The **Blitter** (Block Image Transferrer) is a DMA-driven coprocessor inside Agnus. It performs block copy, fill, and line-draw operations directly on Chip RAM without CPU involvement, at DMA bus speed.
For a 320-pixel wide (40-byte row) bitmap, blitting a 32-pixel (4-byte) wide section:
```
Modulo = 40 - 4 = 36
```
## First/Last Word Masks
`BLTAFWM` (first word mask) and `BLTALWM` (last word mask) mask the A channel for the first and last word of each blit row, allowing sub-word-aligned blitting.
For a fully aligned blit with no partial words:
```asm
move.w #$FFFF, BLTAFWM
move.w #$FFFF, BLTALWM
```
## Line Draw Mode (BLTCON1 bit 0)
In line mode, the blitter draws a line between two points using the Bresenham algorithm:
- A channel provides the single pixel pattern (usually $8000 for MSB)
- D channel is the destination bitmap
- BLTSIZE specifies the line length (height=octant length, width=2)
- BLTCON1 encodes octant, sign flags, and texture data
Line mode is used by `graphics.library``Draw()` calls internally.
## Fill Mode (BLTCON1 IFE/EFE)
**Exclusive fill (EFE):** Each set bit toggles the fill state — produces XOR fill (like polygon rasterisation).
**Inclusive fill (IFE):** Set bit turns fill on, stays on until end of row — used for solid polygon fill.
Fill operates in D channel only (no source channels active). BLTCON1 `DESC` bit = 1 when filling bottom-up.
## Waiting for Blitter Completion
```asm
; Busy-wait on BLTCON0 busy bit
WaitBlit:
btst #6, DMACONR+1 ; test BBUSY bit (bit 14 of DMACONR, byte=bit6)
bne.s WaitBlit
```
Or via exec (preferred):
```c
WaitBlit(); /* graphics.library — waits and resets to safe state */
```
> [!CAUTION]
> Never start a blit while the blitter is busy. Always call `WaitBlit()` or poll `DMACONR[BBUSY]` before setting up new blit registers.
