setjmp/longjmp

[dcbrown]

You don't need any special features in the CPU to do it.

Ummm, actually you do. You really need to have access to the call (program address) stack. Many of the smaller implementations of the PIC architecture don't have program access to the call stack.

There are several commericially available RTOS packages available (PICOS, FreeRTOS, Salvo, Pumpkin, ...) for the PIC that support multi-tasks and some type of round-robin or priority task scheduling. The practical maximum for many of these implementations are 8-12 tasks. The context save area for the task needs to be about 100 bytes for call stack and hardware registers (like PROD, TAB*, FSR, ...) and data sections (.tmpdata, MATH_DATA). In addition, at least a small argument stack is required, so another chunk of memory - say a small 32 byte stack, plus local variable storage. This already puts a task memory structure up over 1/2 of a ram bank in size. This is easy to do, yes, but the costs in terms of chewing up the limited resources on-chip are fairly high.

[Bongle]

Originally Posted by dcbrown

Ummm, actually you do. You really need to have access to the call (program address) stack. Many of the smaller implementations of the PIC architecture don't have program access to the call stack.

Ah, my operating systems course fails me again . Who would've known that learning things on a poorly-simulated CPU rather than on hardware would have given me misconceptions?

Thanks for the info, makes me want to go back to my hardware classes again.

Edit: Just read the wikipedia article. Embedded CPUs are crazy.