OS/816 - Multitasking and System Calls
- smokku
During past weeks I’ve made solid progress on OS/816. The multitasking scheduler and I/O is up and running.
First was a simple test - one task increments its A register, the other decrements its own version of A register. Basic, but it proves context switching works.
Also got the COP-based syscall framework implemented. This part took some effort to get right - making sure system calls don’t interfere with task switching. Now, if a task switch happens during a syscall, the kernel finishes up the syscall work but resumes execution with the next scheduled task instead of the original caller. Works like a proper modern OS should.
For testing this, I got two tasks running:
- A basic counter task to validate context switching.
- A UART echo task using
read() and write() syscalls - reads input into a buffer, then writes it back.
The syscall mechanism itself is a joy to work with. Just push the arguments to the stack, load the API index into A, COP $21, and that’s it. Result comes back in A, and all other registers stay untouched. No need to worry about JSR addresses or register clobbering.
And for the sharp-eyed folks: Yes, $21 was chosen on purpose. 😉
Also started working on the CLI shell. So far, it has help and echo, and adding new commands is really simple.
This was a big step - things are starting to really come together.