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 + 32KiB) of VRAM organised as 16 bits words, which is used to store sprite attributes and the map for the fix layer. Contrary to other systems, the VRAM here does not contain actual graphics.

It isn't mapped in the 68k's address space. The only way to read and write to it is by using 3 Memory mapped registers. Every address points to a word, not a byte.

• VRAM_ADDR ($3C0000) sets the VRAM address
• VRAM_RW ($3C0002) is the data to read or write
• VRAM_MOD ($3C0004) is the signed value added to the VRAM address after a write

Memory map

Start	End	Description
$0000	$6FFF	SCB1
$7000	$7FFF	Fix map
$8000	$81FF	SCB2
$8200	$8400	SCB3
$8400	$8600	SCB4
$8600	?	Inside GPU, display lists ?

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