

For initial register values on all systems, see the table of all CPU registers after power-up.

The SGB2 doesn’t have any extra features which’d require separate SGB2 detection except for curiosity purposes, for example, the game “Tetris DX” chooses to display an alternate SGB border on SGB2s.

Only the SGB2 contains a link port.

SGB hardware has traditionally been detected by sending MLT_REQ commands, but this method is more complicated and slower than checking the value of the A and C registers after startup. The MLT_REQ command enables two (or four) joypads; a normal handheld Game Boy will ignore this command, but an SGB will return incrementing joypad IDs each time when deselecting keypad lines (see MLT_REQ description). The joypad state/IDs can then be read out several times, and if the IDs are changing, then it is an SGB (a normal Game Boy would typically always return $0F as the ID). Finally, when not intending to use more than one joypad, send another MLT_REQ command in order to disable the multi-controller mode. Detection works regardless of how many joypads are physically connected to the SNES. However, unlike the C register method, this detection works only when SGB functions are unlocked from the cartridge header.

Command Packet Transfers

Command packets are transferred from the Game Boy to the SNES by using bits 4 and 5 of of the JOYP register. These lines are normally used to select one of the two button groups (which still works as usual).

Transferring Bits

A command packet transfer must be initiated by setting JOYP bits 4 and 5 both to 0; this will reset and start the ICD2 packet receiving circuit. Data is then transferred (LSB first), setting bit 4 to 0 will indicate a 0 bit, and setting bit 5 to 0 will indicate a 1 bit. For example:

The boot ROM and licensed software keep data and reset pulses LOW for at least 5 M-cycles and leave bit 4 and 5 both to 1 for at least 15 M-cycles after each pulse. Though the hardware is capable of receiving pulses and spaces as short as 2 M-cycles (as tested using sgb-speedtest), following the common practice of 5 M-cycle pulses and 15 M-cycle spaces may improve reliability in some corner case that the community has not yet discovered.

Packet format

Each packet transfer is started by a “Start” pulse, then 16 bytes (128 bits) of data are transferred, the LSB of each byte first; and finally, a 0 bit must be transferred as a stop bit. These 130 bit periods correspond to at least 2600 M-cycles at the recommended rate.

The structure of the first packet in a transmission is:

1. 1 pulse: Start signal
2. 1 byte: Header byte (Command Code × 8 + Length)
3. 15 bytes: Data
4. 1 bit: Stop Bit (0)

The above Length indicates the total number of packets (1-7, including the first packet) which will be sent. If more than 15 data bytes are used, then further packet(s) will follow, as such:

1. 1 pulse: Start signal
2. 16 bytes: Data
3. 1 bit: Stop Bit (0)

By using all 7 packets, up to 111 data bytes (15 + 16 × 6) may be sent.

VRAM Transfers

Overview

Beside for the packet transfer method, larger data blocks of 4KBytes can be transferred by using the video signal. These transfers are invoked by first sending one of the commands with the ending _TRN (by using normal packet transfer), the 4K data block is then read-out by the SNES from Game Boy display memory during the next frame.

Transfer Data

Normally, transfer data should be stored at 8000-8FFF in Game Boy VRAM, even though the SNES receives the data in from display scanlines, it will automatically re-produce the same ordering of bits and bytes, as being originally stored at 8000-8FFF in Game Boy memory.

Preparing the Display

The above method works only when recursing the following things: BG Map must display unsigned characters $00-$FF on the screen; $00..$13 in first line, $14..$27 in next line, etc. The Game Boy display must be enabled, the display may not be scrolled, objects (sprites) should not overlap the background tiles, the BGP palette register must be set to $E4.

Transfer Time

Note that the transfer data should be prepared in VRAM before sending the transfer command packet. The actual transfer starts at the beginning of the next frame after the command has been sent, and the transfer ends at the end of the 5th frame after the command has been sent (not counting the frame in which the command has been sent). The displayed data must not be modified during the transfer, as the SGB reads it in multiple chunks.

Avoiding Screen Garbage

The display will contain “garbage” during the transfer, this dirt-effect can be avoided by freezing the screen (in the state which has been displayed before the transfer) by using the MASK_EN command. Of course, this works only when actually executing the game on an SGB (and not on handheld Game Boy systems), it’d be thus required to detect the presence of SGB hardware before blindly sending VRAM data.

