# Agnus Amiga Custom Chip

{{:amiga:custom_chips:agnus_copypasting_windows.png?200|}}

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

## Blitter

[[:amiga:custom_chips:agnus:blitter|Amiga Agnus Blitter]]

## DMA

### Examples

#### Enabling copper, bitplane rendering, and blitter

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

```asm
DMACON.COPPER   EQU %0000000010000000
DMACON.BITPLANE EQU %0000000100000000
DMACON.BLITTER  EQU %0000000001000000

  MOVE.W #%1000000111000000,DMACON
  MOVE.W #%0000000000111111,DMACON
```

```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;
```

## Copper (co-processor)

[[:amiga:custom_chips:agnus:copper|Amiga Agnus Copper Co-Processor]]


## Setting up a display

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).
```

### Links

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

### Examples

#### Display Setup

PAL Lowres, 5 bitplanes

```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
```

#### Waiting for a vertical blank on PAL

The [code that is everywhere](https://eab.abime.net/showthread.php?t=51928) is:

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

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

This works because you're reading both [`VPOSR`](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0045.html) and [`VHPOSR`](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0044.html) 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

---

## Reference

### DMACON flags

* 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