Hardware

Blitter: The Amiga's Graphics Accelerator

Hardware-powered block transfers

The Blitter moved, combined, and transformed graphics data faster than any CPU could—making the Amiga's smooth scrolling and complex visuals possible.

commodore-amigacustom-chipsgraphicsdmacommodore1985–present

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

The btst #6,$dff002 idiom is the classic Amiga form. It works because btst with a memory operand is byte-sized: it reads the high byte of DMACONR (which is at $dff002) and tests bit 6 of that byte — which is bit 14 of the full 16-bit register, the BBUSY flag. Equivalent and more obvious if you prefer: btst #14,$dff002 with a word read also works but compiles to a longer instruction.

See also