Color Palettes Overview

Available SNES Palettes

The SGB/SNES provides 8 palettes of 16 colors each, each color may be defined out of a selection of 32768 colors (15 bit). Palettes 0-3 are used to colorize the gamescreen, only the first four colors of each of these palettes are used. Palettes 4-7 are used for the SGB Border, all 16 colors of each of these palettes may be used.

Color format

Colors are encoded as 16-bit RGB numbers, in the following way:

The palettes are encoded little-endian, thus, the Red+Green byte comes first in memory.

Here’s a formula to convert 24-bit RGB into SNES format: (color & 0xF8) << 7 | (color & 0xF800) >> 6 | (color & 0xF80000) >> 19

Color 0 Restriction

Color 0 of each of the eight palettes is transparent, causing the backdrop color to be displayed instead. The backdrop color is typically defined by the most recently color being assigned to Color 0 (regardless of the palette number being used for that operation). Effectively, gamescreen palettes can have only three custom colors each, and SGB border palettes only 15 colors each, additionally, color 0 can be used for for all palettes, which will then all share the same color though.

Translation of Grayshades into Colors

Because the SGB/SNES reads out the Game Boy video controllers display signal, it translates the different grayshades from the signal into SNES colors as such:

Note that Game Boy colors 0-3 are assigned to user-selectable grayshades by the Game Boy’s BGP, OBP0, and OBP1 registers. There is thus no fixed relationship between Game Boy colors 0-3 and SNES colors 0-3.

Using Game Boy BGP/OBP Registers

A direct translation of GB color 0-3 into SNES color 0-3 may be produced by setting BGP/OBPx registers to a value of $E4 each. However, in case that your program uses black background for example, then you may internally assign background as “White” at the Game Boy side by BGP/OBP registers (which is then interpreted as SNES color 0, which is shared for all SNES palettes). The advantage is that you may define Color 0 as Black at the SNES side, and may assign custom colors for Colors 1-3 of each SNES palette.

System Color Palette Memory

Beside for the actually visible palettes, up to 512 palettes of 4 colors each may be defined in SNES RAM. The palettes are just stored in RAM without any relationship to the displayed picture; however, these pre-defined colors may be transferred to actually visible palettes slightly faster than when transferring palette data by separate command packets.

Command Summary

SGB System Command Table

Palette Commands

SGB Command $00 — PAL01

Transmit color data for SGB palette 0, color 0-3, and for SGB palette 1, color 1-3 (without separate color 0).

The header byte is $00 << 3 | $01 = $01.

SGB Command $01 — PAL23

Same as PAL01 above, but for Palettes 2 and 3 respectively. The header byte is thus $09.

SGB Command $02 — PAL03

Same as PAL01 above, but for Palettes 0 and 3 respectively. The header byte is thus $11.

SGB Command $03 — PAL12

Same as PAL01 above, but for Palettes 1 and 2 respectively. The header byte is thus $19.

SGB Command $0A — PAL_SET

Used to copy pre-defined palette data from SGB system color palettes to actual SNES palettes.

The header byte is $0A << 3 | $01 = $51. All palette IDs are little-endian.

• Apply ATF: If and only if this is set, then the ATF whose ID is specified by bits 0–5 is applied as if by ATTR_SET.
• Cancel MASK_EN: If this bit is set, then any current MASK_EN “screen freeze” is cancelled.
• ATF number: Index of the ATF to transfer. Values greater than $2C are invalid.

SGB Command $0B — PAL_TRN

Used to initialize SGB system color palettes in SNES RAM. System color palette memory contains 512 pre-defined palettes, these palettes do not directly affect the display, however, the PAL_SET command may be later used to transfer four of these “logical” palettes to actual visible “physical” SGB palettes. Also, the OBJ_TRN feature will use groups of 4 System Color Palettes (4*4 colors) for SNES OBJ palettes (16 colors).

The header byte must be $59.

The palette data is sent by VRAM Transfer.

SGB Command $19 — PAL_PRI

If the player overrides the active palette set (a pre-defined or the custom one), it stays in effect until the smiley face is selected again, or the player presses the X button on their SNES controller.

However, if PAL_PRI is enabled, then changing the palette set (via any PAL_* command besides PAL_TRN) will switch back to the game’s newly-modified palette set, if it wasn’t already active.

Donkey Kong (1994) is one known game that appears to use this.

The header must be $C9.

Bit 0 of the priority byte enables (1) or disables (0) PAL_PRI.

Color Attribute Commands

SGB Command $04 — ATTR_BLK

Used to specify color attributes for the inside or outside of one or more rectangular screen regions.

