Big problem with most RTOS'es is memory requirements. Also they are usually so big (in code) that you will spend more time finding out how they work than designing your system.

I've only seen two that seem good for 8051 type chips. One (SALVO) costs money so I won't talk about it.

The other is called SCREEM 51 and you can find it on Google. It's free.

It is very simple RTOS with round robin scheduling, up to 3 tasks at once. Needs 66 bytes of internal memory as a minimum - you MUST look at each task you write and detemine what the real memory requirement is - be careful about stack usage. Inline functions may help.

The code is so small that you can easily hack it do more or less ... nice. And in doing that you will have a good idea what an RTOS really is (if you didn't before).

Daniel