Emulator JSSpeccy

JSSpeccy — ZX Spectrum

JSSpeccy 3.2 (Matt Westcott / gasman) is the ZX Spectrum bundle. The integration was small — JSSpeccy ships a clean JS API with a documented options object, so the wrapper is mostly just calling it correctly.

Where we started

JSSpeccy 3.2 is GPL-3.0. The version 3 series is a from-scratch WebAssembly rewrite of an earlier pure-JS emulator. The runtime takes a config object ({zoom, sandbox, uiEnabled, machine}) and a JSSpeccy() constructor that wires up to a target div.

The launch flow

play.html reads ?game=<key> from the URL, looks it up in games.json, and constructs JSSpeccy with:

const opts = { zoom: 2, sandbox: false, uiEnabled: false, machine: 48 };
const speccy = JSSpeccy(document.getElementById('jsspeccy'), opts);
speccy.openTAPFile(game.rom);

Three of the four options matter:

• uiEnabled: false — hides JSSpeccy's own toolbar (file picker, machine picker, joystick selector). We're a launcher, not a sandbox.
• sandbox: false — allows the emulator to make real network requests (loading the tape file from the server). With sandbox: true it would only accept tapes uploaded via the file picker.
• machine: 48 — boots the Spectrum 48K by default. JSSpeccy supports 48 / 128 / +2 / +2A / +3; if a future entry needs a specific model, the games.json record can carry the field and play.html reads it.

Layout

JSSpeccy renders to its own canvas inside the #jsspeccy div. CSS pins the canvas with a shadow and pixelated rendering:

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

File formats

JSSpeccy handles .tap, .tzx, .sna, .z80. Most of our entries are .tap (raw tape image) since that's what the contemporary preservation archives serve. .tzx is preferred where copy protection is involved — the format preserves loader timing — but JSSpeccy abstracts the difference, so the wrapper code doesn't care.

Bundle layout

systems/jsspeccy/
├── play.html
├── jsspeccy/            ← upstream JS + WASM
├── controls.html
├── games.json
└── games/
    └── *.tap / *.tzx

USB gamepad support (04/07/2026)

Per-game window.GenXGamepadMaps in play.html drive the shared systems/_shared/genx-gamepad-keys.js shim (pad 0 polled per frame, mapped keys synthesized on edges). Two engine quirks cost real debugging time:

• JSSpeccy re-binds its key listeners. At machine start it calls setKeyboardEventRoot() onto the focusable app-container div it injects into #jsspeccy — synthetic events dispatched at document never reach a descendant's listeners, so the shim's target is a lazy function returning document.querySelector('#jsspeccy > div') (the div doesn't exist until the engine boots). This is also why the real keyboard only responds after a first click: focus.
• No public audio handle. JSSpeccy creates its AudioContext internally with no resume-on-gesture, so direct-URL opens (no window.open activation) stayed silent. play.html patches the AudioContext constructor before the engine loads, captures every context, and resumes suspended ones on load and on real user gestures.

Key decoding is Tr[e.keyCode] with an e.key string fallback (the fallback resolves ,/. as Symbol Shift combos). The engine emulates no joystick hardware — no Kempston; PC arrows land on CAPS SHIFT+5/6/7/8, i.e. the cursor-joystick convention.

All ten games were pad-verified in play (04/07/2026), which doubled as the bundle's first hands-on key audit: Chuckie Egg's documented ,/./A/Z were another platform's keys (real defaults 9/0/2/W, jump Z or M — Space does nothing), and Pssst / Sabre Wulf had up/down swapped relative to each other's docs. Jet Set Willy and Underwurlde also had their snapshots replaced (the old JSW stopped at a code-entry screen and reset; the old Underwurlde booted into a lives-cheat offer).

Related

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