Agnus Amiga Custom Chip

Agnus handles the following on the Amiga:

• Reserving Chip (and Slow) RAM
• Handling Direct Memory Access (DMA) between all the custom chips
• Blitting chunks of Chip RAM data around
  • Copying
  • Shifting
  • Line drawing
  • Filling
• Defining the size of the display
• Sending horizontal and vertical sync signals
• Co-processor is synchronized to the electron gun position and allows setting registers timed to the gun's position
• Genlock

Amiga Agnus Blitter

You need to control DMACON: http://coppershade.org/articles/Code/Reference/DMACON/

snippet.asm

DMACON.COPPER   EQU %0000000010000000
DMACON.BITPLANE EQU %0000000100000000
DMACON.BLITTER  EQU %0000000001000000
 
  MOVE.W #%1000000111000000,DMACON
  MOVE.W #%0000000000111111,DMACON

snippet.c

#include <hardware/dmabits.h>
 
// enable
custom.dmacon = DMAF_SETCLR | DMAF_COPPER | DMAF_RASTER | DMAF_BLITTER;
 
// disable
custom.dmacon = DMAF_AUDIO | DMAF_DISK | DMAF_SPRITE;

Amiga Agnus Copper Co-Processor

There seems to be a real common pattern of setting:

• BPLCON0/1 for bitplanes, resolution, and scroll
• BPL1/2MOD for dealing with how bitplane data is stored
• DIWSTRT/DIWSTOP for setting the corners of the display
  • https://www.amigarealm.com/computing/knowledge/hardref/ch3.htm, SETTING THE DISPLAY WINDOW STOPPING POSITION

    The normal NTSC DIWSTRT is ($2C81).
    The normal NTSC DIWSTOP is ($F4C1).

    The normal PAL DIWSTRT is ($2C81).
    The normal PAL DIWSTOP is ($2CC1).

• https://www.markwrobel.dk/post/amiga-machine-code-letter4/

PAL Lowres, 5 bitplanes

snippet.asm

BPLCON0.COLOR EQU %0000001000000000
BPLCON0.BPU_5 EQU %0101000000000000
 
  MOVE.W #(BPLCON0.COLOR|BPLCON0.BPU_5),BPLCON0
  MOVE.W #0,BPLCON1
  MOVE.W #0,BPL1MOD
  MOVE.W #0,BPL2MOD
  MOVE.W #$2C21,DIWSTRT
  MOVE.W #$2CC1,DIWSTOP
  ;MOVE.W #$38C1,DIWSTOP
  MOVE.W #$0038,DDFSTRT
  MOVE.W #$00D0,DDFSTOP

The code that is everywhere is:

snippet.asm

WaitVBL:
  MOVE.L $DFF004,D0
  AND.L #$1FF00,D0
  CMP.L #300<<8,D0
  BNE.B WaitVBL

snippet.c

void WaitVBL() {
  while ((custom.vposr & 0x1FF00) != (300 << 8)) {}
}

This works because you're reading both `VPOSR` and `VHPOSR` in one operation into D0:

VPOSR                            VHPOSR
| | | | | | | | | | | | | | | | | V8 | V7 | V6 | V5 | V4 | V3 | V2 | V1 | V0 | h | h | h | h | h | h | h | h |

AND.L #$1FF000,D0 masks out the h and keeps the V#. CMP.L #Height<<8 shifts height over 8 bits to the left, then compares the value to the results of the AND.

If you run this on an NTSC Amiga it will lock up!

There's also http://ada.untergrund.net/?p=boardthread&id=31&page=last

• C header hardware/dmabits.h – http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_2._guide/node00C8.html

These are bits that are ORed together. Remember, the more you enable, the more contention there is for RAM and CPU timing, especially on even clock cycles.

• 0-3 -- DMAF_AUDIO – all four Paula audio channels
  • DMAF_AUD(0-3) – the individual channels
• 4 -- DMAF_DISK – the floppy disk
  • There's a funky way to have to enable this: http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0192.html#line14
• 5 -- DMAF_SPRITE – render sprites
• 6 -- DMAF_BLITTER – the blitter
• 7 -- DMAF_COPPER – the copper
• 8 -- DMAF_RASTER – denise screen rendering
• 9 -- DMAF_MASTER – enable all of the DMA not related to blitter settings
• 10 -- DMAF_BLITHOG – blitter wins over CPU
  • b nasty
• 15 -- DMAF_SETCLR – enable these flags

You can also read some blitter-related flags on DMACONR:

• 13 -- DMAF_BLTNZERO – A blit operation happened and only zeroes were written
• 14 -- DMAF_BLTDONE – 0 if not working, 1 if busy