Wouter van Ooijen wrote

> > Ok so the only real
> > problem is the ability
> > to do pre-emptively multitasking
>
> I consider that a serious drawback, and I would add 8 levels is a bit
> short for chips that have up to 8k of code.

Hi Wouter,

I hope you will agree that the greatest benefit in using co-operative
multitasking over pre-emptive multitasking is the ability to control where task
switching occurs. In large embedded systems it is often a real problem that a
task switch can occur anywhere and at any time. Often in such a system, the
tasks need to take serious measures to ensure that critical sections of code are
not interrupted or that access to resources are serialised. Co-operative
multitasking trashes these problems without even trying. I personally consider
the lose of pre-emptively multitasking on the PIC 16 series only a small
annoyance and not a serious drawback.

I also used to think that an 8 level stack was a pain, but after considerable
work with these PICs I have come to realise that it is more of an annoyance than
a real problem. Yes it would be nice to use "call" and "return" instructions
without worrying about the level of the stack, but I now see this in the same
way I see 16 bit arithmetic on the PIC - instead of one instruction to do a 16
bit add I must use a few, instead of one instruction to do a "call" I sometimes
need to use a few. If you use a macro to handle calls to other code pages, why
not use a macro to handle calls off the hardware stack. Treat the hardware stack
as a special resource, one that allows you to execute short sequences of
instructions very quickly with minimal overhead and leave big functions to be
called with your special "out-of-hardware-stack" macro. The only thing we really
need the hardware stack for on the PIC 16 series is interrupt handling and the
god awful table lookups.

>
> NB I totally agree with your static stack argument. Just take a look at
> the bulky code produced by C18 (which uses a real SP-offset local
> variables).
>
> Wouter van Ooijen

Regards
Sergio Masci

http://www.xcprod.com/titan/XCSB - optiming structured PIC BASIC compiler

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