About
Bizmuth is an emulator of the ZX Spectrum Next, and of the 16K, 48K, 128K, +2A and +3 Spectrums, written in Rust for macOS, Windows and Linux. It is built first as a developer’s tool: a program written for the Next is run, stopped, inspected and tested in it, from the debugger window, a text protocol, scripts, or the DeZog extension for VS Code.
Accuracy
The Next’s FPGA design is what Bizmuth reproduces. The VHDL source of the Next’s core defines how the machine behaves, and every register, port and instruction is checked against it. Published descriptions have been wrong on real detail more than once, and other emulators are a second opinion, not a reference.
Where reading the design leaves a question, the design is run in simulation and the answer measured. The Z80N instruction timings and flags on Z80N Instructions were found that way, cycle by cycle. What only the real machine settles, such as how NextZXOS behaves on a board, is tested on a Next.
A behaviour of the hardware that current software does not happen to use is still a behaviour to reproduce: a program written next year may depend on it. A known difference from the hardware is recorded and fixed, not explained away. The ones not yet closed are stated on the pages they affect, such as Machine Timing.
CPU core
The Z80 and Z80N processor is hg80, a separately developed core under the MIT and Apache 2.0 licences, written without reference to any other emulator’s CPU code. Its behaviour is verified against a simulation of the Next’s own CPU design.
ROMs and cards
The ROMs of the classic Spectrums and the Next’s SD card image are not part of Bizmuth: they are copyright of their owners. First-run setup downloads or locates them; see Getting Started.
Credits and licences
Bizmuth is © 2026 TwistedRaven, at twistedraven.net.
It is built on open-source libraries, among them wxWidgets for the window, cpal for sound, gilrs for gamepads, Rhai for scripting and egui for the controller overlay. Help ▸ About Bizmuth ▸ Licences opens the licence of each, and the list ships beside the program as THIRD-PARTY-LICENSES.html.