On Mon, 2 Jun 1997 01:15:17 +0000 John Payson <supercat@MCS.COM> writes:

>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.


        I did this years ago in Turbo Pascal (though the task switch code
was written in assembly).  Worked out quite nicely.  I called my
procedure NextTask, and it did a stack switch.  Besides holding return
addresses, the stack also held local variables for the task.  I generally
called NextTask while in a loop waiting for I/O.  I also called NextTask
in computation intensive tasks that just took a long time.  I'd just
throw a NextTask in there now and then so it wouldn't hog the system.  I
think we had five or six tasks running.


Harold