TRS-80 Color Computer
Tandy's 6809 line
The Tandy/Radio Shack 6809-based home computer line (1980-1991), known as the CoCo, which shared architecture with the Dragon and evolved into the capable CoCo 3.
The TRS-80 Color Computer (commonly “CoCo”) was Tandy/Radio Shack’s 6809-based home computer line, produced from 1980 to 1991. Sharing architecture with the UK’s Dragon computers, the CoCo evolved through three generations, with the CoCo 3 offering significantly enhanced capabilities.
Fast facts
- Released: 1980 (CoCo 1)
- Manufacturer: Tandy Corporation
- CPU: Motorola 6809E — a 68B09E in the CoCo 3
- Sales channel: Radio Shack stores (USA)
- Generations: CoCo 1, 2, 3
Model Comparison
| Model | Year | RAM | Speed | Graphics |
|---|---|---|---|---|
| CoCo 1 | 1980 | 4-64KB | 0.89 MHz | MC6847 |
| CoCo 2 | 1983 | 16-64KB | 0.89 MHz | MC6847 |
| CoCo 3 | 1986 | 128-512KB | 0.89 / 1.78 MHz | ACVC (“GIME”) |
The CoCo 3’s video chip is also its memory manager
The single biggest change in the CoCo 3 is not the extra colours. It is that the new chip carries an MMU, which is how an 8-bit processor with sixteen address lines reaches half a megabyte. Tandy’s service manual:
ACVC can expand memory space up to 512K bytes. Having a built-in MMU (Memory Management Unit), it can support 2 banks of 256K-byte RAM, each with a 9-line address bus, even though the CPU possesses only 16 address lines.
Note the name. Tandy’s own documentation calls the part ACVC throughout; “GIME” is the name that stuck elsewhere.
The palette is likewise simpler in the hardware than in the marketing: sixteen
registers at $FFB0–$FFBF, six bits each. Sixteen colours on screen chosen
from sixty-four, with the same six bits read as R1 G1 B1 R0 G0 B0 for RGB
output and as intensity and phase for composite.
CoCo 3 Enhancements
| Feature | Improvement |
|---|---|
| Speed | Switchable, 0.89 or 1.78 MHz |
| RAM | Up to 512KB |
| Graphics | 320×200 in 16 colours |
| Palette | 64 colours |
| Chip | ACVC replaced the MC6847 — Tandy’s manuals call it ACVC, not GIME |
| MMU | The video chip is also the memory manager, reaching 512 KB from a 16-bit bus |
Dragon Compatibility
The CoCo and Dragon shared:
- Same 6809 CPU
- Same MC6847 video chip (CoCo 1/2)
- Similar memory maps
- Most software cross-compatible
Software library
| Category | Examples |
|---|---|
| Games | Dungeons of Daggorath, Mega-Bug |
| Business | VIP Writer, DynaStar |
| Languages | BASIC, OS-9, C |
| OS | OS-9 multi-tasking |
Legacy
The CoCo line demonstrated the 6809’s potential over its long lifespan. The CoCo 3 particularly showed what the architecture could achieve with enhanced support. Tandy’s Radio Shack distribution gave the platform significant US market penetration.