Emulator Stella

Stella — Atari 2600

The Atari 2600 runs on EmulatorJS + Stella (the stella2014 libretro core), sharing the same framework as the VICE family, ColecoVision and the NES. It got here the hard way — we shipped it on Javatari first, then migrated.

Why we left Javatari

Javatari.js (Paulo Peccin, AGPL-3.0) is a lovely self-contained 2600 emulator and the original integration was a one-liner. Two things eventually pushed us off it:

1. It sizes its own canvas to the window. Every other bundle lets us size the canvas, which is what makes a CRT bezel possible — shrink the picture into the bezel's screen hole. Javatari ignores any container we give it and fills the viewport, so it's the one engine we couldn't bezel. (We tested it: a hard-capped #javatari-screen was completely ignored.)
2. GAME SELECT / RESET were on F11 / F12 — browser-reserved (fullscreen / devtools). The only reliable way to hit them was Javatari's on-screen console faceplate, which is itself chrome we'd otherwise want to hide. Reset starts most 2600 games, so this wasn't optional.

EmulatorJS solves both: we control the canvas (bezel-ready), and the console switches map to plain keys.

The launch flow

stella/play.html is the standard genxBootEJS wrapper. EmulatorJS knows the 2600 as system atari2600, which it resolves to the stella2014 core:

genxBootEJS({
  title: 'Atari 2600',
  coreName: 'atari2600',
  defaultControls: {
    0: {
      0: { value: 'space' },        // FIRE
      2: { value: 'v' },            // GAME SELECT
      3: { value: 'enter' },        // GAME RESET
      4: { value: 'up arrow' }, 5: { value: 'down arrow' },
      6: { value: 'left arrow' }, 7: { value: 'right arrow' }
    }
  }
});

The existing .a26 cart images carried straight over — Stella reads them unchanged.

Controls

EmulatorJS has a dedicated atari2600 control scheme (emulator.js, button-label block): id 0 = FIRE, 2 = SELECT, 3 = RESET, 4–7 = the joystick, and 10–15 = the difficulty A/B and Color/B-W switches (these stay on the EJS defaults Q/E/Tab/R, rarely needed). So on the keyboard:

• Arrows — joystick · Space — fire
• Enter — GAME RESET (starts/restarts most games)
• V — GAME SELECT (cycles the game variant)

A 2600 wrinkle worth documenting per-game: Reset starts the game, but FIRE serves the ball in paddle/serve games. Breakout, for instance, needs Enter to start and then Space to serve each ball.

EJS chrome

Like the other EJS bundles, Stella inherits the shared .ejs_menu_bar, .ejs_context_menu { display: none } rule in genx-frame.css, so EmulatorJS's control bar doesn't pop up on mouse-move — only the screen shows.

Bundle layout

emulators/stella/
├── play.html            ← genxBootEJS wrapper (coreName: 'atari2600')
├── controls.html
├── games.json
└── roms/
    └── *.a26            ← 2600 cart images (reused from the Javatari bundle)

The Stella core itself lives once in emulators/_shared-ejs/ejs/data/cores/stella2014-legacy-wasm.data, shared with the rest of the EJS family.

Related

• Emulator-EmulatorJS-NES-FCEUmm — sibling EJS bundle
• Emulator-gearcoleco-ColecoVision — sibling EJS bundle
• Emulator-JS7800 — sibling Atari machine
• Emulators — index