 Byte  Content
 0     Command*8+Length (length=1..7)
 1     Number of Data Sets ($01..$12)
 2-7   Data Set #1
         Byte 0 - Control Code (0-7)
           Bit 0 - Change Colors inside of surrounded area     (1=Yes)
           Bit 1 - Change Colors of surrounding character line (1=Yes)
           Bit 2 - Change Colors outside of surrounded area    (1=Yes)
           Bit 3-7 - Not used (zero)
           Exception: When changing only the Inside or Outside, then the
           Surrounding line becomes automatically changed to same color.
         Byte 1 - Color Palette Designation
           Bit 0-1 - Palette Number for inside of surrounded area
           Bit 2-3 - Palette Number for surrounding character line
           Bit 4-5 - Palette Number for outside of surrounded area
           Bit 6-7 - Not used (zero)
         Data Set Byte 2 - Coordinate X1 (left)
         Data Set Byte 3 - Coordinate Y1 (upper)
         Data Set Byte 4 - Coordinate X2 (right)
         Data Set Byte 5 - Coordinate Y2 (lower)
           Specifies the coordinates of the surrounding rectangle.
 8-D   Data Set #2 (if any)
 E-F   Data Set #3 (continued at 0-3 in next packet) (if any)

When sending three or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. The format of the separate Data Sets is described below.

SGB Command $05 — ATTR_LIN

Used to specify color attributes of one or more horizontal or vertical character lines.

 Byte  Content
 0     Command*8+Length (length=1..7)
 1     Number of Data Sets ($01..$6E) (one byte each)
 2     Data Set #1
         Bit 0-4 - Line Number    (X- or Y-coordinate, depending on bit 7)
         Bit 5-6 - Palette Number (0-3)
         Bit 7   - H/V Mode Bit   (0=Vertical line, 1=Horizontal Line)
 3     Data Set #2 (if any)
 4     Data Set #3 (if any)
 etc.

When sending 15 or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. The format of the separate Data Sets (one byte each) is described below. The length of each line reaches from one end of the screen to the other end. In case that some lines overlap each other, then lines from lastmost data sets will overwrite lines from previous data sets.

SGB Command $06 — ATTR_DIV

Used to split the screen into two halfes, and to assign separate color attributes to each half, and to the division line between them.

 Byte  Content
 0     Command*8+Length   (fixed length=1)
 1     Color Palette Numbers and H/V Mode Bit
         Bit 0-1  Palette Number below/right of division line
         Bit 2-3  Palette Number above/left of division line
         Bit 4-5  Palette Number for division line
         Bit 6    H/V Mode Bit  (0=split left/right, 1=split above/below)
 2     X- or Y-Coordinate (depending on H/V bit)
 3-F   Not used (zero)

SGB Command $07 — ATTR_CHR

Used to specify color attributes for separate characters.

 Byte  Content
 0     Command*8+Length (length=1..6)
 1     Beginning X-Coordinate
 2     Beginning Y-Coordinate
 3-4   Number of Data Sets (1-360)
 5     Writing Style   (0=Left to Right, 1=Top to Bottom)
 6     Data Sets 1-4   (Set 1 in MSBs, Set 4 in LSBs)
 7     Data Sets 5-8   (if any)
 8     Data Sets 9-12  (if any)
 etc.

When sending 41 or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. Each data set consists of two bits, indicating the palette number for one character. Depending on the writing style, data sets are written from left to right, or from top to bottom. In either case the function wraps to the next row/column when reaching the end of the screen.

SGB Command $15 — ATTR_TRN

Used to initialize Attribute Files (ATFs) in SNES RAM. Each ATF consists of 20×18 color attributes for the Game Boy screen. This function does not directly affect display attributes. Instead, one of the defined ATFs may be copied to actual display memory at a later time by using ATTR_SET or PAL_SET functions.

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1-F   Not used (zero)

The ATF data is sent by VRAM-Transfer (4 KBytes).

 000-FD1  Data for ATF0 through ATF44 (4050 bytes)
 FD2-FFF  Not used

Each ATF consists of 90 bytes, that are 5 bytes (20×2 bits) for each of the 18 character lines of the Game Boy window. The two most significant bits of the first byte define the color attribute (0-3) for the first character of the first line, the next two bits the next character, and so on.

SGB Command $16 — ATTR_SET

Used to transfer attributes from Attribute File (ATF) to Game Boy window.

