Dragon 32/64 and TRS-80 Color Computer: emulator, 6809 assembler and debugger

Both machines start in Microsoft’s BASIC: Dragon BASIC (one 16 KB ROM) or Color BASIC with Extended Color BASIC (two 8 KB ROMs) on the CoCo. They are still copyright — Microsoft’s, Tandy’s and Dragon Data’s successors' — and no rights holder has allowed them to be shared, so this page does not include them. Other emulators work the same way: XRoar, the reference Dragon/CoCo emulator, ships no ROMs and its packages say the images "are still under copyright". The Dragon 64 BASIC source and the CoCo ROM disassemblies people have published are Microsoft’s code too, so they are no way round it. I found no open replacement BASIC for these machines (Microsoft opened its 6502 BASIC in 2025, not the 6809 one).

Load ROM files… on the Machine tab takes the images you have: d32.rom (or the two 8 KB halves), d64_1.rom and d64_2.rom for the Dragon 64, bas12.rom/bas13.rom and extbas11.rom for the CoCo. Each one is recognised by its CRC-32 (as MAME lists them) or by its sign-on text, and kept in IndexedDB in this browser only.

Without ROMs the machine runs the GW 6809 monitor, an open stand-in written for this site (its source is in dragon-coco-data.js, MIT licence). It has no BASIC, but it keeps BASIC’s conventions so machine code written for either ROM runs on it: the indirect jump table at $A000 (POLCAT, CHROUT, CSRDON, BLKIN, BLKOUT, JOYIN, WRTLDR), the interrupt vectors in RAM at $0100–$0111, the screen at $0400 and the cassette and timer variables. At its > prompt:

An SWI in your code shows the registers and returns to the prompt.

With the ROMs loaded, type at the OK prompt, or use the BASIC tab: pick an example or paste a listing and press NEW, type & RUN. The text is handed to the ROM’s own keyboard routine one key at a time, as if typed, so it works for any program; letters go in as capitals unless you untick that. Ctrl+V on the screen (or the Paste button) types the clipboard. The examples — PRINT@, SET blocks, PMODE 4 circles, PLAY (Ode to Joy), JOYSTK and machine code from DATA — run on both machines; the CoCo needs Extended Color BASIC for PMODE, CIRCLE and PLAY.

The Assembly tab has a 6809 assembler written for this site, in Motorola syntax: labels in the first column, ORG, EQU, FCB/FDB/FCC/RMB, SETDP, IF/ELSE/ENDIF; every addressing mode (#imm, <direct, >extended, ,X+, ,--S, A,X, D,Y, 5,U, label,PCR, [,X++], [$A002]), the shortest offsets that fit, and long branches. Assemble & run (Ctrl+Enter) resets the machine, waits for the prompt, puts the code in memory and types EXEC &H4000 (BASIC) or G 4000 (the monitor). The examples print through JSR [$A002] and read keys through JSR [$A000], so they run under either ROM: hello world, reading keys, semigraphics 4 colour blocks, PMODE 4 set up by hand through the SAM and the VDG (on a CoCo its stripes come out in artifact colours), four-colour CG3 graphics, a scale on the DAC, a field-sync interrupt counter hooked into $010C, a joystick-driven block and SWI as a breakpoint. Build .CAS tape, Download .BIN (Disk BASIC’s LOADM format) and Download .DGN (DragonDOS) make files for other emulators or a real machine.

The Assembly tab is a split view: the editor, and beside it (below on a phone) the screen with the registers (A, B, D, X, Y, U, S, DP and the EFHINZVC condition codes), the S stack with likely return addresses, the code around PC and watches.

The Debugger tab works on anything that runs. Its disassembly names the ROM jump table, the RAM vectors and every PIA and SAM register. Views: SAM and VDG (all sixteen SAM bits with their set/clear addresses, the display offset and mode, the CPU rate and map type; the VDG mode from PIA1 port B), PIAs (both chips' ports, control registers and what each line is wired to, keys held, joysticks) and Sound and tape (the DAC, the analogue multiplexer, the motor and the tape position).

Drop files on the screen or the Tapes, programs and cartridges box:

Enter is ENTER, Backspace and ← are the left arrow (BASIC’s back space), Esc is BREAK, Home is CLEAR. Letters type as capitals; Shift+letter gives lower case, which these machines show in inverse video. The mouse over the screen is the right joystick (its button is the fire button), and a gamepad works too. On a phone, Keyboard shows the keyboard.

Full screen fills the screen with the Dragon or CoCo display; ⛶ Present (or F) shows only the screen for a talk. Keys keep working; Esc closes.

The 6809 is emulated instruction by instruction with the data sheet’s cycle counts (including each indexed mode’s extra cycles and PSH/PUL’s per-byte cost), the half carry and DAA, NMI, FIRQ and IRQ with their different stacking, SWI2/SWI3, CWAI and SYNC; it has its own tests, written against Motorola’s documentation (I found no public 6809 test suite with a licence I could use). The SAM counts the video addresses — so its mode and the VDG’s can be mixed into the semigraphics 8/12/24 modes — and sets the CPU rate (the "speed-up poke" works) and the map type. The picture is drawn a scan line at a time from the VDG’s mode on PIA1 port B: 32 × 16 text with semigraphics 4 and 6, and the eight graphics modes from 64 × 64 to 256 × 192, with the NTSC artifact colours of the 256-dot mode (blue or red first: a real CoCo picks one at power-on). The VDG’s 5 × 7 character set was drawn for this site. Horizontal and field sync go to PIA0’s CA1 and CB1, so BASIC’s 50 or 60 Hz timer interrupt runs; the keyboard is the 7 × 8 matrix scanned through PIA0, the joysticks a comparator against the DAC, and sound is the DAC (through the analogue multiplexer and the sound-enable line) plus the 1-bit output, mixed at 44.1 kHz.

Share link copies the model, the tab and the code or listing. Links can be written by hand:

Presets: hello world in 6809 code, PMODE 4 artifact colours on a CoCo, semigraphics 4, PMODE 4 circles in BASIC, the SAM and VDG view.

Related: BBC Micro, Acorn Electron, ZX Spectrum, MSX, 6502 assembler, retro font editor.

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