Blitter: The Amiga's Graphics Accelerator

Overview

The Blitter (block image transferrer) was a DMA engine inside Agnus that performed graphics operations in hardware. It could copy memory, combine multiple sources with logic operations, fill areas, and draw lines—all while the CPU worked on game logic. This division of labour gave the Amiga its characteristic smooth, fast graphics.

Fast facts

Location: integrated into Agnus chip.
Sources: up to 3 inputs (A, B, C) plus 1 output (D).
Operations: copy, logic combine, area fill, line draw.
Speed: moves 1 word (16 pixels) per 4 cycles.

Blitter channels

Channel	Purpose
A	Source data, often used as mask
B	Source data
C	Destination read (for combining)
D	Destination write

Not all channels need be active for every operation.

Basic operations

Copy (A to D)

    move.w  #$09f0,BLTCON0(a6)  ; A->D, no shifts
    move.w  #$0000,BLTCON1(a6)  ; No special modes
    move.l  #source,BLTAPT(a6)  ; Source address
    move.l  #dest,BLTDPT(a6)    ; Destination address
    move.w  #$0000,BLTAMOD(a6)  ; Source modulo
    move.w  #$0000,BLTDMOD(a6)  ; Dest modulo
    move.w  #64*64+40,BLTSIZE(a6) ; 64 lines × 40 words

Masked copy (A masks B to D)

Cookie-cutter operation for sprites:

    move.w  #$0fca,BLTCON0(a6)  ; A=mask, B=source, C=bg, D=output

Minterm $CA: D = (A AND B) OR (NOT A AND C)

Fill

Fill enclosed areas:

    move.w  #$0001,BLTCON1(a6)  ; Fill mode enabled

Draw outline first, then fill—Blitter toggles fill state at each edge pixel.

Line drawing

Draw lines in hardware:

    move.w  #$0001,BLTCON1(a6)  ; Line mode + octant
    move.w  #dx,BLTBMOD(a6)     ; Delta X
    move.w  #dy,BLTAMOD(a6)     ; Delta Y
    ; ... additional setup
    move.w  #1*64+width,BLTSIZE(a6) ; Start line draw

Minterm logic

The Blitter combines sources using an 8-bit minterm:

Bit	Condition
7	A AND B AND C
6	A AND B AND NOT C
5	A AND NOT B AND C
4	A AND NOT B AND NOT C
3	NOT A AND B AND C
2	NOT A AND B AND NOT C
1	NOT A AND NOT B AND C
0	NOT A AND NOT B AND NOT C

Common minterms

Minterm	Operation
$F0	D = A (copy A)
$CC	D = B (copy B)
$AA	D = C (copy C)
$CA	D = (A∧B)∨(¬A∧C) (cookie cut)
$5A	D = A XOR C
$0A	D = A AND C

Shifting

The Blitter can shift data horizontally:

    move.w  #$n9f0,BLTCON0(a6)  ; n = shift count (0-15)

Essential for pixel-precise positioning in planar graphics.

Blitter registers

Register	Address	Purpose
BLTCON0	$DFF040	Control: minterm, shift, channels
BLTCON1	$DFF042	Control: fill, line mode
BLTAFWM	$DFF044	First word mask for A
BLTALWM	$DFF046	Last word mask for A
BLTCPT	$DFF048	Channel C pointer
BLTBPT	$DFF04C	Channel B pointer
BLTAPT	$DFF050	Channel A pointer
BLTDPT	$DFF054	Channel D pointer
BLTSIZE	$DFF058	Size and start operation
BLTCMOD	$DFF060	Channel C modulo
BLTBMOD	$DFF062	Channel B modulo
BLTAMOD	$DFF064	Channel A modulo
BLTDMOD	$DFF066	Channel D modulo
BLTCDAT	$DFF070	Channel C data
BLTBDAT	$DFF072	Channel B data
BLTADAT	$DFF074	Channel A data

Modulo explained

Modulo values handle non-contiguous memory:

Source width = 320 pixels = 40 bytes = 20 words
Screen width = 320 pixels = 40 bytes = 20 words
Blit width = 32 pixels = 4 bytes = 2 words
Modulo = screen width - blit width = 20 - 2 = 18 words

Waiting for Blitter

Always wait before using Blitter results:

wait_blit:
    btst    #6,$dff002          ; Check BLTBUSY in DMACONR
    bne.s   wait_blit           ; Loop until done

Or check bit 14 of DMACONR for Blitter-done flag.