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Mezzano, an operating system written in Common Lisp.


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Pre-built images

Demo releases are available through GitHub.

These releases are designed to be run in VirtualBox, though QEMU is also supported. 2GB of RAM, a virtio-net NIC and an Intel HDA audio controller are recommended.

Building from source

See the MBuild repo: (

For help & support or to follow development, join the #mezzano IRC channel on Libera Chat (

Major changes since Demo 4

  • USB stack by fittestbits
  • Improved overall file system support by fittestbits
  • EXT2/3/4 support has been implemented by Bruno Cichon (ebrasca)
  • GMA950 modesetting display driver
  • Hardware accelerated 3D support via qemu's Virgl device
  • Multicore/SMP support
  • Improved atomic operations
  • Async APIs: wait-for-objects, dispatch, and thread pools
  • Networking improvements: Server support, DHCP, TCP retransmit
  • Source locations are tracked for many kinds of definitions
  • Weak hash tables and other weak objects
  • Cleanup of object representation and unifcation of standard-object/structure-object
  • Unboxed structure slots
  • Short floats implemented using IEEE half floats
  • Unboxed (unsigned-byte 64) arithmetic
  • Stack overflows and memory faults are trapped and can be recovered from
  • Support for building on Windows
  • Major improvements to CLOS and MOP conformance
  • Keymap picker
  • More bug fixes, performance improvements and features

Major changes since Demo 3

  • FAT32 support has been implemented by Bruno Cichon (ebrasca).
  • McCLIM has been ported by fittestbits.
  • Quicklisp has been ported by Peter S. Housel.
  • Improved introspection tools: DISASSEMBLE and ED have been implemented.
  • Generational collection has been added to the garbage collector.
  • New SSA-based compiler backend, supporting unboxed value representations.
  • Gray streams support has been overhauled.

Major changes since Demo 2

  • Trentino, a media player, has been implemented by Eugene Zaikonnikov.
  • Further improvements to conformance, stability and performance.
  • The CLOS implementation follows the MOP much more closely.
  • More traditional window management.
  • Booting from CD/USB on real hardware is now possible.
  • Driver support for Intel HDA audio devices.
  • VirtualBox guest (mouse & display) integration.

Major changes since Demo 1

  • Many improvements to conformance, stability and performance.
  • The editor has been greatly improved, thanks to Burton Samograd.
  • The system now functions correctly on computers with more than 1GB of RAM.
  • The allocator and garbage collector now make much better use of available memory, with far fewer GC cycles occuring.
  • (ROOM T) prints more detailed information about allocated objects.
  • Transparency and premultiplied alpha support in the GUI.
  • And more!

Additional information

"Hypothymis azurea - Kaeng Krachan" by JJ Harrison ( [CC BY-SA 3.0 (], via Wikimedia Commons

"Mandarin Pair" by © Francis C. Franklin / CC-BY-SA-3.0. Licensed under CC BY-SA 3.0 via Wikimedia Commons -

"Handsome" by Andy Morffew - [CC BY 2.0 (]

Includes Dejavu Fonts 2.37 (

Some icons from Icojam (