ZX84

A browser emulator for the Sinclair ZX80, ZX81 and ZX Spectrum, Amstrad CPC, Tatung Einstein, MSX, and Memotech MTX, with an MCP server for automated testing.

https://zx84.envytech.workers.dev

ZX84 is an old-computer emulator with machine-specific hardware models, browser-based media management, inspection tools, and a configurable CRT presentation layer.

Supported Machines

Related MCP server: gdb_mcp

Features

Hardware And Peripherals

• Spectrum ULA timing, floating bus, contention, 128K paging, AY sound, and +3 floppy support.

• Spectrum peripherals: Multiface 1/128/3, Interface 2 cartridges, VTX-5000 Viewdata modem, ZX Interface 1 with up to eight Microdrives, MGT +D, and Beta Disk/TR-DOS.

• CPC cassette support, disk-capable 664/6128 models, Multiface Two, and optional ParaDOS ROM.

• Einstein disk mounting and optional Xtal DOS boot-disk behavior.

• HX-10 cartridge loading and BIOS-level .cas cassette loading.

• MTX CP/M 2.2 boot through the real FDX hardware using a hosted Type 07 system disk.

• MTX logical cassette loading, banked ROM packs, FDX/SDX Type 03 and Type 07 floppy mounting, optional 512 KiB RAM expansion, and optional 80×24 colour display.

• The bundled MTX CP/M profile installs the native SIDISC module and exposes fitted expansion RAM as an empty 512 KiB type-51 drive F:.

Spectrum ROM-overlay peripherals are model-dependent. Interface 1, MGT +D, and Beta Disk are mutually exclusive; Beta Disk takes precedence when enabled.

Media

Load by picker or drag-and-drop. ZIP archives are unpacked and routed to compatible machines where supported.

The tape deck provides block navigation, transport controls, fast ROM loading, turbo loading, loading sound where applicable, and original-media download. The disk UI supports drive selection, write protection, disk sounds, changed-image saving, blank image creation, and flippy disks.

Media can also be mounted at startup from HTTP(S) URLs. URL-encode each value; disk units are zero-based:

?snap=https%3A%2F%2Fexample.com%2Fstate.sna
&disk0=https%3A%2F%2Fexample.com%2Fsystem.dsk
&disk1=https%3A%2F%2Fexample.com%2Fdata.dsk
&tape=https%3A%2F%2Fexample.com%2Fgame.tap

Snapshots are applied first, then disks in unit order, then tape. Relative URLs are supported. Cross-origin hosts must permit browser CORS access, and the URL path or Content-Disposition response header must supply a recognised filename extension.

Display And Audio

• WebGL CRT renderer with a Canvas fallback.

• Integer scaling plus HQx and xBR upscalers.

• Scanline accuracy controls, Spectrum rainbow rendering, selectable palettes, border cropping, and CPC pixel-aspect correction.

• CRT controls for brightness, contrast, saturation, gamma, scanlines, softness, noise, dot pitch, curvature, masks, and monitor presets.

• Shadow-mask, aperture-grille, slot-mask, LCD-grid, and attribute-mask options.

• Web Audio output with AudioWorklet and SharedArrayBuffer paths where available, plus a ScriptProcessor fallback.

• Master volume, beeper-to-AY mix, AY stereo placement, DC blocking, and ultrasonic-tone filtering.

Input And Development Tools

• Keyboard mapping, configurable two-player joystick mappings, physical gamepads, and touch/mouse D-pads.

• Spectrum joystick interfaces: Kempston, Cursor, Sinclair 1, and Sinclair 2.

• Kempston and AMX mouse modes on supported machines.

• Pause, frame stepping, step into/over/out, breakpoints, run-to-cursor, disassembly, registers, memory views, and clipboard export.

• Spectrum and Einstein tracing: full execution, port I/O, and ZXTrace. Spectrum traces coalesce repeated loops.

• Spectrum-specific BASIC, BASIC variables, system variables, font, memory-bank, screen-text, and OCR tools.

• Customizable pane ordering, placement, visibility, collapse state, and persistent per-pane settings.

Saving, Library, And Persistence

• Export Spectrum .szx and .z80 snapshots, CPC snapshots, screenshots, and supported screen/RAM exports.

• Persist settings, pane layout, custom ROMs and fonts, and supported media in browser storage.

• Spectrum and ZX80/ZX81 software libraries with ZXDB-derived search, screenshots, automatic model selection, and cached catalog data.

MCP Server

The included stdio MCP server drives Spectrum models (48K, 128K, +2, +2A, and +3) for automated testing and reverse engineering. It supports execution control, memory and register inspection, disassembly, breakpoints and watchpoints, traps, keyboard input, port I/O, OCR, snapshots, tape/disk mounting, tracing, disk inspection, and Multiface/VTX-5000 control.

See mcp/README.md for setup, the complete tool reference, and workflows.

Getting Started

npm install
npm run dev

Open http://localhost:5174, choose a machine in the Hardware pane, then load compatible ROMs and media. The hosted version supplies the standard machine ROM sets; local builds can load replacement ROM images from the ROM pane.

Useful commands:

npm test              # full Vitest suite
npx tsc --noEmit      # type-check without output
npm run depcheck      # enforce architecture boundaries
npm run build         # production build
npm run mcp           # start the Spectrum MCP server

Current Scope

Hardware and media support varies by machine and model. In particular, the HX-10 has no floppy controller, and some Einstein subsystems remain incomplete. See docs/todo.md for planned formats, devices, and machine work.

License

MIT

Acknowledgments

Built with inspiration from the ZX Spectrum, CPC, Einstein, and MSX communities and their hardware documentation.