Des Bromilow writes:
>My thesis was based on writing a virtual instruments package under win =
>3.x/95.
>I could get the 55ms interupt operable, but anything faster than that is =
>impossible under the normal PC architecture.

        This pretty well goes along with what I have found in DOS.

        You can try an experiment to see just how busy things are.
Get a P.C. hardware manual and look at the section on the
timer/counter which makes tones for the speaker.  There are two
control bits, one to turn the path to the speaker on and off and
another to enable the square wave output.  In this case, that bit
should be off so that the speaker pops once when the path is turned on
and once when it is turned off.  If you write a timing loop to cycle
the speaker 500 times per second in a square wave, it should sound
just like the square wave one gets from the normal counter.  On a
P.C., it sounds like a sick buzzer which is about to die.  This is due
to all the little lapses in timing which elongate a cycle here and a
cycle there.  We aren't trying to measure time, but generating timing
and it is obvious that in an interrupt-driven world, it is hard to do
either accurately.  The buzz loop simply accentuates how rough it can
be.

        You can have interrupts and accurate timing, but the
interrupts, themselves, are where one needs to take measurements and
let the foreground activities happen asynchronously.

        I once wrote an experimental routine to pop the speaker every
time the timer ticked and it gave the P.C. a nice little fast ticking
sound like a 16-MM movie projector.  The 55-ms timer tick has one of
the highest priorities on a P.C. so you can literally count on it.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group