gmc64

gmc64 is a JavaScript reimplementation of Garry Kitchen’s GameMaker for the Commodore 64. This project documents the graphic and sound formats, virtual machine, instruction set and runtime behavior, and includes a browser-based version that works with original GameMaker projects.

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If you used GameMaker back then

You probably remember the fun of animating sprites pixel by pixel, drawing background scenes, coding up little games and demos, tweaking SID sounds, and writing music.

If you tried to relive that fun on a C64 emulator, you probably noticed the joystick based UI on the original is a tough sell today, and the slow disk operations are a drag.

gmc64 solves this. It's a fully modern JavaScript recreation of GameMaker with modern convenience: use your mouse or trackpad, instantly preview, load, and save files, copy, paste, undo, and more. Import and export GIF, PNG, and MIDI files.

It's not an emulator, it's a recreation, with the stumbling blocks gone.

If you have your original work

Do you have your original work on a disk? gmc64 reads .d64 disk images and all GameMaker file formats. Drag and drop a disk or use the built-in file browser.

Then you can open the editor, add a sprite, change the music, and save it back to the disk image. You can even export your creations as a single self-contained HTML file that you can post online.

gmc64 can save files back to .d64 images as well, and you can even import them back to the original GameMaker.

If you're new to GameMaker

GameMaker was a game-creation tool for the Commodore 64 created by Garry Kitchen and published by Activision in 1985. It let users build real, runnable C64 games without learning assembly or having to hand code graphics and sounds. An early graphical IDE, you drew sprites and backgrounds, composed music, recorded sound effects, and wrote game logic from a list of simple instructions.

sprite 1 is player
sprite 1 x position = 160
sprite 1 y position = 100
if joystick 1 is left then
    sprite 1 direction = left
endif

Many people who grew up to be game developers got their start here. It's been largely inaccessible in the modern age, and even through emulation the usability suffers from the limitations of the original C64.

gmc64 brings it back. The same sprites, same instructions, same sounds, all instantly in your browser.

Two-way compatibility

gmc64 doesn't just open old files. It implements the same formats: /PRG, /SPR, /PIC, /SND, and /SNG — and they round-trip cleanly in both directions.

A game built here saves to a real disk image that loads on an actual Commodore 64 running the original 1985 GameMaker disk. Anything authored on that hardware loads here without translation. New creations can ship to a 1541. Old creations can ship to the web.

Quick start

Click any of the image links at the top of this page to dive in.

Then:

• Try the demos - disks/gmc64-demo.d64 ships with the project and includes runnable programs and editable sprites, scenes, sounds, and songs
• Start from scratch — author sprites, scenes, sounds, and music, of your own
• Drop a .d64 into any gmc64 tool - editor, sprite-maker, scene-maker, sound-maker, and music-maker will all open a pop-up file picker.
• Host a .d64 - you can point to an online d64 as well: program-editor.html?disk=https://your-host/game.d64 opens the same file picker

When your game is ready, hit Export Game to HTML to get a single HTML file of your creation.

Sharing your creations

Every editor takes URL parameters, so you can hand someone a direct link to a specific program or drop the player into a page you're building. And every editor accepts a .d64 dropped onto its window — the file picker will open filtered to that tool's file type (.PRG in the editor, .SPR in sprite-maker, etc.).

Player (play.html) — chrome-free, iframe-friendly

Param	Purpose
disk	.d64 URL, or the magic value demo for the bundled demo disk
file	Program name on the disk (case-insensitive)
nocredit=1	Hide the gmc64.com corner link
poster_seconds	How many seconds to simulate the game for the preview frame behind the play button. Default 2, max 10, 0 skips the poster entirely. Decimals allowed.

Example — direct link:

https://gmc64.com/play.html?disk=demo&file=aliens/prg&nocredit=1

Program editor (program-editor.html) — loads into the editor, optionally plays

Param	Purpose
disk	Same as above
file	Same as above
play=1	Show a play-button overlay with a poster preview when the page opens. Visitor clicks play to run, clicks the stop button to drop into the editor. Without this flag the file just opens for editing.
poster_seconds	Same as above; only meaningful with play=1
play_demo=1	Alias — expands to disk=demo&file=gmc64i/prg&play=1&poster_seconds=8.5

Example — send someone a runnable link that still lets them drop into the editor and see the code:

https://gmc64.com/program-editor.html?disk=https://your-host.com/game.d64&file=GAME/PRG&play=1

The other editors (sprite-maker.html, scene-maker.html, sound-maker.html, music-maker.html) accept disk and file too, deep-linking straight to a specific asset for editing.

Iframe embed

Two ways to get an embeddable game, depending on what you're willing to host.

Option A — self-contained HTML. Use the editor's Export Game to HTML button under file. It downloads a single HTML file containing your program, the disk image, and the runtime. Host it anywhere (GitHub Pages, Netlify, S3, your own server), then embed it. The Export dialog also generates the iframe snippet for you — just paste the URL where you'll host the file:

<iframe src="https://your-site.com/mygame.html"
        width="640" height="500"
        allow="autoplay" loading="lazy"
        frameborder="0"></iframe>

This is the beginning of the README. Read the rest at the original. Read the full README ↗