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Re: Flight Computers



Dennis,

>The SHARC had so much horsepower that our analysis of the processor
>bandwidth loading is that with all of the above running we used about
>15% of the processor bandwidth.  Most of the time was spent waiting
>for the next task switch.

Most interesting. What kind of response deadlines did you have, and
did you ever have any problems with excessive interrupt latencies?

As far as I can tell, the distinguishing feature of a "real-time" OS
is that it bounds the interrupt latencies associated with protecting
critical sections of code in the kernel. But as you say, just using a
faster CPU with a "conventional" OS may work just as well.

>This is a reason that I am a very strong advocate of doing whatever it
>takes in radiation shielding to protect the processors.  This brings
>so much greater reliability to the system than trying to use the last
>ounce of power from an ancient processor.

Agreed. Problem is, the energy distribution of charged particles in
the Van Allen belts has a very long tail. It's easy to stop nearly all
of the particles with relatively little shielding, but those really
high energy particles are almost impossible to stop. So you need
rad-hard parts and error-correcting memory. Or you can stay in LEO,
where the radiation environment is a lot more benign.

Phil


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