This is a work-in-progress operation system for Buri, my homebrew 6502 microcomputer system. The hardware design can be found at https://github.com/rjw57/buri-6502-hardware.
The OS is in the form of an 8K ROM image. Compilation of this software requires
cc65. Assuming that cc65 is installed under
$HOME/opt/cc65, you can clone the source and make the image:
$ git clone https://github.com/rjw57/buri-6502-os
$ cd buri-6502-os
$ CC65_DIR=$HOME/opt/cc65 makeSyscalls are made via the BRK software-interrupt instruction. When a BRK is
issued, the OS uses the value of the X register to index an OS call. Return
values, if any, are stored in the A register.
The default throughout is for the M and X flags to be set (8-bit accumulator and index) which allows for more compact immediate addressing instructions. Code which enables a 16-bit register should take care to reset it before calling other public code. Generally, "public" means code in a separate source file. Functions should preserve the X and Y registers but are free to corrupt the A register (or use it as a return value).