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1     Attribute File Number ($00-$2C), Bit 6=Cancel Mask
 2-F   Not used (zero)

When above Bit 6 is set, the Game Boy screen becomes re-enabled after the transfer (in case it has been disabled/frozen by MASK_EN command). Note: The same functions may be (optionally) also included in PAL_SET commands, as described in the chapter about Color Palette Commands.

Sound Functions

SGB Command $08 — SOUND

Used to start/stop internal sound effect, start/stop sound using internal tone data.

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1     Sound Effect A (Port 1) Decrescendo 8-bit Sound Code
 2     Sound Effect B (Port 2) Sustain     8-bit Sound Code
 3     Sound Effect Attributes
         Bit 0-1 - Sound Effect A Pitch  (0..3=Low..High)
         Bit 2-3 - Sound Effect A Volume (0..2=High..Low, 3=Mute on)
         Bit 4-5 - Sound Effect B Pitch  (0..3=Low..High)
         Bit 6-7 - Sound Effect B Volume (0..2=High..Low, 3=Not used)
 4     Music Score Code (must be zero if not used)
 5-F   Not used (zero)

See Sound Effect Tables below for a list of available pre-defined effects.

Notes:

1. Mute is only active when both bits D2 and D3 are 1.
2. When the volume is set for either Sound Effect A or Sound Effect B, mute is turned off.
3. When Mute on/off has been executed, the sound fades out/fades in.
4. Mute on/off operates on the (BGM) which is reproduced by Sound Effect A, Sound Effect B, and the Super NES APU. A “mute off” flag does not exist by itself. When mute flag is set, volume and pitch of Sound Effect A (port 1) and Sound Effect B (port 2) must be set.

SGB Command $09 — SOU_TRN

Used to transfer sound code or data to SNES Audio Processing Unit memory (APU-RAM).

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1-F   Not used (zero)

The sound code/data is sent by VRAM transfer as a contiguous list of “packets”.

All 16-bit values are little-endian.

Data transfer packet format:

 0-1    Size of data below (N); if zero, this is instead a jump packet
 2-3    Destination address in S-APU RAM (typically $2B00, see below)
 4-N+3  Data to be transferred

Jump packet format:

 0-1  Must be $0000
 2-3  S-APU jump address, use $0400 to safely restart the built-in SGB BIOS' N-SPC sound engine

Possible destinations in APU-RAM are:

This function may be used to take control of the SNES sound chip, and/or to access the SNES MIDI engine. In either case it requires deeper knowledge of SNES sound programming.

SGB Sound Effect A/B Tables

Below lists the digital sound effects that are pre-defined in the SGB BIOS, and which can be used with the SGB “SOUND” Command. Effect A and B may be simultaneously used. Sound Effect A uses channels 6 and 7, Sound Effect B uses channels 0, 1, 4 and 5. Effects that use less channels will use only the upper channels (eg. 4 and 5 for a B Effect with only two channels).

Sound Effect A Flag Table

Sound effect A is used for formanto sounds (percussion sounds).

Sound Effect B Flag Table

Sound effect B is mainly used for looping sounds (sustained sounds).

System Control Commands

SGB Command $17 — MASK_EN

Used to mask the Game Boy window, among others this can be used to freeze the Game Boy screen before transferring data through VRAM (the SNES then keeps displaying the Game Boy screen, even though VRAM doesn’t contain meaningful display information during the transfer).

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1     Game Boy Screen Mask (0-3)
         0  Cancel Mask   (Display activated)
         1  Freeze Screen (Keep displaying current picture)
         2  Blank Screen  (Black)
         3  Blank Screen  (Color 0)
 2-F   Not used (zero)

Freezing works only if the SNES has stored a picture, that is, if necessary wait one or two frames before freezing (rather than freezing directly after having displayed the picture). The Cancel Mask function may be also invoked (optionally) by completion of PAL_SET and ATTR_SET commands.

SGB Command $0C — ATRC_EN

Used to enable/disable Attraction mode, which is enabled by default.

Built-in borders other than the Game Boy frame and the plain black border have a “screen saver” activated by pressing R, L, L, L, L, R or by leaving the controller alone for roughly 7 minutes (tested with 144p Test Suite). It is speculated that the animation may have interfered with rarely-used SGB features, such as OBJ_TRN or JUMP, and that Attraction Disable disables this animation.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     Attraction Disable  (0=Enable, 1=Disable)
 2-F   Not used (zero)

SGB Command $0D — TEST_EN

Used to enable/disable test mode for “SGB-CPU variable clock speed function”. This function is disabled by default.

