VRAM: Difference between revisions

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|$7500
|$7500
|$7FFF
|$7FFF
|[[Fix bankswiching|Extension]]
|[[Fix bankswitching|Extension]]
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|$8000
|$8000

Revision as of 20:02, 24 December 2012

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.

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).

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.