Simulate 6502-based microcomputer systems in Python.
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CHANGES.txt Fixed two monitor tests that were broken under Windows. Sep 12, 2010
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Py65 provides tools for simulating hardware based on 6502-like microprocessors. It has the following goals:

  • Focus on ease of use and modularity rather than performance. Py65 is written in the Python programming language for productivity, while similar programs are written in C for performance.

  • Enable simulations to be created for systems where it might have otherwise not been practical, such as homebuilt computers.

  • Rigorously unit test all of the components. While the tools provided by Py65 may not always be perfect, their behavior is verified through tests so unexpected results are minimized.


Py65 packages are available on the Python Package Index (PyPI). You download them from there or you can use easy_install to automatically install or upgrade Py65:

$ easy_install -U py65

Alternatively, you can download a package from GitHub in .tar.gz or .zip format. After extracting the package, use the following command to install Py65:

$ python install


The following devices are simulated at this time:

  • mpu6502 simulates the original NMOS 6502 microprocessor from MOS Technology, later known as Commodore Semiconductor Group (CSG). At this time, all of the documented opcodes are supported. Support for the illegal opcodes is planned for the future.

  • mpu65c02 simulates a generic CMOS 65C02 microprocessor. There were several 65C02 versions from various manufacturers, some with more opcodes than others. This simulation is based on the W65C02S from the Western Design Center (WDC).


Py65 includes a console-based machine language monitor (sometimes also called a debugger). This program, py65mon, allows you to interact with the simulations that you build. Its features include:

  • Commands that are largely compatible with those used in the monitor of the popular VICE emulator for Commodore computers.

  • Ability to load, dump, and fill memory.

  • Simple assemble and disassemble capability, including support for labels and labels with offsets.


Py65 documentation is written using Sphinx and is periodically published to


These people are responsible for Py65:

  • Mike Naberezny is the original author of Py65 and is the primary maintainer.

  • Oscar Lindberg started the 65C02 simulation module and contributed greatly to its implementation.

  • Ed Spittles helped with testing and provided many useful issue reports and patches.