Tandy Color Computer — the colour computer with a 6809 heart

01 · ORIGINSHow the CoCo was born

The story does not begin at Radio Shack but in agriculture. In 1977 the University of Kentucky and its agricultural extension service were looking for a cheap videotex terminal for farmers. The contract went to Motorola in Austin, Texas, with Tandy joining later as the manufacturer. The project was called “Green Thumb”. The terminal had to connect over the telephone line to an ordinary colour television [3].

The first prototype (1978) had too many chips to be economical. Motorola gathered the functions of many of them into one — the MC6883 SAM. Together with the MC6847 video chip and the 6809 processor it became the core of the AgVision terminal, sold through Radio Shack and also as “VideoTex” around 1980. From there it was only a step to a home computer: remove the modem, add ports for a cassette recorder, serial input and joysticks, and the slot on the right for ROM cartridges (“Program Paks”) [3][5].

On 31 July 1980 Tandy announced the TRS-80 Color Computer, with the same case, keyboard and layout as the AgVision. The first model (catalogue no. 26-3001) had 4 KB of dynamic memory and 8 KB of Microsoft BASIC in ROM, and cost $399. It appeared in Radio Shack stores in the US and Canada in the following months. According to 80 Micro, the computer was Tandy’s answer to the success of the Commodore VIC-20 [3].

02 · HARDWAREWhat is inside

The computer connects only to a television. The case is large, silver-grey, with a “calculator-style” keyboard. Almost all of the main chips are from Motorola [3]. One remarkable feature is the “artifact colours” mode: at the highest resolution, 256×192, on an NTSC television you get orange and blue, but which of the two is which depends on the moment of power-on — so games often ask you to press “Reset” until the colours come out right [3].

03 · THE PROCESSORWhy the 6809

The Motorola 6809 was designed by Terry Ritter and Joel Boney and introduced in 1978. The reference article calls it “one of the most powerful 8-bit processors of its time”, but also one of the most expensive: in 1981 a single unit cost $37, against $9 for a Z80 and $6 for a 6502 [5].

• two 8-bit accumulators, A and B, which can work as one 16-bit register, D;
• two index registers (X and Y) and two stack pointers (U for the user, S for the system);
• a hardware multiply instruction (MUL): 8×8 bits gives a 16-bit result;
• program-counter-relative addressing — code that can be loaded at any place in memory [5].

These very decisions make multitasking systems such as OS-9 and UniFLEX possible. Besides the CoCo, the 6809 is found in the Dragon 32/64, the Japanese Fujitsu FM-7, the French Thomson machines and arcade cabinets such as Williams’ Defender and Joust [5].

04 · BASICColor BASIC and Extended Color BASIC

Color BASIC is a variant of Microsoft BASIC that fits in 8 KB of ROM and runs on every CoCo from 1980 to 1991. The extended variant (Extended Color BASIC) requires at least 16 KB of memory; Radio Shack offered an upgrade for $99. BYTE magazine described it in May 1981 as “the first truly easy-to-use and inexpensive system that produces colour graphics” [3][12].

A small program for the 256×192 graphics mode with a circle and a rectangle. It has not been run on a machine or an emulator — it serves as an illustration:

10 PMODE 4,1
20 PCLS
30 SCREEN 1,1
40 CIRCLE(128,96),50
50 LINE(10,10)-(100,50),PSET,BF
60 GOTO 60

The sound side of BASIC is just one command, PLAY, which takes the notes as text (for example “CDEFGAB”). The music is output through the built-in 6-bit converter [3].

05 · OS-9A multitasking system for 8 bits

OS-9 is a family of multitasking, multi-user operating systems created by Microware for the 6809 processor in 1979–1980 as support for the BASIC09 project for Motorola. The first version, “Level One”, is written in assembly language and all its processes live in a 64 KB address space, with no memory-management unit. The later “Level Two” uses memory-mapping hardware and can address up to 2 MB [4].

