I've been looking at Barry Kauler's SCREEM RTOS a lot recently. I've built it with Franklin C and run it on their sim, it seems pretty good. It's extremely small, and the source is only a few pages.

By default it does round robin scheduling on up to 3 tasks (their can be as many tasks as you want in your source, but only 3 can run at once). But thos tasks can be created or killed from anywhere in your code with OS macros.

It looks like it will not be too hard to change the scheduling rules. I want a serial RX and serial TX task to be fired from serial ints, and another task to be scheduled periodically from the timer int. It should also be possible to make some tasks have priority by using flags.

The beauty of this is that it does a true context switch. You would not be depending on tasks always being complete within x microseconds to avoid system death.

The penalty is ram usage. It maintains a seperate stack per task. I will need to optimise each task to get stack usage down as much as possible.

Has anyone else tried this OS?

If I get any solid results maybe I'll put up a web page and post it here.