Hardware

Denise: The Amiga's Display Engine

Playfields, sprites, and HAM mode magic

Denise transformed raw bitplane data into stunning visuals with dual playfields, eight hardware sprites, and the famous Hold-And-Modify mode.

commodore-amigacustom-chipsgraphicsvideocommodore1985–1992

Overview

Denise handled the final stage of Amiga graphics: converting bitplane data into colour values and mixing in sprites. While Agnus fetched the data, Denise determined what appeared on screen. Its flexibility allowed programmers to create effects impossible on contemporary hardware.

Fast facts

  • Function: display generation, sprite output, collision detection.
  • Versions: 8362 (OCS), 8373 (ECS), Lisa (AGA).
  • Palette: 32 colours from 4096 (OCS/ECS), 256 from 16.7M (AGA).
  • Resolutions: 320×200 to 1280×200 (interlaced doubles vertical).

Display modes

Mode Colours Resolution Bitplanes
Lowres 32 320×200 5
Hires 16 640×200 4
HAM 4096 320×200 6
EHB 64 320×200 6
SuperHires (ECS+ only) 4 1280×200 2

Bitplane architecture

The Amiga uses planar graphics:

  • Each bit of a pixel comes from a different bitplane.
  • 5 bitplanes = 32 colours (2^5).
  • Bitplanes can be toggled independently.
  • Different depths suit different needs.

Colour calculation

Pixel colour = (BPL5 × 16) + (BPL4 × 8) + (BPL3 × 4) + (BPL2 × 2) + BPL1

Dual playfield mode

Two independent scrolling layers:

Playfield Bitplanes Colours
Odd 1, 3, 5 8 (including transparent)
Even 2, 4, 6 8 (including transparent)

Each playfield scrolls independently—perfect for parallax effects.

Hardware sprites

Denise provides 8 hardware sprites:

Feature Specification
Width 16 pixels (lowres)
Height 16 pixels per data block; can be reused mid-frame via the Copper to span the full display
Colours 3 + transparent each
Attachment pairs combine for 15 colours + transparent

Sprite registers

Sprite Position Data
0 SPR0POS SPR0DATA
1 SPR1POS SPR1DATA
… … …
7 SPR7POS SPR7DATA

Sprite multiplexing

The Copper can reposition sprites mid-frame:

  1. Wait for sprite’s last scanline.
  2. Update position registers.
  3. Load new data.
  4. Sprite appears at new location.

This technique multiplies effective sprite count.

HAM mode

Hold-And-Modify displays 4096 colours:

Bits 5-4 Action
00 Set pixel from palette (16 base colours)
01 Hold R & G, modify B
10 Hold G & B, modify R
11 Hold R & B, modify G

HAM limitations

  • Colour changes ripple horizontally.
  • Fast horizontal colour transitions cause fringing.
  • Best for photographs and gradients.

Collision detection

Denise detects sprite and playfield collisions in hardware. Two registers cooperate:

Register Address Purpose
CLXCON $DFF098 Configures which collisions to detect (which sprite pairs, which bitplane bits, even/odd playfield masks)
CLXDAT $DFF00E Reports detected collisions; reading clears the bits

Bit flags indicate which objects overlap—no CPU calculation needed. Reading CLXDAT clears the latches, so games typically read it once per frame after the relevant scanlines.

Key registers

Register Address Purpose
BPLCON0 $DFF100 Bitplane control (depth, modes)
BPLCON1 $DFF102 Scroll delays
BPLCON2 $DFF104 Sprite priority
BPL1DAT $DFF110 Bitplane 1 data
COLOR00 $DFF180 Background colour
COLOR01-31 $DFF182+ Palette entries
SPR0POS $DFF140 Sprite 0 position

See also