LCD Status Register

TERMINOLOGY

A dot is the shortest period over which the PPU can output one pixel: is it equivalent to 1 T-state on DMG or on CGB single-speed mode or 2 T-states on CGB double-speed mode. On each dot during mode 3, either the PPU outputs a pixel or the fetcher is stalling the FIFOs.

FF41 - STAT (LCD Status) (R/W)

Bit 6 - LYC=LY STAT Interrupt source         (1=Enable) (Read/Write)
Bit 5 - Mode 2 OAM STAT Interrupt source     (1=Enable) (Read/Write)
Bit 4 - Mode 1 VBlank STAT Interrupt source  (1=Enable) (Read/Write)
Bit 3 - Mode 0 HBlank STAT Interrupt source  (1=Enable) (Read/Write)
Bit 2 - LYC=LY Flag                          (0=Different, 1=Equal) (Read Only)
Bit 1-0 - Mode Flag                          (Mode 0-3, see below) (Read Only)
          0: In HBlank
          1: In VBlank
          2: Searching OAM
          3: Transferring Data to LCD Controller

The two lower STAT bits show the current status of the PPU.

Bit 2 is set when LY contains the same value as LYC. It is constantly updated.

Bits 3-6 select which sources are used for the STAT interrupt.

The LCD controller operates on a 2^22 Hz = 4.194 MHz dot clock. An entire frame is 154 scanlines, 70224 dots, or 16.74 ms. On scanlines 0 through 143, the PPU cycles through modes 2, 3, and 0 once every 456 dots. Scanlines 144 through 153 are mode 1.

The following sequence is typical when the display is enabled:

Mode 2  2_____2_____2_____2_____2_____2___________________2____
Mode 3  _33____33____33____33____33____33__________________3___
Mode 0  ___000___000___000___000___000___000________________000
Mode 1  ____________________________________11111111111111_____

When the PPU is reading a particular part of video memory, that memory is inaccessible to the CPU.

  • During modes 2 and 3, the CPU cannot access OAM (FE00h-FE9Fh).
  • During mode 3, the CPU cannot access VRAM or CGB Palette Data (FF69,FF6B).
ModeActionDurationAccessible video memory
2Searching OAM for OBJs whose (X,Y) coordinates overlap this line80 dots (19 µs)VRAM, CGB palettes
3Reading OAM and VRAM to generate the picture168 to 291 dots (40 to 60 µs) depending on sprite countNone
0Horizontal blanking85 to 208 dots (20 to 49 µs) depending on previous mode 3 durationVRAM, OAM, CGB palettes
1Vertical blanking4560 dots (1087 µs, 10 scanlines)VRAM, OAM, CGB palettes

Properties of STAT modes

Unlike most game consoles, the Game Boy can pause the dot clock briefly, adding dots to mode 3’s duration. It routinely takes a 6 to 11 dot break to fetch sprite patterns between background tile pattern fetches. On DMG and GBC in DMG mode, mid-scanline writes to BGP allow observing this behavior, as a sprite delay shifts the effect of a write to the left by that many dots.

Three things are known to pause the dot clock:

  • Background scrolling: If SCX mod 8 is not zero at the start of the scanline, rendering is paused for that many dots while the shifter discards that many pixels from the leftmost tile.
  • Window: An active window pauses for at least 6 dots, as the background fetching mechanism starts over at the left side of the window.
  • Sprites: Each sprite usually pauses for 11 - min(5, (x + SCX) mod 8) dots. Because sprite fetch waits for background fetch to finish, a sprite’s cost depends on its position relative to the left side of the background tile under it. It’s greater if a sprite is directly aligned over the background tile, less if the sprite is to the right. If the sprite’s left side is over the window, use 255 - WX for SCX in this formula.

TO BE VERIFIED

The exact pause duration for window start is not confirmed; it may have the same background fetch finish delay as a sprite. If two sprites’ left sides are over the same background or window tile, the second may pause for fewer dots.

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 cycle, and then the written value were written the next 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.