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From: RayeR <glaux AT centrum DOT cz>
Newsgroups: comp.os.msdos.djgpp
Subject: bad delay() precision on intel C2D machine under pure DOS
Date: Wed, 2 Apr 2008 09:00:06 -0700 (PDT)
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

In my CPUID utility I measure CPU frequency via TSC using delay as a
time base, code looks like this

    tsc=rdtsc();                       // precti time stamp counter z
CPU
    delay(500);                        // pockej chvilku
    tsc=rdtsc()-tsc;                   // precti time stamp counter z
CPU a spocti rozdil

But I found on faster machine intel C2D it takes longer time than
should. Same for pure DOS and Win9x dos box. Then I found it helps
when I split one delay to two or more delay calls:

    tsc=rdtsc();                       // precti time stamp counter z
CPU
    delay(250);                        // pockej chvilku (kvuli nejake
chybe v DJGPP je nutno rozlozit
    delay(250);                        // pockej chvilku
    tsc=rdtsc()-tsc;                   // precti time stamp counter z
CPU a spocti rozdil

Then it seems to be OK but why? I also tried my udelay() function
based on uclock() which has perfect
precision in pure DOS but it's shorter in win9x. I use DJGPP 2.04, I
know that delay code was changed but hoped it doesn't affect DOS.