>Unfortunately, no PIC (not even the 17Cxx) can at present handle the
>general case of cooperative multi-tasking, where either/any process
>may
>execute a yield() from within nested procedures.  The normal method
>for
>handling such task switching is to split the stack area into two or
>more
>pieces; on the 8x51, a task switch (assuming two tasks) would then be
>performed as:
>
>Yield:
>        mov     a,other_sp
>        xch     a,sp
>        mov     other_sp,a
>        ret
>
>
>Calling Yield() will place the address following the call onto the
>stack,
>save the stack pointer and point it to the other stack, and then work
>on
>the other task.  When that task calls Yield() the stack pointer will
>be
>restored with its old value, so RETurning will resume execution of the
>former task.  Simple and very efficient.