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From: Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: DPMI Interrupt latency ?
Date: Tue, 29 Jan 2002 21:26:03 CST
Organization: Rice University, Houston TX
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> Tried the way you suggested - but it doesn't seem to work if I try
> invoking the hardware interrupt through a software (INT XX) call. I
> hooked INT0D (IRQ5) from protected mode. But on reaching the shell, The
> interrupt vector of Int 0d was pointing to the initial Real mode value
> in ROM. 

Right, it reprograms the pic.  Sorry, forgott to warn you about that.

> I then tried calling Int8D (where the master PIC vector seemed
> to be redirected to), but still my interrupt handler didn't seem to get
> called. Does the code check if the interrupt is a genuine hardware IRQ
> and skip calling the handler if it isn't ?

It shouldn't.  You can look at the sources.

> Anyways, managed to do the tests by hooking IRQ8 (timer interrupt) and
> writing a small Real mode routine which hangs on in Real mode till a
> fixed number of Timer ticks ahve passed. On return from the Real mode
> call, the delays were averaged. The results were as follows (for 100
> samples, values are rounded to 50 clocks - CPU is P3 - 933 MHz)
> 
> With CWSDPMI  : Maximum 6500 clocks, minimum 3200, Average 3500 clocks
> 
> With PMODE/DJ : Maximum 3100 clocks, minimum 1200, Average 1250 clocks

Very interesting.  I'm surprised there is that much variability (nested
interrupts?)  The averages are pretty close to what I had tested or
had reported to me in the past (around 2-4K for CWSD, 1-2K for PMDJ),
I had expected a little over 2X speed difference.  The Min/average
show the maximums are fairly rare - in pmodes case about 2/100 or less?

> I assume the reason for the maximum numbers being much larger than the
> average could be because of instruction cache misses ??

A few cache misses wouldn't add up that badly.  It's something more
terrible...

> Hope this information is useful to someone.

Very much so.  Hard facts make good references.  I don't suppose you
could re-run the test for CWSDPR0? :-)  For completeness/documentation.

> The PMODE/DJ numbers seem to be more or less acceptable for my
> application, and so not investigating the "Unreal" mode approach
> immediately. May be later when I have some spare time.. 
> 
> If anyone is interested in the source code of the test program used to
> do the above tests, pl. let  me know, I'll be glad to post it.

Plese send, I'd like for my test suite.

> Thanks for all the help.

Certainly.  Thanks for the test results.





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