This command does nothing on some SGB revisions. (SGBv2 confirmed, unknown on others)

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     Test Mode Enable    (0=Disable, 1=Enable)
 2-F   Not used (zero)

Maybe intended to determine whether SNES operates at 50Hz or 60Hz display refresh rate ??? Possibly result can be read-out from joypad register ???

SGB Command $0E — ICON_EN

Used to enable/disable ICON function. Possibly meant to enable/disable SGB/SNES popup menues which might otherwise activated during Game Boy game play. By default all functions are enabled (0).

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     Disable Bits
         Bit 0 - Use of SGB-Built-in Color Palettes    (1=Disable)
         Bit 1 - Controller Set-up Screen    (0=Enable, 1=Disable)
         Bit 2 - SGB Register File Transfer (0=Receive, 1=Disable)
         Bit 3-6 - Not used (zero)
 2-F   Not used (zero)

Above Bit 2 will suppress all further packets/commands when set, this might be useful when starting a monochrome game from inside of the SGB-menu of a multi-gamepak which contains a collection of different games.

SGB Command $0F — DATA_SND

Used to write one or more bytes directly into SNES Work RAM.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     SNES Destination Address, low
 2     SNES Destination Address, high
 3     SNES Destination Address, bank number
 4     Number of bytes to write ($01-$0B)
 5     Data Byte #1
 6     Data Byte #2 (if any)
 7     Data Byte #3 (if any)
 etc.

Unused bytes at the end of the packet should be set to zero, this function is restricted to a single packet, so that not more than 11 bytes can be defined at once. Free Addresses in SNES memory are Bank 0 1800-1FFF, Bank $7F 0000-FFFF.

SGB Command $10 — DATA_TRN

Used to transfer binary code or data directly into SNES RAM.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     SNES Destination Address, low
 2     SNES Destination Address, high
 3     SNES Destination Address, bank number
 4-F   Not used (zero)

The data is sent by VRAM-Transfer (4 KBytes).

 000-FFF  Data

Free Addresses in SNES memory are Bank 0 1800-1FFF, Bank $7F 0000-FFFF. The transfer length is fixed at 4KBytes ???, so that directly writing to the free 2KBytes at 0:1800 would be a not so good idea ???

SGB Command $12 — JUMP

Used to set the SNES program counter and NMI (vblank interrupt) handler to specific addresses.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     SNES Program Counter, low
 2     SNES Program Counter, high
 3     SNES Program Counter, bank number
 4     SNES NMI Handler, low
 5     SNES NMI Handler, high
 6     SNES NMI Handler, bank number
 7-F   Not used, zero

The game Space Invaders uses this function when selecting “Arcade mode” to execute SNES program code which has been previously transferred from the SGB to the SNES. The SNES CPU is a Ricoh 5A22, which combines a 65C816 core licensed from WDC with a custom memory controller. For more information, see “fullsnes” by nocash.

Some notes for intrepid Super NES programmers seeking to use a flash cartridge in a Super Game Boy as a storage server:

• JUMP overwrites the NMI handler even if it is $000000.
• The SGB system software does not appear to use NMIs.
• JUMP can return to SGB system software via a 16-bit RTS. To do this, JML to a location in bank $00 containing byte value $60, such as any of the stubbed commands.
• IRQs and COP and BRK instructions are not useful because their handlers still point into SGB ROM. Use SEI WAI.
• If a program called through JUMP does not intend to return to SGB system software, it can overwrite all Super NES RAM except $0000BB through $0000BD, the NMI vector.
• To enter APU boot ROM, write $FE to $2140. Echo will still be on though.

Multiplayer Command

SGB Command $11 — MLT_REQ

Used to request multiplayer mode (that is, input from more than one joypad). Because this function provides feedback from the SGB/SNES to the Game Boy program, it can also be used to detect SGB hardware.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     Multiplayer Control (0-3) (Bit0=Enable, Bit1=Two/Four Players)
         0 = One player
         1 = Two players
         3 = Four players
 2-F   Not used (zero)

In one-player mode, the second joypad (if any) can only be used for the SGB BIOS. In two-player mode, both joypads are used for the game. Because SNES only has two joypad sockets, four-player mode requires an external “Multiplayer 5” adapter.

Changing the number of active players ANDs the currently selected player minus one with the number of players in that mode minus one. For example, if you go from four players to two players while player 4 was active, player 2 will then be active because 3 & 1 = 1. However, sending the MLT_REQ command will increment the counter several times so results may not be exactly as expected. The most frequent case is going from one player to two-or-four player which will always start with player 1 active.

