Techniques & Technology

Character Graphics

Building worlds from 8×8 blocks

Character graphics used redefined text characters as game graphics, enabling colourful displays with minimal memory on systems designed primarily for text.

commodore-64sinclair-zx-spectrumbbc-microcommodore-vic-20graphicsmemorytechnique1977–present

Overview

Before dedicated graphics hardware, computers displayed text using character sets—grids of 8×8 pixel patterns. Programmers realised they could redefine these characters to create game graphics, building entire worlds from 256 small tiles. The technique was memory-efficient and surprisingly flexible.

How it works

Standard text mode

Element Description
Screen memory One byte per character position
Character ROM 256 pre-defined 8×8 patterns
Display ROM pattern at screen position

Custom characters

Change Result
Point to RAM Use custom definitions
Define 8 bytes Create 8×8 tile
Reference by number Display anywhere

Memory efficiency

Display method Memory for 40×25 screen
Bitmap (mono) 8,000 bytes
Bitmap (colour) 8,000 + 1,000 colour RAM
Character mode 1,000 (screen) + 1,000 (colour RAM at $D800-$DBE7) + 2,048 (charset) = 4,048 bytes

Character mode uses far less RAM. The C64 has dedicated colour RAM at $D800-$DBE7 (one nibble per cell, hard-wired and not configurable) that holds the per-character colour for text and multicolour modes.

C64 implementation

Pointing to custom charset

    lda #$1c        ; Bits 1-3 select charset
    sta $d018       ; Character at $3000

Defining a character

; Character 0 = solid block
    lda #$ff
    sta $3000       ; Row 0
    sta $3001       ; Row 1
    ; ... continue for 8 rows

Building game screens

Tile-based levels

Tile number Represents
0 Empty space
1 Ground
2 Brick
3 Ladder
4-7 Decorations

Screen composition

Level data: 32 × 8 = 256 bytes per screen
Charset: 64 tiles × 8 = 512 bytes
Total: 768 bytes vs 8KB bitmap

Multicolour character mode

On C64:

  • Each character uses 4 colours
  • Half horizontal resolution
  • Richer graphics
  • 160×200 effective pixels

Animation techniques

Character cycling

Animate by changing character definitions:

  • Water ripples
  • Fire flickering
  • Blinking lights

Character swapping

Change which character displays:

  • Player animation frames
  • Different tiles same position

Platform variations

Platform Characters Colours Custom-character method
C64 256 Per-character (text mode) or per-cell colour RAM Point $D018 bits 1-3 to RAM-based charset
Spectrum 256 Per-8×8 attribute block Point CHARS to RAM, OR use the 21 UDGs (codes 144-164) — the cleaner approach for small sets
BBC Micro 256 Mode-dependent OSWORD calls or direct VDU 23 redefine commands
VIC-20 256 Per-character Point character pointer to RAM

The Spectrum’s UDG (User-Defined Graphics) system gives 21 redefinable characters in the upper printable range — sufficient for small games without disturbing the rest of the character set. Larger graphics need full charset replacement.

Limitations

Limitation Workaround
8×8 grid alignment Sprite overlay
256 character max Bank switching
Colour restrictions Careful palette

Notable uses

Game Character graphics use
Boulder Dash Entire playfield
Impossible Mission Backgrounds
Manic Miner Level tiles
Most platformers Background scenery

See also