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.

FF4C — KEY0 (CGB Mode only): CPU mode select

This GBC-only register (which is not officially documented) is written only by the CGB boot ROM, as it gets locked after the bootrom finish execution (by a write to the BANK register).

Once it is locked, the behavior of the system can’t be changed without a reset (this behavior can be observed using this test ROM).

As a result of the above most of the behavior is not directly testable without hardware manipulation. Even though we can’t test its behavior directly we can inspect the disassembly of the CGB bootrom and infer the following:

• DMG compatibility mode: 0 = Disabled (full CGB mode, for regular CGB cartridges), 1 = Enabled (for DMG only cartridges)

PGB mode

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.