The best-known platform for OS-9 is precisely the CoCo, thanks to its low price and wide distribution. According to the reference article, even on the minimal CoCo, under Level One, more than one user can work at the same time (one at the console, another on a background process, a third over a serial line) — this claim has no citation there, and we pass it on with reservation [4]. For the CoCo 3, Microware offered Level 2 soon after its launch. It provides a windowed screen, a full development environment with BASIC09 and compilers for C and Pascal [3].

The file system is hierarchical and resembles UNIX, but is not UNIX: the tree is a “forest”, with one tree per device, and instead of fork there is a system call that starts a specified program [4]. In the mid-1980s OS-9 was chosen as the basis of the CD-i — the CD-RTOS operating system [4]. The community additionally rewrote Level 2 for the CoCo 3 as NitrOS-9, optimised for the Hitachi 6309 processor [3][4].

06 · GAMESThe games — from dungeons to robots

07 · MODELSFrom the CoCo 1 to the CoCo 3 — and the relatives

The CoCo 3 was announced on 30 July 1986 at the Waldorf-Astoria hotel in New York. The GIME chip replaces the graphics and memory of the older models, adds text of up to 80 columns with lower-case letters, up to 16 colours from a palette of 64, and memory management that later allowed RAM upgrades of up to 2 MB. It is compatible with most of the old programs [2][3]. The original specifications also mention a 256-colour mode, but according to the reference article it has never been found [3].

There is also a British relative — the Dragon 32/64, built on the same Motorola reference design, with video output and a parallel port. In 1983 Tano began making it in New Orleans for the North American market, but sales fell short of expectations and production stopped after about a year [3].

08 · LEGACYThe end and what remains

In 1985 CoCo owners feared that Tandy would abandon the computer in favour of the Tandy 1000. The manager Ed Juge then claimed that “no home computer today has the potential of the Color Computer” [3]. Even so, on 26 October 1990 Tandy announced that the CoCo 3 would be withdrawn from sale; production ended in 1991 [3]. Many owners believed the company had never taken this line seriously [3].

The community, however, stayed. NitrOS-9 was made, there are emulators such as VCC (Virtual Color Computer), and hobbyists have recreated the CoCo 3 in an FPGA chip that adds 256-colour modes [3][4]. The annual CoCoFEST convention in Chicago was held at least until 2018 [4].

The CoCo is the computer of contrasts: cheap, but with a refined processor; a games machine, but with a UNIX-like system; released by a chain of electronics-hobbyist shops, but born in an agricultural project. It never achieved the mass success of the Commodore 64 or the Spectrum, and we have not confirmed the exact sales. But whoever got to know it remembers it fondly.

09 · SOURCESSources

1. BYTE, March 1981, “What’s Inside Radio Shack’s Color Computer?” (cited in [3]; not read directly) — archive.org
2. The New York Times, Calvin Sims, “5 Models Introduced By Tandy”, 31 July 1986 (cited in [3]; not read directly) — nytimes.com
3. Wikipedia, “TRS-80 Color Computer” (secondary) — en.wikipedia.org/wiki/TRS-80_Color_Computer
4. Wikipedia, “OS-9” (secondary) — en.wikipedia.org/wiki/OS-9
5. Wikipedia, “Motorola 6809” (secondary) — en.wikipedia.org/wiki/Motorola_6809
6. Wikipedia, “TRS-80 MC-10” (secondary) — en.wikipedia.org/wiki/TRS-80_MC-10
7. Wikipedia, “Motorola 6847” (secondary) — en.wikipedia.org/wiki/Motorola_6847
8. Wikipedia, “Dungeons of Daggorath” (secondary) — en.wikipedia.org
9. Wikipedia, “Robot Odyssey” (secondary) — en.wikipedia.org/wiki/Robot_Odyssey
10. Wikipedia, “List of TRS-80 Color Computer games” (secondary; cites the King’s Quest III box) — en.wikipedia.org
11. CoCopedia, “Downland” (community wiki) — cocopedia.com/wiki/Downland
12. Wikipedia, “Color BASIC” (secondary) — en.wikipedia.org/wiki/Color_BASIC

Checked on 3 October 2026. Sources 3–12 are secondary or community-made; the primary sources 1 and 2 are cited through them and have not been read directly.

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