Reading Multiple Controllers (Joypads)

When having enabled multiple controllers by MLT_REQ, data for each joypad can be read out through the P1 register as follows: First set P14 and P15 both HIGH (deselect both Buttons and Cursor keys), you can now read the lower 4 bits of P1, which indicate the joypad ID for the following joypad input:

Next, read joypad state normally. The next joypad is automatically selected when P15 goes from LOW (0) to HIGH (1) (source), so you can simply repeat reading the joypad state normally until all two (or four) joypads have been read out.

Border and OBJ Commands

SGB Command $13 — CHR_TRN

Used to transfer tile data (characters) to SNES Tile memory in VRAM. This normally used to define BG tiles for the SGB Border (see PCT_TRN), but might be also used to define moveable SNES foreground sprites (see OBJ_TRN).

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1     Tile Transfer Destination
         Bit 0   - Tile Numbers   (0=Tiles $00-$7F, 1=Tiles $80-$FF)
         Bit 1   - Tile Type      (0=BG Tiles, 1=OBJ Tiles)
         Bit 2-7 - Not used (zero)
 2-F   Not used (zero)

The tile data is sent by VRAM transfer (4 KiB).

 000-FFF  Bitmap data for 128 Tiles

Each tile occupies 32 bytes (8×8 pixels, 16 colors each). When intending to transfer more than 128 tiles, call this function twice (once for tiles $00-$7F, and once for tiles $80-$FF). Note: The BG/OBJ Bit seems to have no effect and writes to the same VRAM addresses for both BG and OBJ ???

Each tile is stored in 4-bit-per-pixel format consisting of bit planes 0 and 1 interleaved by row, followed by bit planes 2 and 3 interleaved by row. In effect, each tile consists of two Game Boy tiles, the first to determine bits 0 and 1 (choosing among color 0, 1, 2, or 3 within a 4-color subpalette), and the second to determine bits 2 and 3 (choosing among colors 0-3, 4-7, 8-11, or 12-15).

SGB Command $14 — PCT_TRN

Used to transfer tile map data and palette data to SNES BG Map memory in VRAM to be used for the SGB border. The actual tiles must be separately transferred by using the CHR_TRN function.

 Byte  Content
 0     Command*8+Length    (fixed length=1)
 1-F   Not used (zero)

The map data is sent by VRAM transfer (4 KiB).

 000-6FF  BG Map 32×28 Entries of 16 bits each (1792 bytes)
 700-73F  BG Map 1×28 extra row, 32 entries of 16 bits each (64 bytes)
 740-7FF  Not used, don't care
 800-85F  BG Palette Data (Palettes 4-6, 16 little-endian RGB555 colors each)
 860-FFF  Not used, don't care

Each BG Map Entry consists of a 16-bit value as such: `VH01 PP00 NNNN NNNN```

 Bit 0-9   - Character Number (use only $00-$FF, upper 2 bits zero)
 Bit 10-12 - Palette Number   (use only 4-6)
 Bit 13    - BG Priority      (use only 0)
 Bit 14    - X-Flip           (0=Normal, 1=Mirror horizontally)
 Bit 15    - Y-Flip           (0=Normal, 1=Mirror vertically)

The 32×28 map entries correspond to 256×224 pixels of the Super NES screen. The 20×18 entries in the center of the 32×28 area should be set to a blank (solid color 0) tile as transparent space for the Game Boy window to be displayed inside. Non-transparent border data will cover the Game Boy window (for example, Mario’s Picross does this, as does WildSnake to a lesser extent).

A border designed for a modern (post-2006) widescreen television may use the center 256×176 pixels and leave the top and bottom 24 lines blank. Using letterbox allows more tile variety in the portion of the border that a widescreen TV’s zoom mode does not cut off.

All borders repeat tiles. Assuming that the blank space for the GB screen is a blank tile, and the letterbox (if any) is a solid tile, a border defining all unique tiles would have to define this many tiles:

• (256*224-160*144)/64+1 = 537 tiles in full-screen border
• (256*176-160*144)/64+2 = 346 tiles in letterboxed border

Because the CHR RAM allocated by SGB for border holds only 256 tiles, a full-screen border must repeat at least 281 tiles and a letterboxed border at least 90.

