VRAM

VRAM stand for Video RAM.

File:Aes cxk5814.jpg

One of two CXK5814 2KiB RAM chips used for the upper zone of the VRAM ($8000~$87FF) on a AES system

The NeoGeo has 68KiB (physically 64KiB + 4KiB, called lower and upper, or slow and fast) of VRAM organised as 16 bits words, which is used to store sprite attributes, the fix layer tile map and sprite priority buffers for video rendering. Contrary to other systems, the VRAM here does not contain actual graphics.

Acces to VRAM is done through 3 Memory mapped registers handled by the GPU, it isn't mapped in the 68k address space. Every VRAM address points to a word, not a byte.

• REG_VRAMADDR ($3C0000) sets the VRAM address for the next read/write operation
• REG_VRAMRW ($3C0002) is the data read or to write
• REG_VRAMMOD ($3C0004) is the signed value added to the VRAM address after a write

SNK specifies that the address register should be set directly instead of using auto-incrementation when changing between VRAM zones ($0000~$7FFF to/from $8000~$FFFF). This is because the adder is only 15 bit and not 16.

Memory map

Start	End	Description
$0000	$6FFF	SCB1
$7000	$74FF	Fix map
$7500	$7FFF	Extension
$8000	$81FF	SCB2
$8200	$83FF	SCB3
$8400	$85FF	SCB4
$8600	$867F	Sprite list for even scanlines
$8680	$86FF	Sprite list for odd scanlines

VRAM can be modified at any time, even during active display.

Timing considerations

After a write:

• Another write should be made after at least 12 cycles.
• An address change should be made after at least 16 cycles.

After an address change, reads should be made after at least 16 cycles.

Note that these timings only affect the VRAM access, and not the LSPC registers.