JNEXT — a ZX Spectrum Next emulator for developers

JNEXT is a real-time ZX Spectrum Next emulator, built for the people who write Next software.

A comfortable machine to develop on: a debugger that shows you everything the hardware is doing — every layer, every sprite, every register, and the ability to step backwards — plus a headless mode that puts your program under CI.

_{Click either image for full size.}

• A debugger that sees the whole machine. Z80/Z80N disassembly, memory, the 8-slot MMU, all 256 NextREGs, 128 sprites, the Copper, the tilemap, the layer compositor, the AY chips — plus backwards execution: step back, frame back, scrub a rewind slider.
• Every layer, on its own. See the ULA, Layer 2, sprites and the tilemap rendered separately, next to the composite the hardware would show you
• Headless and scriptable. Deterministic PNG screenshots (frame-accurate, per-layer, with a pinnable RTC), keypress injection, a T-state profiler, RZX and MP4 recording: enough to put a Next program under CI.
• It boots the real thing. NextZXOS cold-boots through the authentic chain (FPGA boot ROM → TBBLUE.FW → NextZXOS) off an SD-card image, like real hardware — all machine and peripheral ROMs come from the SD card, not from the emulator.
• The VHDL is the specification. Where the hardware's behaviour is ambiguous, it is resolved against the official FPGA core sources and pinned there by a unit test — so the emulator's answer is the silicon's answer.

• Machines — 48K, 128K, +3 and ZX Spectrum Next (Issue 2)
• CPU — Z80N, with IM1/IM2 interrupts
• Video — ULA (48K + Timex modes), Layer 2, 128 hardware sprites, tilemap, Copper, and the layer compositor
• Audio — 3 × AY-3-8910 (TurboSound), DAC, beeper
• Storage and peripherals — DivMMC, Multiface, DMA, UART, CTC, SPI / I²C / RTC, keyboard and USB gamepads, ESP01 Wifi subsystem
• Formats — NEX, SNA, SZX, Z80, TAP, TZX, WAV, RZX

Status: 1.0, not finished — fixes are still pending and many features are on the roadmap. It works well enough for its author, and hopefully for you; the 1.0 release is meant to bring more users and more bug reports, so please report what you find at GitHub issues. Extreme cycle-exactness is explicitly not a goal; "good enough to develop games on" is. Some things are still rough or missing; they are listed, not hidden:

• GitHub issues — known issues and pending features
• KNOWN-FUNCTIONALITY-GAPS-AND-PLAN.md — historical 2026-04 gap audit (frozen 2026-07-24; anything pending lives in GitHub issues)
• SUBSYSTEM-TESTS-STATUS.md — live per-subsystem unit-test dashboard: exactly what is verified against the VHDL

Linux, Windows (x64 and 32-bit x86), and macOS are supported.

Installing a package is the recommended way to get JNEXT. Download the one for your system from the latest release:

The Linux packages put a jnext command on your PATH.

Note (Flatpak only): the Flatpak build ships without LTO — its runtime's compiler miscompiles the GUI startup under LTO. This only reduces top turbo-speed throughput; normal (100 %) speed is unaffected. For maximum turbo performance use the native RPM/DEB packages, which keep LTO.

The Windows builds are self-contained, portable zips — no installer, with all the Qt and SDL runtime DLLs bundled. Unzip one anywhere and run jnext.exe; delete the folder to uninstall. The executable is deliberately not code-signed — an Authenticode certificate is a recurring annual cost — so on first launch Windows SmartScreen warns about an unrecognized publisher. Click More info → Run anyway; or clear the download's Mark of the Web first, by right-clicking the downloaded zip and ticking Properties → Unblock. Details, and what a signature would and would not prove: user guide § Windows.

All Windows packages are self-contained portable zips — no installer, no redistributables, every runtime DLL bundled, and the SD-card image self-provisioning built in. Pick by your Windows version:

Details and the audited compatibility floors: packaging/README.md and doc/design/WINDOWS-COMPAT-PLAN.md.

The macOS build is a .dmg; open it and drag jnext to Applications. The app is ad-hoc signed but deliberately not notarised — notarisation requires a paid Apple Developer Program membership — so Gatekeeper blocks the first launch. Try to open it once, then allow it under System Settings → Privacy & Security → Open Anyway; or clear the download flag with xattr -dr com.apple.quarantine /Applications/jnext.app. The old Control-click → Open bypass no longer covers this case on macOS 15 and later. Details, and how to check the bundle is intact: user guide § macOS.

Prefer to build it yourself? See BUILD.md — it also covers building the packages above (make package-rpm / package-deb / package-flatpak / package-win / package-macos).

# First launch boots NextZXOS; jnext offers to download an SD-card image
jnext

# Run a program — a bare filename is all it takes
jnext game.tap

Everything you can do with it — every CLI option, the GUI, the debugger, the SD card and ROMs — is in USAGE.md, or in man jnext once installed from a package: both are generated from the same source, so they never disagree.

The user guide is the place to start. You can access it online

It also ships rendered in this repository, so there is nothing to install and nothing to build — serve it and read it:

make read-userguide

That prints a http://localhost:8000/ URL; open it in your browser. (Or open doc/user-guide/index.html directly.) Chapters 1-5 cover running programs; 6 and 7 are for developers — every debugger panel and function, and how to automate JNEXT the way its own test suite does.

Contributions are welcome — shared effort is how software gets better, and every good fix or feature makes the program better for everyone who uses it.

Bug reports, feature requests and pull requests are all appreciated. Pull requests are reviewed strictly against a fixed protocol — a PR that does not comply is not merged. In short, every PR must:

• ship discriminative tests for its change (they fail without the change and pass with it);
• not modify existing tests (without owner approval);
• use only license-clean fixtures;
• match the project's code style;
• not add dependencies (without owner approval).

A bugfix PR also needs a full bug description (or a linked bug issue); a feature PR also needs an explicit use case and a design document under doc/.

Full rules: PULL-REQUEST-PROTOCOL.md.

JNEXT is being fully developed using Claude (Anthropic's AI), with human guidance and supervision. The complete prompt history, design documents, daily task files and development documentation are in the repository — which makes JNEXT not just an emulator, but a practical case study in building a large, complex piece of software with AI-assisted programming.

Copyright (C) 2026 Jorge Gonzalez Villalonga, aka ZXjogv

JNEXT is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

JNEXT is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the LICENSE file for the full license text.