I don't see it listed anywhere in the 0000-ffff address space. Is it controled by the CPU? Or something else entirely?

  • 1
    Would it be possible to link the source you're reffering to?
    – Raffzahn
    Commented Feb 16, 2020 at 17:33

2 Answers 2


It's controlled by the programmer, not the CPU. The LCDC is located at memory address &FF40 and can be read from/written to just like any other piece of memory. This memory location is one byte long and each bit represents a specific hardware feature related to the screen. In the list below, bit 7 is the "leftmost" bit, bit 0 is the "rightmost."

  • Bit 7: If this equals 1, the screen is on. If it's 0, the screen is off.
  • Bit 6: If this equals 1, the "window" gets its tile data from addresses &9C00-&9FFF. If this equals 0, the "window" gets its tile data from addresses &9800-&9BFF.
  • Bit 5: Turns the "window" on or off. The window is a separate layer from the background and sprites, which uses its own tilemap. (You have two choices for where that tilemap can be stored, Bit 6 decides which one is active.)
  • Bit 4: The Game Boy will pull its sprite graphics data from memory area &8000-&8FFF if this bit equals 1. If 0, the sprite graphics come from &8800-&97FF.
  • Bit 3: The Game Boy will pull background graphics from memory area &9C00-&9FFF if this bit equals 1. If this bit equals 0, they come from &9800-&9BFF instead.
  • Bit 2: If this bit equals 1, sprites are 8x16 pixels in size. If 0, they are 8x8.
  • Bit 1: Sprites are only visible if this bit equals 1.
  • Bit 0: The background is only visible if this bit equals 1. (This is only true for the original Game Boy - on the Game Boy Color, the background is always visible.)

There are a few ways to write to this register, I'll quickly list some.

SET 7,(HL)   ;turn on the screen. This method can only alter one setting at a time,
             ;but it won't affect any other settings.
LD A,%11100111
LD (&FF40),A
;turn on screen, window tiles come from &9C00+, window on,
;sprites from &8800, background from &9800, 8x16 sprites, sprites on, background on

;This method quickly changes a bunch of settings at once, but any old settings 
;are replaced with the new ones.

Just as a warning: If you clear bit 7 of LCDC outside of VBlank, you run the risk of damaging the screen. Emulators aren't affected by this of course, but a real Game Boy is. If you're going to shut off the screen during the game (which you will need to do to load graphics) you need to call a procedure that waits for VBlank first, then as soon as you return clear bit 7 of LCDC.

There's more explained in the Pan Docs.

  • Just out of curiosity: Why do you think will the screen be damaged? Commented Oct 25, 2021 at 6:46
  • The PanDocs says it will happen. I haven't tested it personally but I'd rather not find out. Supposedly Nintendo knew this was a problem and wouldn't publish a game that didn't wait for vBlank to clear bit 7 of &FF40. Commented Oct 27, 2021 at 14:32

The LCD control block is located at FF40h and the LCDC is the first thereof, see as well the Pandocs.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .