CGB Registers

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This chapter describes only CGB (Color Gameboy) 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 0143h in the cartridge header. Typically use a value of 80h for games which support both CGB and monochrome gameboys, and C0h 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 examing the CPU accumulator (A-register) directly after startup. A value of 11h indicates CGB (or GBA) hardware, if so, CGB functions can be used (if unlocked, see above). When A=11h, 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

FF4D - KEY1 - CGB Mode Only - Prepare Speed Switch

 Bit 7: Current Speed     (0=Normal, 1=Double) (Read Only)
 Bit 0: Prepare Speed Switch (0=No, 1=Prepare) (Read/Write)

This register is used to prepare the gameboy to switch between CGB Double Speed Mode and Normal Speed Mode. The actual speed switch is performed by executing a STOP command after Bit 0 has been set. After that Bit 0 will be cleared automatically, and the gameboy will operate at the 'other' speed. The recommended speed switching procedure in pseudo code would be:

 IF KEY1_BIT7 <> DESIRED_SPEED THEN
   IE=00H       ;(FFFF)=00h
   JOYP=30H     ;(FF00)=30h
   KEY1=01H     ;(FF4D)=01h
   STOP         ;STOP
 ENDIF

The CGB is operating in Normal Speed Mode when it is turned on. Note that using the Double Speed Mode increases the power consumption, it would be recommended to use Single Speed whenever possible. However, the display will flicker (white) for a moment during speed switches, so this cannot be done permanentely. In Double Speed Mode the following will operate twice as fast as normal:

 The CPU (2.10 MHz, 1 Cycle = approx. 0.5us)
 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


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 gameboys IR signal). After sending or receiving data you should reset the register to 00h to reduce battery power consumption again.

 Bit 0:   Write Data   (0=LED Off, 1=LED On)             (Read/Write)
 Bit 1:   Read Data    (0=Receiving IR Signal, 1=Normal) (Read Only)
 Bit 6-7: Data Read Enable (0=Disable, 3=Enable)         (Read/Write)

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.

FF70 - SVBK - CGB Mode Only - WRAM Bank

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

 Bit 0-2  Select WRAM Bank (Read/Write)

Writing a value of 01h-07h will select Bank 1-7, writing a value of 00h will select Bank 1 either.

Undocumented registers

These are undocumented CGB Registers. 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.

FF6C - Bit 0 (Read/Write) - CGB Mode Only

Only the least significant bit of this register can be written to. It defaults to 0, so this register's initial value is $FE.

In non-CGB mode, it isn't writable, and its value is locked at $FF.

FF72 - Bits 0-7 (Read/Write)

FF73 - Bits 0-7 (Read/Write)

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

FF74 - Bits 0-7 (Read/Write) - 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 (Read/Write)

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

FF76 - PCM amplitudes 1 & 2 (Read Only)

This register is read-only. The low nibble is a copy of sound channel #1's PCM amplitude, the high byte a copy of sound channel #2's.

FF77 - PCM amplitudes 3 & 4 (Read Only)

Same, but with channels 3 and 4.