It turns out that 29 rows of the border tilemap sent through PCT_TRN are at least partly visible in some situations. The SGB system software sets the border layer’s vertical scroll position (BG1VOFS) to 0. Because the S-PPU normally displays lines BGxVOFS+1 through BGxVOFS+224 of each layer, this hides the first scanline of the top row of tiles and adds one scanline of the nominally invisible 29th row at the bottom. Most of the time, SGB hides this extra line with forced blanking (writing $80 to INIDISP at address $012100). While SGB is busy processing some packets, such as fading out the border’s palette or loading a new scene’s palette and attributes, it neglects to force blanking, making the line flicker on some TVs. This can be seen even with some built-in borders.

To fully eliminate flicker, write a row of all-black tilemap entries after the bottom row of the border ($8700-$873F in VRAM in a PCT_TRN), or at least a row of tiles whose top row of pixels is blank. If that is not convenient, such as if a border data format doesn’t guarantee an all-black tile ID, you can make the flicker less noticeable by repeating the last scanline. Take the bottommost row (at $86C0-$86FF in VRAM) and copy it to the extra row, flipped vertically (XOR with $8000).

The Super NES supports 8 background palettes. The SGB system software (when run in a LLE such as Mesen 2) has been observed to use background palette 0 for the GB screen, palettes 1, 2, 3, and 7 for the menus, and palettes 4, 5, and 6 for the border. Thus a border can use three 15-color palettes.

SGB Command $18 — OBJ_TRN

Used to start transferring object attributes to SNES object attribute memory (OAM). Unlike all other functions with names ending in “_TRN”, this function does not use the usual one-time 4 KiB VRAM transfer method. Instead, when enabled (below execute bit set to 1), data is continuously (each frame) read out from the lower character line of the Game Boy screen. To suppress garbage on the display, the lower line is masked, and only the upper 20×17 characters of the Game Boy screen are used - the masking method is unknown - frozen, black, or recommended to be covered by the SGB border, or else ??? Also, when the function is enabled, attract mode (built-in borders’ screen saver on idle) is not performed.

This command does nothing on some SGB revisions. (SGBv2, SGB2?)

 Byte  Content
 0     Command*8+Length (fixed length=1)
 1     Control Bits
         Bit 0   - SNES OBJ Mode enable (0=Cancel, 1=Enable)
         Bit 1   - Change OBJ Color     (0=No, 1=Use definitions below)
         Bit 2-7 - Not used (zero)
 2-3   System Color Palette Number for OBJ Palette 4 (0-511)
 4-5   System Color Palette Number for OBJ Palette 5 (0-511)
 6-7   System Color Palette Number for OBJ Palette 6 (0-511)
 8-9   System Color Palette Number for OBJ Palette 7 (0-511)
         These color entries are ignored if above Control Bit 1 is zero.
         Because each OBJ palette consists of 16 colors, four system
         palette entries (of 4 colors each) are transferred into each
         OBJ palette. The system palette numbers are not required to be
         aligned to a multiple of four, and will wrap to palette number
         0 when exceeding 511. For example, a value of 511 would copy
         system palettes 511, 0, 1, 2 to the SNES OBJ palette.
 A-F   Not used (zero)

The recommended method is to “display” Game Boy BG tiles $F9..$FF from left to right as first 7 characters of the bottom-most character line of the Game Boy screen. As for normal 4 KiB VRAM transfers, this area should not be scrolled, should not be overlapped by Game Boy objects, and the Game Boy BGP palette register should be set up properly. By following that method, SNES OAM data can be defined in the $70 bytes of the Game Boy BG tile memory at following addresses:

 8F90-8FEF  SNES OAM, 24 Entries of 4 bytes each (96 bytes)
 8FF0-8FF5  SNES OAM MSBs, 24 Entries of 2 bits each (6 bytes)
 8FF6-8FFF  Not used, don't care (10 bytes)

The format of SNES OAM entries is that of the SNES PPU, as described in the OAM section of Fullsnes. Notice that X and Y are swapped compared to GB PPU OAM entries, and byte 3 is shifted left by 1 bit compared to GB and GBC OAM.

  Byte 0  OBJ X-Position (0-511, MSB is separately stored, see below)
  Byte 1  OBJ Y-Position (0-255)
  Byte 2  Tile Number
  Byte 3  Attributes
    Bit 7    Y-Flip (0=Normal, 1=Mirror Vertically)
    Bit 6    X-Flip (0=Normal, 1=Mirror Horizontally)
    Bit 5-4  Priority relative to BG (use only 3 on SGB)
    Bit 3-1  Palette Number (4-7)
    Bit 0    Tile Page (use only 0 on SGB)

The format of SNES OAM MSB Entries packs 2 bits for each of 4 objects into one byte.

