TRS-80 Color Computer Extended BASIC Emulator

A complete emulator for the TRS-80 Color Computer (CoCo) Extended Color BASIC, written in JavaScript. Run classic BASIC programs in your browser with authentic graphics and sound.

The TRS-80 Color Computer Extended BASIC emulator in action

About the TRS-80 Color Computer

The TRS-80 Color Computer (affectionately known as the "CoCo") was a home computer produced by Tandy Corporation from 1980 to 1991. It was one of the most popular home computers of the early 1980s, competing with the Commodore 64 and Apple II.

Key historical facts:

• Based on the Motorola 6809 processor (one of the most advanced 8-bit CPUs)
• Featured Extended Color BASIC in ROM, developed by Microsoft
• Supported vibrant color graphics (up to 256x192 resolution)
• Had a distinctive "chiclet" keyboard in early models
• Three main generations: CoCo 1 (1980), CoCo 2 (1983), CoCo 3 (1986)
• Strong educational and hobbyist community that remains active today

This emulator faithfully recreates the Extended Color BASIC environment, allowing you to write and run programs just like on the original hardware.

Demo

Animated bouncing ball demo using PMODE graphics

Installation

Browser (No Installation Required)

Simply clone the repository and open the HTML file:

git clone https://github.com/bshichman/trs80-coco-basic.git
cd trs80-coco-basic
open ui/coco-emulator.html
# Or on Linux: xdg-open ui/coco-emulator.html
# Or on Windows: start ui/coco-emulator.html

Alternatively, serve it with any static file server:

# Using Python
python -m http.server 8000
# Then open http://localhost:8000/ui/coco-emulator.html

# Using Node.js (npx)
npx serve .

Node.js (Command Line)

git clone https://github.com/bshichman/trs80-coco-basic.git
cd trs80-coco-basic

# Interactive mode
node index.js

# Run a program file
node index.js programs/test.bas --run

No npm dependencies required - it's pure JavaScript!

Features

Program Control

• PRINT, INPUT, LET
• IF...THEN...ELSE
• FOR...TO...STEP...NEXT
• GOTO, GOSUB, RETURN
• ON...GOTO, ON...GOSUB
• END, STOP, REM

Data Handling

• DATA, READ, RESTORE
• DIM (multi-dimensional arrays)
• DEF FN (user-defined functions)

Math Functions

• ABS, SGN, INT, FIX
• SQR, LOG, EXP
• SIN, COS, TAN, ATN
• RND

String Functions

• LEFT$, RIGHT$, MID$
• LEN, CHR$, ASC
• STR$, VAL, STRING$
• INSTR, INKEY$

Extended Color BASIC Graphics

• PMODE (0-4) - Set graphics mode
• SCREEN - Switch between text and graphics
• PCLS - Clear graphics screen
• COLOR - Set foreground/background colors
• PSET, PRESET - Plot/unplot pixels
• LINE - Draw lines and boxes (with B, BF options)
• CIRCLE - Draw circles and ellipses
• PAINT - Flood fill
• DRAW - Turtle graphics commands
• PCOPY - Copy graphics pages

Sound

• SOUND - Generate tones (frequency, duration)
• PLAY - Music macro language
  • Notes: C, D, E, F, G, A, B
  • Octave: O1-O6, >, <
  • Length: L1-L64
  • Tempo: T32-T255
  • Volume: V0-V15
  • Pause: P

Memory & System

• PEEK, POKE - Direct memory access
• MEM - Available memory
• TIMER - System timer
• 64KB address space emulation

Usage

Web Browser

Open ui/coco-emulator.html in a web browser. The interface includes:

• Terminal: Green phosphor CRT-style display with command input
• Graphics Canvas: Authentic CRT effect with scanlines
• Program Editor: Load, edit, and save BASIC programs
• Sample Programs: Pre-loaded demos to try

Command Line (Node.js)

# Interactive mode - type BASIC commands directly
node index.js

# Run a program file
node index.js program.bas --run

Programmatic API

const { CoCoEmulator } = require('./index.js');

const emulator = new CoCoEmulator({
  outputCallback: (text) => console.log(text)
});

emulator.loadProgram(`
10 FOR I = 1 TO 10
20 PRINT I * I
30 NEXT I
40 END
`);

await emulator.run();

Sample Programs

The programs/ directory contains example programs:

File	Description
graphics-demo.bas	Demonstrates PMODE graphics, lines, circles
music-demo.bas	PLAY command music examples
bouncing-ball.bas	Animation using graphics primitives
test.bas	Basic functionality test

