Rendering logic

On the NeoGeo hardware, the GPU (Graphics Processing Unit) a.k.a. VDP, may refer to a chip or a group of different chips used to generate the video signal.


 * LSPC-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 an overview of the interconnections between chips and cartridges.

Temporary notes

 * PCK2 rises with BNKB and CHBL
 * The first valid rendering cycle is 32mclk after CHBL low ?
 * Fix and sprite pixels are rendered at the same speed because sprite pixels are written by pairs
 * Tile pixel lines are rendered in halves:


 * For the fix (32mclk = 8 pixels corresponds to 6MHz pixel clock):
 * Full address is ...1**** (PCK2 pulse)
 * 2H1 is 0 for 2 pixels (columns 0 & 1), then 1 for 2 pixels (columns 2 & 3)
 * Full address is ...0**** (PCK2 pulse)
 * 2H1 is 0 for 2 pixels (columns 4 & 5), then 1 for 2 pixels (columns 6 & 7)


 * For sprites (32mclk = 16 pixels):
 * Full address is ...1***** (PCK1 pulse)
 * CA4 is 0 for 4 pixels (columns 0~3), then 1 for 4 pixels (columns 4~7)
 * Full address is ...0***** (PCK1 pulse)
 * CA4 is 0 for 4 pixels (columns 8~11), then 1 for 4 pixels (columns 12~15)


 * As fix is rendered in realtime, the fix tile address is set before sprites (on a new line PCK2 pulses before PCK1)
 * X position to B1, just before each PCK2 pulse (SP during 1mclk), for 20 sprites next to each other (X+16px each time):
 * Start of line: 0000,0808,1010,1838,2000,2808,3010,3838,40C0,48E8,50F0,58F8,60C0,68E8,70F0,78F8,8000,8808,9010,9838,0,0,0...

=Video generation=

See Display timing for the sync signal's timing.

NEO-B1 is used for double-buffering scanlines. While a buffer is output to the screen, the other one is filled up. They're swapped each new scanline. Each of the two line buffers are actually 2 buffers of even/odd pixels. They will be named (1 & 2), and (3 & 4).


 * The TMS0 signal from LSPC tells B1 how the pair of buffers are used:
 * 0: Buffers 1&2 are output to the TV. Buffers 3&4 are written to.
 * 1: Buffers 1&2 are written to. Buffers 3&4 are output to the TV.


 * CSK1~4 signals are used to step to the next pixel (rising edge ?), periodic for video output, VRAM-dependent when filling up. Inactive during H-blank.
 * WSE1~4 signals are used to indicate if the pixel color from GAD/GBD needs to be written to the buffer (falling edge ?), matches CSK for video output (ignored ?), depends on DOTA/DOTB (opaque pixel signal) when filling up.
 * SS1~2 signals are used to reset the pixel pointers on falling edge ? (probably wrong)


 * The rising edge of PCK2 latches the X position of the sprite (and something else in a byte ?)
 * 1H1 (6MHz / 2 pixels per byte = 3MHz) is used to clock in the pixels of FIXD into the pixel buffers (or directly to the output ?) if they're not 0000.

=Sprite parsing=

This is a draft. The following information shouldn't be considered as exact.


 * LSPC runs at 24MHz
 * Fast VRAM is 35ns
 * The reads always occur 1mclk (41.6ns) after address is set


 * FIXT: P23~16 are 0, P15~0 are S ROM address (+ external 2H1)
 * SPRT: P23~0 are C ROM address (+ external CA4)
 * LO: P23~16 are LO ROM data, P15~0 are LO address
 * FP: P19~16 is the fix tile palette, rest is 0
 * SP: P23~16 is the sprite tile palette, P15~8 is X position, P7~0 is ?


 * LSPC always starts filling up active sprite list A ($8600) each new frame
 * Read sequence (100p capacitor delay on AES too on PCKxB ?):

Timing diagram when no sprites fall in the next scanline (no writes to sprite list): Parse       ################################                                ################################ Render                                      ##########################                                      ########################## 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... Pos         00111122223333444455556666777788889999AAAABBBBCCCCDDDDEEEEFFFF00 Neg         0000111122223333444455556666777788889999AAAABBBBCCCCDDDDEEEEFFFF State       0000000011111122222233333344444455555555666666777777888888999999


 * 1: No idea. Why read 600 all the time ?!
 * 2: Read sprite Y position from SCB3 to see if it's in next scanline
 * 3: Read sprite list to get sprite #
 * 4: Read SCB2 zoom values
 * 5: Read SCB3 Y/size/chain
 * 6: Read SCB4 X

10 states in 16 cycles (or 5 in 8 cycles: 4-3-3-3-3).

Matched sprite = 6 or 7 (3 or 3.5mclk) = 2 slots Unmatched sprite = 3 or 4 (1.5 or 2mclk) = 1 slot 8mclk = 5 slots available for parsing out of 16mclk: 768mclk per scanline: 480 slots (unmatched). (240 matched)

SNK says max. 380 sprites can be parsed, and max 96 per line:

If we have 380-96 = 284 empty/unmatched sprites, then 96 matched sprites, we have 480-284 = 196 slots free, which allow 196/2 = 98 active sprites. Maybe 2 less because of scanline start delay or something ?

So the 380 sprites max. limit can be explained.

Can sprite parsing go over 380 ? For example:
 * Is an active sprite displayed after 400 unmatched sprites ?
 * Is an active sprite displayed after 474 unmatched sprites ? Does that hit the limit ? (((480-474)/2)-2) ?
 * Are 2 active sprites displayed after 474 unmatched sprites ? Only the first one ?
 * Is an active sprite displayed after 476 unmatched sprites ? Never?

SNK says min. 12 68kclk between writes (so 24mclk). 1 write every 24mclk = 64 per scanline.

Why 12 and not 8 ?

All parsing cycles are taken into consideration by SNK to give the "380 sprites max" limit, so CPU I/O must be done in the "1" states (address 600 by default when no I/O request ?).

Timing diagram when the sprite list is being filled: +5/8:

0  5   2   7   4   1   6   3  5   2   7   4   1   6   3  0      |       |   |       |  | 0 1 2 3 4 5 6 7 8 9 A B C D E F

LLLLLLLLHHHHHHHHLLLLHHHHHHHHLLLL HHHHHHLLLLLLLLHHHHLLLLLLLLHHHLLL

0 1 2 3 4 5 6 7 8 9 A B C D E F            |       |     |     |     |

Parse       ################################                                ################################ Render                                      ##########################                                      ########################## 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


 * Available CPU R/W slots depending on parsing progress, safest is ? cycles

Slow (lower) VRAM

 * Slow VRAM is 100ns (10MHz) and is read at ?
 * 4 slots per render cycle, 1 slot for CPU R/W (1 each 16 68k cycles)