Re:
>This is exactly the kind of code where you don't want a compiler between you
>and the machine.  It may be fine to write the application in C on the RTOS,
>but I certainly wouldn't want to see critical parts of the RTOS written in
>anything but assembler.  When you are writing an RTOS, you have to assume
>that the task switching code is both speed and space critical.  You also
>have to perform some fancy stack diddling which would confuse most
>compilers.  Since you want a pre-emptive RTOS, a lot of the task switching
>code will be in the interrupt routine.  This is even more reason to write it
>in assembler.

As a data point, Salvo on a PIC16C77 doing fully priority-and
event-based cooperative multitasking context-switches at around 2500
context switches/sec with a 4MHz crystal / 1us instruction cycle.
That's written entirely in C. I suspect the performance would improve
to about 3000/sec if it were written in assembler. This is Salvo's
Demo #1, available in hex form for programming into a PIC16C77 or
equivalent in the freeware download from our website.


>Are you really sure you need a pre-emptive RTOS?  Is it really that hard to
>break your tasks into "small" chunks of no-wait code?  Pre-emptive can look
>like the more elegant solution, but adds more overhead and trouble than you
>may realize.  The big gotcha is that any thread has to be prepared to be
>swapped out at any time.  This means you either need to disable task
>swithing during critical parts or add interlocks around global state.
>
>I've done a lot of embedded system designs (It's what I do for a living).
>While I have used multiple tasks or pseudo-tasks (can't do real tasks on
>PICs other than 18 family, for example) many times, I have never yet needed
>to do pre-emptive task swithing on a small resource-limited system.


Yes, it's pretty astonishing what you can do in a cooperative system.
For highest performance, you do need to perhaps tailor your tasks to
ensure minimal latency for higher-priority tasks, but for many
PICmicro apps we're seeing, it's not an issue.

Check out our website for more info.
--

  ______________________________________
Andrew E. Kalman, Ph.D.


Salvo(TM), The RTOS that runs in tiny places(TM)
Pumpkin, Inc.
750 Naples Street
San Francisco, CA 94112
tel: (415) 584-6360
fax: (415) 585-7948
web: http://www.pumpkininc.com
email: aek@pumpkininc.com

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