Dragon 32

The Dragon 32 was a Welsh home computer released in 1982 by Dragon Data Ltd. Based on the superior Motorola 6809 CPU with 32KB RAM, it offered strong programming capabilities and TRS-80 Color Computer compatibility, but struggled commercially against the cheaper Spectrum and more capable C64.

Fast facts

Released: 1982
Price: £199
CPU: Motorola 6809E @ 0.89 MHz
RAM: 32 KB
ROM: 16 KB (Microsoft BASIC)
Manufacturer: Dragon Data, Port Talbot, Wales

Three chips, and a register you set by address

Inside the Dragon (1983) names the display hardware as three parts: the VDG (6847) which generates the video signal, the SAM (6883) — a Synchronous Address Multiplexor — and a PIA (6821), of which the machine has two.

SAM is the interesting one. It “has been specifically designed to provide the necessary control and timing signals for the M6809, the VDG chip, and the video RAM”: the processor’s timing comes out of the chip that also feeds the screen, the same arrangement the Archimedes uses when MEMC clocks the ARM.

Its control register is addressed in a way that is genuinely unusual. The book poses the question itself:

The SAM control register is memory mapped at address FFC0 and occupies the address range FFC0 to FFDF. As the control register is 16 bits wide, why are 32 bytes allocated in memory to that register?

Because each bit gets two addresses. Writing to the even one clears the bit; writing to the odd one sets it. The data written is irrelevant — the address is the instruction. In the book’s own mode-setting routine the accumulator holds a value being preserved for something else entirely while the stores go out:

SAMMOD    PSHS A           ; Preserve VDG pattern
          STA SAMV0C       ; Clear V0
          STA SAMV1C       ; Clear V1
          STA SAMV2C       ; Clear V2

Three STA instructions with the same register, clearing three different bits, because they go to three different places.

One display mode, two registers, three exceptions

The mode is not held in one place. SAM has bits V0–V2 and the VDG has GM0–GM2, and both must be set, consistently, for a mode to work. Usually they correspond directly. Three times they do not:

Condition	SAM V0V1V2
`A/G = 0` (text)	`000`
`A/G = 1` and `GM0GM1GM2 = 000`	`100`
`A/G = 1` and `GM0GM1GM2 = 111`	`011`

A programmer changing screen mode on a Dragon is keeping two chips in agreement by hand, with a short table of cases where the obvious mapping is wrong.

Eight colours, but you choose a set

The VDG “has the capability of generating eight colours”, but not freely. In the colour graphics modes “two colour sets each of four colours are available”, and a program selects a set rather than individual colours. The four-colour 128×192 mode is four colours from one of two fixed groups — which is why Dragon and Tandy CoCo screens of the period share a family resemblance whether or not their authors wanted one.

Specifications

Component	Detail
Graphics	MC6847 VDG
Resolution	256×192 (2 colours) or 128×192 (4)
Text	32×16 characters
Sound	1-bit DAC
Storage	Cassette, optional disk

Graphics Modes

Mode	Resolution	Colours
Text	32×16	Green/orange
Semigraphics	64×32	8
High-res	256×192	2
Medium-res	128×192	4

CoCo Compatibility

The Dragon was largely compatible with the TRS-80 Color Computer:

Same 6809 CPU
Same MC6847 video chip
Similar memory map
Most CoCo software ran with minor changes

Strengths and Weaknesses

Strengths	Weaknesses
Superior 6809 CPU	More expensive than Spectrum
Good keyboard	Limited sound (1-bit)
CoCo compatibility	VDG graphics restrictive
Solid construction	Smaller software library

Legacy

The Dragon represented Welsh ambition in the home computer market. Though Dragon Data folded in 1984, the machine introduced UK users to the elegant 6809 architecture and maintains an active preservation community.

Welsh 6809 computer