Emulator JtyOne

JtyOne — Sinclair ZX81

JtyOne (Simon Holdsworth) is a port of Mike Wynne's EightyOne. The ZX81 bundle is small and quiet — the smallest screen, the smallest emulator, the smallest games.

Where we started

JtyOne is GPL-2.0. Pure JavaScript, no WASM, no build step. The runtime exports a global object and writes to an emulator canvas + status span.

The launch flow

play.html reads ?game=<key> from the URL, fetches games.json, gets a {rom, type} record per entry, and hands it off to JtyOne's loader. rom is the file path; type distinguishes .p files (snapshot, loads instantly) from .tap files (real tape image, real load time).

The display

ZX81's native screen is 256×192 monochrome (one screen = 1 KB of character memory). JtyOne renders to a canvas that we constrain via standard genx-frame CSS:

#emulator_canvas {
  box-shadow: 2px 2px 10px rgba(0, 0, 0, .7);
  image-rendering: pixelated;
}

Below the canvas, the upstream code expects an #emulator_status span for tape position / loader messages. We kept it (small, low-vis text) for tape-loading feedback.

Joystick / keyboard input

ZX81 was keyboard-only — no joystick port on the original hardware. JtyOne maps the joystick directions to the period-correct ZX81 keys (Q/A/O/P for the four directions, on Sinclair games that use them) but most of our entries respond to direct key input from qaop + Enter for fire.

File formats

• .p — ZX81 snapshot. Instant load.
• .p81 — alternative snapshot format with a filename header.
• .tap — tape image. Real-time loading; the screen flickers for ~30 seconds while LOAD "" runs.

We default to .p where available.

Bundle layout

systems/jtyone/
├── play.html
├── zx81_emu.js          ← JtyOne runtime
├── controls.html
├── games.json
├── roms/                ← system ROM
└── tapes/
    └── *.p / *.tap

USB gamepad support + key filtering (04-05/07/2026)

The seven action games have per-game window.GenXGamepadMaps on the shared shim (the three typed-command games are keyboard by design). JtyOne listens on document and decodes e.key single chars with e.keyCode fallbacks, so the shim's defaults work unmodified. The pad-map block must run before the loader script, which rewrites the URL for the engine — and the rewrite now keeps ?game= alongside ?tzx= (deleting it raced the deferred shared controls-link script and could cost the gamedoc link).

Because every ZX81 key is live BASIC (Space is BREAK), each action game also gets a key allowlist: a capture-phase listener swallows any key with no in-game function before the engine sees it. Games that define a quit key (3D Monster Maze's A) close the game tab via window.close() — prompt-launched windows are script-opened, so this is allowed; hand-typed URLs fall through to the tape's patched behaviour.

3D Monster Maze tape surgery (05/07/2026)

Working from a full disassembly, the intro's inverse-video keyword labels (STOP / LIST / CONT — keycap legends a PC keyboard doesn't have) were patched byte-for-byte in tapes/monstmze.p.hex to the real keys "A" / "K" / "C", and the program's two NEW tokens (0xE6) became RUN (0xF7), so no keypress can strand the player at a dead BASIC prompt. The .p.hex format is plain hex of the ZX81 memory image from 0x4009; display text uses the ZX81 charset (letters 0x26-0x3F, +0x80 for inverse), BASIC lines are big-endian line number + little-endian length + tokens. In-game keys (5 turn left, 7 forward, 8 turn right) were confirmed from the disassembly's game loop.

Related

• Emulator-JSSpeccy — sibling Sinclair machine
• Emulators — index