IBM Color Graphics Adapter
The IBM Color Graphics Adapter (CGA) is the original 8-bit ISA colour graphics card IBM released alongside the IBM PC (5150) in 1981. It uses a Motorola 6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the IBM 5153) or a composite TV through an NTSC encoder.
Video Modes
| Mode | Resolution | Colours | Notes |
|---|---|---|---|
| 0 | 40 × 25 text | 16 fg / 8 bg | Composite or RGBI |
| 1 | 40 × 25 text | 16 fg / 8 bg | Colour burst suppressed |
| 2 | 80 × 25 text | 16 fg / 8 bg | 8 × 8 character cell |
| 3 | 80 × 25 text | 16 fg / 8 bg | Colour burst suppressed |
| 4 | 320 × 200 graphics | 4 from two fixed palettes | Palette 0: cyan/magenta/white; Palette 1: green/red/yellow |
| 5 | 320 × 200 graphics | 4 (greyscale on composite) | Composite "alternate" palette |
| 6 | 640 × 200 graphics | 2 (background + foreground) | Highest resolution |
Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block "mode 7", composite NTSC artefact colour, "8088 MPH"-style techniques), but they are not part of the BIOS interface.
Hardware
- 6845 CRTC — clock and sync generation, addressing.
- 16 KB video RAM (8 × 4116 DRAM on early boards, later consolidated).
- TTL RGBI output on DB-9 (drives the IBM 5153).
- Composite NTSC output on RCA jack (drives any NTSC TV).
- Light-pen connector (rarely used).
Common Faults
- Snow — the famous CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Disable by waiting for the horizontal retrace (most CGA-aware games already do this).
- No video, beep code 1 long + 2 short — the CGA card is dead or unseated. Reseat first.
- Missing one colour channel — bad output buffer at the DB-9, or cracked solder joint on the connector.
- Smoke from a discoloured tantalum — replace the failed tantalum (typically a 22 µF / 16 V) and any series resistor that may have died with it. See ISA card capacitor positions.
Related Pages
- IBM PC (5150)
- IBM 5153 — the matching colour monitor
- IBM Monochrome Display Adapter — the monochrome alternative
- IBM PC (5150) Capacitor Replacement Guide