A framework for emulating 8-bit processors, written in C++.

The currently supported processors are:

• Motorola MC6809
• MOS 6502 (NMOS version)

The primary repository for the code is https://github.com/raybellis/usim/.

The code requires a C++20 compatible compiler.

This code was first started in 1993 and was specifically for the Motorola 6809 processor. The initial released versions had some serious known bugs in the handling of some 6809 instructions and addressing modes. Many thanks are due to B. Armstrong and Soren Roug for finding and resolving those.

As of January 2021 support for interrupts was implemented, and the code was reorganised so that complete machines are generated by creating instances of a CPU, memory devices and peripherals and then "attaching" those to each other. See main.cpp for an example of how this is done.

(In earler versions it was necessary to create a subclass of the CPU object and overload the read and write functions to direct memory and I/O accesses accordingly).

While this framework was always intended to be extensible for other CPU types, this didn't happen until December 2025 when MOS 6502 support was added.