Rendering logic

On the NeoGeo hardware, the term GPU (Graphics Processing Unit) may refer to pairs of different chips used to generate the video signal.

• PRO-A0, PRO-B0 (early)
• LSPC2-A2, NEO-B1 (most common)
• NEO-GRC, NEO-OFC (CD systems)
• NEO-GRZ (CDZ, MV-1C ?)

See graphics pipeline for details and interconnections between chips and cartridges.

Internal

• LSPC runs at 24MHz.
• Fast VRAM is 35ns (28MHz) and is read at 16MHz (24/1.5).
• The read occurs 41.6ns (1clk) after address is set

24M  |'|_|'|_|'|_|'|_|'|_|'|_
Addr |     |     |     |
Read     !     !     !     !

• LSPC always starts in sprite list A ($8600) each new frame
• Read sequence (100p capacitor delay on AES too on PCKxB ?):
• Timing diagram when the sprite list for the actual line is already filled (no writes):

24M    |'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_
Addr   | 600 |  200  | 201 | 202 | 203 | 204 |  681  | 00E | 20E | 40E | 600 |  205  | 206 | 207 | 208 | 209 |  682  | 00F | 20F | 40F
PCK1   ______|'''|___________________________________________________________|'''|_____________________________________________________
PCK1B  '''''''|____|''''''''''''''''''''''''''''''''''''''''''''''''''''''''''|___|''''''''''''''''''''''''''''''''''''''''''''''''''''
LOAD   |'''''''|_______________________|'''''''|_______________________|'''''''|_______________________|'''''''|_______________________
12M    __|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|_
2Pixel       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
Read       ?       !     !     !     !     !       !     !     !     !     ?       !     !     !     !     !       !     !     !     !
What      1      2      2     2     2     2      3      4     5     6     1      2      2     2     2     2      3      4     5     6...

• 1: ?
• 2: Read SCB3 to see if sprite is in next scanline (just increments), starts frame at sprite 1 ?
• 3: Read sprite list to get sprite #
• 4: Read SCB2 zoom values
• 5: Read SCB3 Y/size/chain
• 6: Read SCB4 X

• Timing diagram when the sprite list is being filled:

24M    |'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_|'|_
Addr   | 600 |  20F  | 210 | 211 | 600 | 601 |  684  | 005 | 205 | 405 | 600 |  212  | 213 | 602 | 603 | 214 |  685  | 006 | 206 | 406
PCK1   ______|'''|___________________________________________________________|'''|_____________________________________________________
PCK1B  '''''''|____|''''''''''''''''''''''''''''''''''''''''''''''''''''''''''|___|''''''''''''''''''''''''''''''''''''''''''''''''''''
LOAD   |'''''''|_______________________|'''''''|_______________________|'''''''|_______________________|'''''''|_______________________
12M    __|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|___|'''|_
2Pixel       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
/WE    ''''''''''''''''''''''''''|___|'|___|'''''''''''''''''''''''''''''''''''''''''''''''|___|'|___|'''''''''''''''''''''''''''''''''
Read       ?       !     !     !                   !     !     !     !     ?       !     !                 !       !     !     !     !

• R/W sequences: (2 write buffers ?)
• 600 RRRWW... 600 RRWWR...
• 600 WWRRW... 600 WRRWW... 600 RRWWR ... 600 RWWRR

• Even lines: Write to list A, Read from list B (Start of display)
• Odd lines: Write to list B, Read from list A
• In 16clk, 2 sprites SCB3 max. are checked to fill up sprite list , and 1 sprite's attributes are read for output
• 384px * 4clk/px = 1536clk/line
• 1536clk / 16clk = 96 sprites max/line
• This means that there's at least 2 sprite SCB3 checked each 16clk, 4 writes to sprite list can be done max per 16clk ?

• Slow VRAM is 100ns (10MHz) and is read at ?