Additional demos in ui/:

File	Description
bouncing-balls.bas	Multiple bouncing balls animation
breakout.bas	Classic Breakout game clone

Quick Start Example

10 REM SIMPLE GRAPHICS DEMO
20 PMODE 4,1
30 SCREEN 1,1
40 PCLS
50 FOR I = 0 TO 255 STEP 8
60   LINE (I,0)-(255-I,191),1
70 NEXT I
80 GOTO 80

Graphics Modes (PMODE)

Mode	Resolution	Colors	Bytes/Page
0	128x96	2	1,536
1	128x96	4	3,072
2	128x192	2	3,072
3	128x192	4	6,144
4	256x192	2	6,144

Color Sets

Set 0 (CSS=0): Green, Yellow, Blue, Red Set 1 (CSS=1): Buff, Cyan, Magenta, Orange

Architecture

trs80-coco-basic/
├── core/
│   ├── tokens.js      # Token definitions for lexer
│   ├── lexer.js       # Tokenizer (source → tokens)
│   ├── parser.js      # Parser (tokens → AST)
│   ├── memory.js      # 64KB memory manager
│   └── interpreter.js # BASIC interpreter engine
├── graphics/
│   └── graphics.js    # VDG graphics emulation
├── sound/
│   └── sound.js       # Web Audio API sound system
├── storage/
│   └── storage.js     # Program storage (localStorage)
├── ui/
│   ├── coco-emulator.html  # Main browser interface
│   ├── coco-browser.js     # Browser integration
│   ├── coco-display.js     # CRT display rendering
│   └── coco-font.js        # Authentic CoCo font
├── programs/          # Sample BASIC programs
└── index.js           # Main entry point / API

Browser Compatibility

Browser	Version	Status
Chrome	66+	Full support
Firefox	60+	Full support
Safari	12+	Full support
Edge	79+	Full support

Required Web APIs:

• Canvas 2D Context
• Web Audio API (for sound)
• ES6+ JavaScript features (classes, async/await, template literals)
• localStorage (for program storage)

Note: Sound requires user interaction to start (browser autoplay policy).

Known Limitations

This emulator focuses on Extended Color BASIC programming. The following features from the original hardware are not implemented:

Feature	Status	Notes
Cassette I/O	Not implemented	Use the program editor instead
Disk BASIC	Not implemented	No LOADM, SAVEM, disk commands
Joystick input	Not implemented	JOYSTK() returns 0
RS-232 serial	Not implemented	-
Semigraphics modes	Partial	SG4 text mode only
Machine language	Limited	EXEC not supported
CLOAD/CSAVE	Not implemented	Use browser storage

BASIC limitations:

• TRON/TROFF (trace) not implemented
• EDIT command not implemented (use browser editor)
• No line renumbering (RENUM)
• String garbage collection differs from original

Contributing

Contributions are welcome! Here's how you can help:

Getting Started

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Make your changes
4. Test thoroughly in both browser and Node.js
5. Commit your changes (git commit -m 'Add amazing feature')
6. Push to the branch (git push origin feature/amazing-feature)
7. Open a Pull Request

Areas for Contribution

• Additional BASIC commands: TRON/TROFF, EDIT, RENUM
• Disk BASIC support: File operations, LOADM/SAVEM
• More sample programs: Games, utilities, demos
• Documentation: Tutorials, command reference
• Testing: Unit tests, compatibility tests
• Bug fixes: See Issues tab

Code Style

• Use ES6+ features
• Maintain compatibility with both browser and Node.js
• Comment complex algorithms
• Follow existing code patterns

Reporting Bugs

Please include:

• Browser/Node.js version
• Steps to reproduce
• Expected vs actual behavior
• Sample BASIC code that triggers the bug

Resources

• CoCo Wiki - Comprehensive CoCo information
• Color Computer Archive - Software and documentation
• Extended Color BASIC Unravelled - Technical reference
• TRS-80 Color Computer Facebook Group - Active community

License

This project is licensed under the MIT License - see the LICENSE file for details.

Acknowledgments

• Tandy/Radio Shack for creating the original Color Computer
• Microsoft for Extended Color BASIC
• The CoCo community for keeping the platform alive
• Contributors and testers

---

Ready to relive the 1980s? Clone the repo and RUN your first program!

10 PRINT "HELLO, COCO WORLD!"
20 GOTO 10