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FuzixOS: Even tiny computers deserve Unix
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Latest commit f5be6be Jan 22, 2017

FuzixOS: Because Small Is Beautiful

This is the initial public tree for the FuzixOS project. It is not yet useful although you can build and boot it and run test application code. A lot of work is needed on the utilities and libraries.


FUZIX is a fusion of various elements from the assorted UZI forks and branches beaten together into some kind of semi-coherent platform and then extended from V7 to somewhere in the SYS3 to SYS5.x world with bits of POSIX thrown in for good measure. Various learnings and tricks from ELKS and from OMU also got blended in

What does FUZIX have over UZI

  • Support for multiple processes in banked memory (as per UZI180) but with Minix style chmem and efficient use of bank allocations.
  • Support for multiple processes via hard disk or non mappable RAM drive switching (as per UZI, UZIX).
  • The ability to run single tasking on small devices, for bring up and for standalone tool execution
  • Support for "real" swapping combined with banked memory.
  • Proper sane off_t and lseek
  • Normal dev_t
  • 30 character filenames
  • Proper sane time_t
  • System 5 signals (half baked)
  • Posix termios (does all the original UZI tty did but much can be added)
  • Blocking on carrier for terminals
  • Optimisations to avoid bogus uarea copying compared to UZI180
  • More modern system call API: 3 argument open, mkdir, rmdir, rename, chroot (with correct .. semantics), fchdir, fchmod, fchown, fstat, fcntl, setpgrp, sighold and friends, waitpid, setpgrp, nice O_NDELAY, O_CLOEXEC, F_SETFL, F_DUPFD etc
  • Address validation checks on all syscall copies
  • Builds with a modern ANSI C compiler (SDCC)
  • Kernel boots to userspace on both 6502 (bitrotted), 68000, 6809, MSP430 and Z80/Z180
  • Core code can be built for 6502, 6809, 68000, 8086, MSP430, pdp11 and Z80/Z180 so should be far more portable
  • Core architecture designed to support building and maintaining multiple target machines without forking each one
  • Helpers to make many bits of implementation wrappers to core code
  • Lots more bugs right now

What does UZI have over FUZIX

  • Can run in 64K of RAM (32K kernel/32K user). FUZIX would need banked ROM or similar to pull this off. If you have banked ROM then our kernel footprint in RAM is about 8K plus userspace plus any framebuffers and similar overhead. On a 6809 it's just about possible to run in a straight 64K

What do the UZI branches have that FUZIX has not yet integrated

  • Minimal TCP/IP (UZIX 2.0). Unfortunately the original TCP was never released openly.
  • Symbolic links (UZIX)
  • Various clever fusions of syscalls that may save a few bytes (UZIX)
  • setprio (UZIX)
  • Rather crude loadable drivers (UZIX)
  • Use of __naked and __asm for Z80 specific bits to avoid more .S files than are needed (UMZIX)

Plus OMU has a really clever function passing trick for open/creat and friends, while UMZIX has a neat unified "make anything" function.

What Key Features Are Missing Still

  • ptrace, ulimit
  • root reserved disk blocks
  • banked executables
  • TCP/IP (in progress)
  • select/poll() (in progress)
  • Support for > 32MB filesystems (but first figure out how to fsck a giant fs on a slow 8bit micro!)
  • Smarter scheduler
  • Optimisations for disk block/inode allocator (2.11BSD)

Tool Issues

  • No useful 8086 compiler option (started work on pcc 8086 but help needed, coherent may provide a suitable cc but that also needs work)
  • 6809 gcc and cc65 don't have long long 64bit (for sane time_t)
  • SDCC can generate ROMmable binaries but not banked ones (hack fixes done)
  • SDCC has no register passing function call support, and for some stuff it really shows
  • SDCC generates quite bloaty small applications. Needs research on how to improve.
  • None of the above have an O88 style common sequence compressor


  • Amstrad NC100/NC200 - real hardware sanity check
  • Amstrad PCW8256 - fork and memory management needs some fixing up
  • Atari 520ST - 68000 core code build test only at this point
  • Dragon64 - in progress to see how small we can go
  • Dragon Nx32 - Dragon with Tormod's memory expansion card (SPInx)
  • Epson PX4/4Plus - WIP port to a very early Z80 laptop
  • Memotech MTX512 - boots to userspace in emulation, should run on real hardware
  • MSP430FR5969 - decent functionality; see Documentation/
  • MSX - basic functionality
  • Multicomp09 - decent functionality. This is an FPGA 6809 system extended from the design by Grant Searle.
  • N8VEM-MarkIV - Supports the on-board RTC, RS232, RS422, IDE and SD interfaces, on the ECB expansion bus only the PropIO V2 serial port is supported at this time.
  • P112 - Supports the floppy disk controller, ESCC serial ports, and optional G-IDE interface.
  • SocZ80 - 128MHz extreme Z80 FPGA machine. Boots to shell, drivers need work
  • Tandy COCO3 - boots to userspace and supports drivewire
  • TGL6502 - Test 6502ish environment
  • TRS80 - boots to userspace in emulation, swapping, floppy and hard disc done
  • V68 - 68K emulation/debug platform. Being used for bring up
  • Z80Pack - used as a dev and test environment for both large swapping multiprocess and for small single tasking
  • Zeta v2 - initial port running to user space
  • ZX Spectrum 128K - can boot to shell but needs swap debugging to get further

Various other platforms are partly filled out to sanity check assumptions and start making progress on them. The main need there is now to tackle all the billion different ways of interfacing the floppy controllers.

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