VRAM: Difference between revisions

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[[File:Aes_cxk5814.jpg|thumb|One of two CXK5814 2KiB RAM chips used for the upper zone of the VRAM ($8000~$87FF) on a AES system]]
[[File:Aes_cxk5814.jpg|thumb|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, respectively called lower and upper, or slow and fast) of VRAM accessible as 16 bits words, which is used to store [[Sprites|sprite]] attributes, the [[fix layer]] tile map and sprite lists for video rendering. Contrary to other systems, the VRAM here '''does not contain actual graphics'''.
The NeoGeo has 68KiB of VRAM split in two: a 64KiB lower zone, and 4KiB higher zone. Contrary to other systems, the VRAM '''does not contain actual graphics'''. It is used to store [[Sprites|sprite]] attributes, the [[fix layer]] tile map and sprite lists for video rendering.


Access 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.
Access to VRAM is always done through 3 [[memory mapped registers]] handled by the [[LSPC]], it does not appear in the [[68k]] address space. Every VRAM address points to a 16-bit word, not a byte.


* {{Reg|REG_VRAMADDR ($3C0000)}} sets the VRAM address for the next read/write operation
* {{Reg|REG_VRAMADDR ($3C0000)}} sets the VRAM address for the next read/write operation.
* {{Reg|REG_VRAMRW ($3C0002)}} is the data read or to write
* {{Reg|REG_VRAMRW ($3C0002)}} is the data read or to write.
* {{Reg|REG_VRAMMOD ($3C0004)}} is the signed value added to the VRAM address after a write
* {{Reg|REG_VRAMMOD ($3C0004)}} is the signed value automatically added to the VRAM address after a write.


The original SNK documentation specifies that the address register should be set directly instead of using {{Reg|REG_VRAMMOD}} when crossing VRAM zones ($0000~$7FFF to/from $8000~$FFFF). This is probably because the GPU's internal adder is 15 bit only.
Due to the different access slots used internally for each of the VRAM zones, the address register must be set directly instead of relying on {{Reg|REG_VRAMMOD}} to cross zones ($0000~$7FFF to/from $8000~$FFFF).


== Memory map ==
==Memory map==
[[File:Vrammap.png|right|frame]]
[[File:Vrammap.png|right|frame]]


Line 19: Line 19:
|'''Start'''
|'''Start'''
|'''End'''
|'''End'''
|'''Words'''
|'''Zone'''
|'''Description'''
|'''Description'''
|-
|-
|$0000
|$0000
|$6FFF
|$6FFF
|28K
|rowspan=3|Lower
|[[Sprites#SCB1|SCB1]]
|[[Sprites#SCB1|SCB1]]
|-
|-
|$7000
|$7000
|$74FF
|$74FF
|rowspan=2|4K
|[[Fix layer|Fix map]]
|[[Fix layer|Fix map]]
|-
|-
Line 35: Line 40:
|$8000
|$8000
|$81FF
|$81FF
|512
|rowspan=5|Upper
|[[Sprites#SCB2|SCB2]]
|[[Sprites#SCB2|SCB2]]
|-
|-
|$8200
|$8200
|$83FF
|$83FF
|512
|[[Sprites#SCB3|SCB3]]
|[[Sprites#SCB3|SCB3]]
|-
|-
|$8400
|$8400
|$85FF
|$85FF
|512
|[[Sprites#SCB4|SCB4]]
|[[Sprites#SCB4|SCB4]]
|-
|-
|$8600
|$8600
|$867F
|$867F
|128
|Sprite list for even scanlines
|Sprite list for even scanlines
|-
|-
|$8680
|$8680
|$86FF
|$86FF
|128
|Sprite list for odd scanlines
|Sprite list for odd scanlines
|-
|$8700
|$87FF
|256
|Unused (free)
|}
|}


Even if VRAM can be modified at any time (even during active display), since it is not dual-ported, some timing restrictions have to be met in order to avoid skipping writes or reading data too early after setting the address. This is because the LSPC allocates a number of "access slots" per video line for the CPU between rendering steps.
==Access timing==
 
Even if VRAM can be modified even during active display, some timing restrictions have to be met in order to avoid skipping writes or reading incorrect data. This is because the [[LSPC]] can only provide a limited number of access slots for the CPU between the video rendering steps.
 
SNK imposed minimum waiting times to always be sure that no read or write operation to VRAM will fail:
 
* After writing the VRAM address, a read will return valid data after 16 CPU cycles or more (>32mclk).
* After writing the VRAM data, a new address can be set after 16 CPU cycles or more (>32mclk).
* After writing the VRAM data, another write can be done after 12 CPU cycles or more (>24mclk).


== Timing considerations ==
Note that these restrictions only concern the VRAM access, and not the other LSPC registers.
The following timings are given to be 100% sure that no read or write to VRAM will fail. It is possible to sometimes shorten them with extreme guessing methods on the CPU side, but it's practically useless.


After a write:
[[Overclocking|Overclocked]] systems often produce video glitches because games are trying to access VRAM too quickly.
*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 CPU cycles.
==Speed==


Note that these timings only affect the VRAM access, and not the other LSPC registers.
* Lower (slow) VRAM must be 120ns or less (3mclk).
* Upper (fast) VRAM must be 35ns or less (1mclk).


[[Category:Video system]]
[[Category:Video system]]

Latest revision as of 00:42, 15 March 2024

VRAM stands 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 of VRAM split in two: a 64KiB lower zone, and 4KiB higher zone. Contrary to other systems, the VRAM does not contain actual graphics. It is used to store sprite attributes, the fix layer tile map and sprite lists for video rendering.

Access to VRAM is always done through 3 memory mapped registers handled by the LSPC, it does not appear in the 68k address space. Every VRAM address points to a 16-bit word, not a byte.

Due to the different access slots used internally for each of the VRAM zones, the address register must be set directly instead of relying on REG_VRAMMOD to cross zones ($0000~$7FFF to/from $8000~$FFFF).

Memory map

Start End Words Zone Description
$0000 $6FFF 28K Lower SCB1
$7000 $74FF 4K Fix map
$7500 $7FFF Extension
$8000 $81FF 512 Upper SCB2
$8200 $83FF 512 SCB3
$8400 $85FF 512 SCB4
$8600 $867F 128 Sprite list for even scanlines
$8680 $86FF 128 Sprite list for odd scanlines
$8700 $87FF 256 Unused (free)

Access timing

Even if VRAM can be modified even during active display, some timing restrictions have to be met in order to avoid skipping writes or reading incorrect data. This is because the LSPC can only provide a limited number of access slots for the CPU between the video rendering steps.

SNK imposed minimum waiting times to always be sure that no read or write operation to VRAM will fail:

  • After writing the VRAM address, a read will return valid data after 16 CPU cycles or more (>32mclk).
  • After writing the VRAM data, a new address can be set after 16 CPU cycles or more (>32mclk).
  • After writing the VRAM data, another write can be done after 12 CPU cycles or more (>24mclk).

Note that these restrictions only concern the VRAM access, and not the other LSPC registers.

Overclocked systems often produce video glitches because games are trying to access VRAM too quickly.

Speed

  • Lower (slow) VRAM must be 120ns or less (3mclk).
  • Upper (fast) VRAM must be 35ns or less (1mclk).
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