Emulator EmulatorJS 7800 ProSystem

ProSystem — Atari 7800

The Atari 7800 runs on EmulatorJS + ProSystem (the prosystem libretro core), sharing the same framework as the VICE family, ColecoVision, the NES and the 2600. Like the 2600, it got here the hard way — we shipped it on JS7800 first, then migrated for the bezel.

Why we left JS7800

JS7800 (raz0red, GPL-2.0) is a solid 7800 core and the original integration just meant backing out its demo-style UI (header, cart-picker, leaderboard/snackbar toasts, controls toolbar). What it couldn't do was take a CRT bezel:

• It drives its own requestAnimationFrame loop and sizes its own canvas. Pinned into a bezel's screen hole it stalled on the 7800 BIOS splash — the cart loaded (the headless console confirmed the cartridge header) but the render loop never advanced. Constrained to the hole, the picture wouldn't come up. Every other bundle lets us size the canvas, which is what makes the bezel possible.

EmulatorJS solves it the same way it did for the 2600: we control the canvas (bezel-ready), and the console switches map to plain keys. The 80s.png wood-cabinet TV bezel — shared with the Atari 800XL — went on at the same time.

The launch flow

js7800/play.html is the standard EmulatorJS wrapper. EmulatorJS knows the 7800 as system atari7800, which it resolves to the prosystem core:

window.EJS_core = 'atari7800';
window.EJS_defaultControls = {
  0: {
    0: { value: 'z' },      // BUTTON 1 (left fire)
    8: { value: 'x' },      // BUTTON 2 (right fire)
    9: { value: 'enter' },  // RESET  → starts the game from the title
    2: { value: 'shift' },  // SELECT → cycles game variations
    3: { value: 'p' }       // PAUSE
  }
};

The existing .a78 cart images and games.json carried straight over — ProSystem reads them unchanged.

Controls — the RESET gotcha

The one trap: on the EmulatorJS atari7800 scheme, RESET is button id 9, not START (id 3) — and on the 7800, RESET is what starts a game from the title screen. Mapping START there left every game stuck on its title. So Enter is wired to id 9 (RESET → start), Shift to id 2 (SELECT → variations), P to PAUSE. The two fire buttons are Z (left) / X (right); arrows are the d-pad.

Bundle layout

systems/js7800/
├── play.html            ← EmulatorJS wrapper (EJS_core: 'atari7800')
├── controls.html
├── games.json
└── roms/
    └── *.a78            ← 7800 cart images (reused unchanged)

The core itself lives in the shared framework at systems/_shared-ejs/ejs/data/cores/prosystem-legacy-wasm.data; the old js7800/js/ engine (JS7800 + bupboop) is gone. Save / load state work through the GenX save/load buttons like the other EmulatorJS bundles.

Related

• Emulator-Stella — sibling Atari console, same Javatari→EmulatorJS migration story
• Emulator-VICE-family — the shared EmulatorJS gotchas
• Emulators — index