Dragon 32 / TRS-80 Color Computer 2

The Dragon 32 (1982, Wales) and the TRS-80 Color Computer 2 (CoCo 2, 1983, USA) are closely related machines — both built around the Motorola 6809 CPU and the MC6847 Video Display Generator, with similar ROM BASICs and nearly identical hardware. The Dragon was designed and built by Dragon Data in South Wales; the CoCo was made by Tandy/Radio Shack and sold through their stores.

The 6809 is a remarkable CPU — see the 6809 assembly guide for why it's considered the most elegant 8-bit processor ever made.

Languages in the IDE

Language	Notes
Color BASIC	Built-in ROM BASIC — line-numbered, interpreted
CMOC C	Cross-compiler producing 6809 machine code
6809 assembly	LWASM assembler — maximum control

Quick start

Select Dragon 32 or TRS-80 CoCo 2 as the platform.
Choose your language preset.
Build and run in the emulator.

Dragon vs CoCo differences

The Dragon 32 and CoCo 2 are similar but not identical. Key differences: the Dragon uses a slightly different memory map, different SAM (Synchronous Address Multiplexer) chip, and the BASIC ROM has minor differences. The IDE emulates each separately — pick the right one for your target.

Hardware overview

CPU — Motorola 6809E at ~0.89 MHz

The 6809 is a 16-bit capable 8-bit processor — two accumulators (A and B), a combined 16-bit D register, two 16-bit index registers (X and Y), two stack pointers (S and U), and a direct page register. See 6809 assembly for the full story.

Both machines run at approximately 0.89 MHz (the 3.58 MHz colour burst oscillator divided by 4). This is slower than the BBC Micro or ZX Spectrum, but the 6809's efficiency more than compensates.

MC6847 — Video Display Generator

The MC6847 VDG generates the video signal with no CPU involvement once configured. It reads directly from RAM and produces the display. Graphics modes:

Mode	Command	Resolution	Colours
Text	`TEXT`	32×16 chars	2 (green/black or white/black)
Graphics 1C	`GR(0)`	64×64	4
Graphics 2C		128×64	4
Graphics 3C		128×96	4
Graphics 6C	`HGR(0)`	128×192	4
Graphics 6R	`HGR(1)`	256×192	2

The MC6847 has a fixed colour palette based on an NTSC phosphor palette — greens, yellows, blues, reds — without the custom colour mixing of the C64's VIC-II or Spectrum's ULA.

SAM — Synchronous Address Multiplexer

The MC6883 SAM manages memory and video timing. It controls which RAM area the VDG reads from, the CPU clock rate, and the VDG mode. SAM registers are write-only and live at $FFC0–$FFDF.

Sound

Both machines have a single-bit DAC connected to the cassette port — basic but capable of reasonable music with careful timing. The CoCo also has a 6-bit DAC via the MC6821 PIA.

Memory map (Dragon 32 / CoCo 2)

Range	Contents
$0000–$001F	Direct page / system variables
$0020–$007F	More system variables
$0080–$00FF	More system area
$0100–$7FFF	User RAM (up to ~32 KB)
$8000–$BFFF	Cartridge ROM slot
$A000–$BFFF	Extended Color BASIC ROM (CoCo)
$C000–$DFFF	Color BASIC ROM
$E000–$FFFF	Color BASIC ROM / OS

VDG screen RAM is configurable via the SAM — by default it starts at $0400 (Dragon) or $0400 (CoCo).

PIA — I/O

Two MC6821 PIAs handle: - PIA 0: Keyboard matrix, joystick comparators, cassette, horizontal sync - PIA 1: 6-bit DAC (sound), VDG mode bits, vertical sync, motor control

Color BASIC

Color BASIC on both machines is a Microsoft-licensed BASIC very similar to other Microsoft BASICs of the era. See the dedicated Color BASIC guide for a full reference.

Quick overview:

10 CLS             : REM Clear screen (green on black in text mode)
20 PRINT "HELLO, DRAGON!"
30 FOR I=1 TO 10
40   PRINT I
50 NEXT I
60 POKE &H1000, 0  : REM POKE in hex (& prefix)
70 X = PEEK(&H1000)
80 PLAY "CDEFGAB"  : REM Play musical notes!
90 JOYSTK(0)       : REM Read joystick 0 (0-63)

Color BASIC includes PLAY for music, DRAW for graphics primitives, GET and PUT for sprite-style graphics, and MOTOR to control the cassette motor — a nicely featured BASIC for its era.

CMOC C

CMOC is a C cross-compiler that targets the 6809 — it takes a subset of C and produces 6809 assembly via LWASM. See CMOC guide for full details.

#include <cmoc.h>

int main() {
    // Clear screen
    cls();

    // Print text
    printf("HELLO FROM CMOC!\n");

    // Hardware access
    byte *screen = (byte *) 0x0400;
    *screen = 0xBF;   /* Block graphic character */

    return 0;
}

CMOC supports printf, malloc, basic string functions, and inline assembly — enough for real games.

6809 assembly quick reference

The 6809's powerful addressing modes make it particularly pleasant for Dragon/CoCo programming:

; Dragon 32 — write a character to the screen
; Screen RAM at $0400 (32 cols × 16 rows text mode)

        ORG $7E00

start:
        LDX #$0400      ; Screen RAM start
        LDA #$C1        ; 'A' in MC6847 semi-graphics set
        STA ,X          ; Write to top-left
        RTS

        END start

Call from BASIC with EXEC &H7E00.

Key differences from 6502/Z80

No zero page (but moveable Direct Page — set DP register)
16-bit registers throughout — index registers, stack, and D are all 16-bit
Two stacks — use U freely as a second stack or frame pointer
MUL instruction — 8×8=16 bit multiply in one opcode
LEA instructions — Load Effective Address (like a 68000-style LEA)

LEAX 5,X        ; X = X + 5  (no memory access — just address arithmetic)
LEAY D,X        ; Y = X + D  (very useful for table lookups)
LEAS -4,S       ; Allocate 4 bytes on stack (move SP down)

Joystick input

Both machines have two analog joysticks (0–63 range) read through the PIA's comparator:

10 X = JOYSTK(0)   : REM Left joystick horizontal (0-63)
20 Y = JOYSTK(1)   : REM Left joystick vertical
30 PRINT X, Y

In assembly, joystick reading involves the PIA's comparator and a DAC — the BASIC ROM's JOYSTK routine handles the timing. Calling it via JSR is the easiest approach.

Practical tips

Dragon vs CoCo BASIC compatibility — the two BASICs are very similar but have subtle differences in some PLAY/DRAW command syntax and a few POKE addresses. Always test on the target machine.

The 6809 DP register — both machines use $00 as the Direct Page on startup (same as 6502 zero page). You can change it, but ROM routines expect DP=$00.

Screen RAM and SAM — the SAM chip controls where the VDG reads screen data from. If you need a custom screen address, you must program the SAM as well as use that address in your code.

Cassette/tape format — the Dragon and CoCo use the same basic tape encoding. Programs saved from one can sometimes be loaded on the other, depending on BASIC version compatibility.

Extended BASIC — the CoCo's Extended Color BASIC (in the separate $A000–$BFFF ROM) adds DRAW, PAINT, GET/PUT, and other graphics commands not in the base Dragon BASIC.