From:	"Tom St Denis" <tomstdenis AT yahoo DOT com>
Newsgroups:	comp.os.msdos.djgpp
References:	<3ACABB23 AT MailAndNews DOT com>
Subject:	Re: Any comments about using RDTSC to sync code ?
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Date:	Sun, 01 Apr 2001 05:30:22 GMT
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"Eduardo TUDA Lorenzetti Pellini" <dochell AT MailAndNews DOT com> wrote in message
news:3ACABB23 AT MailAndNews DOT com...
> Hi DJGPP friends,
>
> I'm doing a real time app using DJGPP that runs for almost 2 hours polling
> and
> reading some hardware ports in loops of 50 usec (*micro*seconds).
> I'm using the Pentium TSC counter to sync the data acquision since the RTC
> and
> system timer don't have this desired resolution or have some undesired
> granularity.
>
> The first problem i am living is to accurately calculate the CPU clock to
> stablish the relationship between system clocks and real time.
>
> The second problem is that (i think) PC hardware don't have any
> stabilization
> method for the CPU internal clock (Xtal voltage and temperature
regulation).
> So i think that the CPU clock may vary, but i don't know how much.

Typically it's about 10%.  My athlon 1200mhz runs at "supposedly" 1211mhz
according to various programs.

> The third problem is about the TSC reading routine (RDTSC) and the 64 bits
> arithmetic involved to calculate the elapsed time. They impact on the CPU,
> but
> i don't know how much. On a Pentium 100 system I estimated that RDTSC
takes
> from 12 to 24 clocks to finnish. And about the 64 bits arithmetic ? DJGPP
> supports it, but i don't think that the operations are somehow hardware
> accelerated. Am i right ? Are there any WWX (the oposite of MMX 8)
> instructions on any Intel/AMD class processor that could be used by DJGPP
or
> inline assembler ?

Well you can do 64-bit math on an x86 several ways.  The x86 set does have
"add/sub" with carries so you can add a 64-bit word in two cycles.  You can
multiply one in about 10 or so... etc..

Using RDTSC to sync programs is a bad idea since RDTSC may take a variable
amount of cycles to complete.

Tom

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