  Bit7    OBJ 3 OBJ Size     (0=Small, 1=Large)
  Bit6    OBJ 3 X-Coordinate (upper 1bit)
  Bit5    OBJ 2 OBJ Size     (0=Small, 1=Large)
  Bit4    OBJ 2 X-Coordinate (upper 1bit)
  Bit3    OBJ 1 OBJ Size     (0=Small, 1=Large)
  Bit2    OBJ 1 X-Coordinate (upper 1bit)
  Bit1    OBJ 0 OBJ Size     (0=Small, 1=Large)
  Bit0    OBJ 0 X-Coordinate (upper 1bit)

Undocumented SGB commands

The following information has been extracted from disassembling a SGB1v2 firmware; it should be verified on other SGB revisions.

The SGB firmware explicitly ignores all commands with ID >= $1E. This leaves undocumented commands $1A to $1D inclusive.

Stubbed commands

Commands $1A to $1F (inclusive)’s handlers are stubs (only contain an RTS). This is interesting, since the command-processing function explicitly ignores commands $1E and $1F.

CPU registers and flags

Registers

As shown above, most registers can be accessed either as one 16-bit register, or as two separate 8-bit registers.

The Flags Register (lower 8 bits of AF register)

Contains information about the result of the most recent instruction that has affected flags.

The Zero Flag (Z)

This bit is set if and only if the result of an operation is zero. Used by conditional jumps.

The Carry Flag (C, or Cy)

Is set in these cases:

• When the result of an 8-bit addition is higher than $FF.
• When the result of a 16-bit addition is higher than $FFFF.
• When the result of a subtraction or comparison is lower than zero (like in Z80 and x86 CPUs, but unlike in 65XX and ARM CPUs).
• When a rotate/shift operation shifts out a “1” bit.

Used by conditional jumps and instructions such as ADC, SBC, RL, RLA, etc.

The BCD Flags (N, H)

These flags are used by the DAA instruction only. N indicates whether the previous instruction has been a subtraction, and H indicates carry for the lower 4 bits of the result. DAA also uses the C flag, which must indicate carry for the upper 4 bits. After adding/subtracting two BCD numbers, DAA is used to convert the result to BCD format. BCD numbers range from $00 to $99 rather than $00 to $FF. Because only two flags (C and H) exist to indicate carry-outs of BCD digits, DAA is ineffective for 16-bit operations (which have 4 digits), and use for INC/DEC operations (which do not affect C-flag) has limits.

CPU Instruction Set

The Game Boy’s SM83 processor possesses a CISC, variable-length instruction set. This page attempts to shed some light on how the CPU decodes the raw bytes fed into it into instructions.

The first byte of each instruction is typically called the “opcode” (for “operation code”). By noticing that some instructions perform identical operations but with different parameters, they can be grouped together; for example, inc bc, inc de, inc hl, and inc sp differ only in what 16-bit register they modify.

In each table, one line represents one such grouping. Since many groupings have some variation, the variation has to be encoded in the instruction; for example, the above four instructions will be collectively referred to as inc r16. Here are the possible placeholders and their values:

These last two are a little special: if they are present in the instruction’s mnemonic, it means that the instruction is 1 (imm8) / 2 (imm16) extra bytes long.

Groupings have been loosely associated based on what they do into separate tables; those have no particular ordering, and are purely for readability and convenience. Finally, the instruction “families” have been further grouped into four “blocks”, differentiated by the first two bits of the opcode.

Block 0

stop is often considered a two-byte instruction, though the second byte is not always ignored.

Block 1: 8-bit register-to-register loads

Exception: trying to encode ld [hl], [hl] instead yields the halt instruction:

Block 2: 8-bit arithmetic

Block 3

The following opcodes are invalid, and hard-lock the CPU until the console is powered off: $D3, $DB, $DD, $E3, $E4, $EB, $EC, $ED, $F4, $FC, and $FD.

$CB prefix instructions

CPU Comparison with Z80

Comparison with 8080

The Game Boy CPU has a bit more in common with an older Intel 8080 CPU than the more powerful Zilog Z80 CPU. It is missing a handful of 8080 instructions but does support JR and almost all CB-prefixed instructions. Also, all known Game Boy assemblers use the more obvious Z80-style syntax, rather than the chaotic 8080-style syntax.

Unlike the 8080 and Z80, the Game Boy has no dedicated I/O bus and no IN/OUT opcodes. Instead, I/O ports are accessed directly by normal LD instructions, or by new LD (FF00+n) opcodes.