Re: Ada Microkernel?

[Ted Dennison<dennison@telepath.com>]

In article <9hi6fv$lop$1@nh.pace.co.uk>, Marin David Condic says...
>Suppose the rest of the world wants to timeslice. The timeslice thing is
>going to require some sort of setting of a clock/interrupt/timer that will
>stop your run-until-blocked process to give control to the scheduler. The
>secheduler may look and say "Ted's job is still the most important thing to
>do" and give you back the processor, but in the mean time, you've been
>interrupted and had control given to the OS for some duration. even if the
>duration is predictable, its still undsirable because it takes time from you

But that is already happening. After all, the scheduler has to wake up
periodicly to verify that no higher-priority task is ready to run.

The way vxWorks operates, there's a PC Real-time clock interrupt that can be
modified but by default goes off at 60hz on IRQ 0 (or was it 2?). The OS handles
that, and immediately invokes the scheduler to see if anything was sitting on a
"delay" (actually, a taskDelay) that has just expired. If so, the appropriate
scheduling data structures are modified, and a task switch is initiated if
needed. 

For schedulability analysis, I presume one would handle this either by adding
the appropriate multiple of the max scheduling delay (which the OS vendor would
hopefully provide), or by showing that the task in question completes in less
than one scheduling interval. Being able to provide that max scheduling delay
(and have it be a resonable number) is where I think most consumer OS's would
have trouble.

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