MOSnix
This repository contains an experiment in writing a POSIX-ish operating system for 6502 microprocessors in C using llvm-mos.
Why?
This project started with some observations. The C programming language was invented to implement Unix and we now have a decent C compiler for 6502, llvm-mos. So, can we implement an operating system in C using llvm-mos, just like the original Unix? How much of POSIX could we do?
Note that this isn't a port of someone else's fully fledged Unix. And obviously it will be nowhere near as capable as Linux and other modern OS designs in the Unix family. There will of course be shortcomings due to limited memory and no MMU.
There have been many attempts over the decades to create a Unix-like OS for 6502. But due to the difficulty of high-level programming on the 6502, they have all fallen back on writing the OS and user space applications in assembly. This limits the power of the resulting system and can be difficult to maintain.
This experiment is about finding out if a useful operating system can be built in mostly C using llvm-mos, using a small amount of assembly code for context switching and hardware interfacing. The experiment may fail, but I think it is worth doing.
What state is it in?
Very early days!
- System call dispatching works.
- No more than 6 user space processes, 5 excluding the shell.
- A single user space process for the shell with very basic commands.
- Pre-emption has not been fully implemented yet, but some support is in place.
- RAM filesystem for the root directory skeleton.
- Support for FAT32 filesystems on SD cards for the main storage,
mounted at
/mnt/sd. - The FAT32 filesystem is currently read-only.
- Fork/exec of non-shell processes doesn't work yet - soon.
- Reading from stdin and writing to stdout/stderr basically works.
- System call numbering is not set in stone, will probably change.
The target is to get a useful POSIX-like operating system in less than 32K of ROM and 16K of RAM that can run on Ben Eater's Breadboard 6502 Computer. Other platforms may follow later if the experiment is successful.
Building
You will need the following to build MOSnix:
- LLVM-MOS SDK,
with the
bindirectory somewhere on yourPATH. elf2o65utility from o65utils for building relocatable user space application binaries. This must also be somewhere on yourPATH.
Then configure and build the code as follows:
mkdir build
cd build
cmake ..
make
Two kernels are built by the default configuration, for the mos-sim
emulator, and the Ben Eater Breadboard 6502 computer. After building,
you can run the kernel in the emulator with one of the following:
make boot
mos-sim build/os/mosnix-sim
To run it on a breadboard computer, you will need some modifications for the serial port and SD card. More information coming on this soon.
Contact
For more information on this code, to report bugs, or to suggest improvements, please contact the author Rhys Weatherley via email.