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From: "Rod Pemberton" <do_not_have AT bitfoad DOT cmm>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Performance enhancement for gettimeofday()?
Date: Tue, 9 Jan 2007 03:46:03 -0500
Organization: Aioe.org NNTP Server
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References: <OF1B107D53 DOT 1922E890-ON8725725D DOT 005F267B-8725725D DOT 00633021 AT seagate DOT com>
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<Gordon DOT Schumacher AT seagate DOT com> wrote in message
news:OF1B107D53 DOT 1922E890-ON8725725D DOT 005F267B-8725725D DOT 00633021 AT seagate DOT com...
> I was indeed talking about the 18.2 per second timer - which appears to be
> the one that DOS is using internally.  So that staleness of the hundredths
> field is already an issue just using the CMOS.  Here's a snippet of
> documentation on this: http://www.merlyn.demon.co.uk/prog2000.htm#MSF
> I've also dug through the FreeDOS source code and confirmed that this is
> how FD behaves.

Interesting... Thanks for that.  Looks like some more good info there too.

> Use the following for proof:
>
> #include <stdio.h>
> #include <time.h>
>
> int main()
> {
>   int i;
>   for (i = 0; i < 400; i++) {
>     uclock_t start;
>     struct timeval tp;
>
>     gettimeofday(&tp, NULL);
>     printf("%ld\t", tp.tv_usec);
> // Uncomment if your machine is too fast...
> //    start = uclock();
> //    while (uclock() < start + UCLOCKS_PER_SEC / 500);
>   }
>   printf("\n");
>   return 0;
> }
>

I'm sure I'll get around to trying it eventually.  Seems like I have some
need for everything six months or so later...   :-)

> The clock frequency should be (1.193*10^6) / 65536 Hz, which works out to
> 18.203 or so.  I'm modifying my code to use the more accurate calculation
> starting with the crystal frequency instead of CLOCKS_PER_SEC (it means
> doing floating-point math but it's still so much faster than the original
> that it's still worth it).

I believe this it the math you'll need:

14.318Mhz=4*3.58Mhz=4*(4.5Mhz*455/572)
    (4.5Mhz US TV bandwith/channel, 455 colorburst phase changes/line, 572
total lines/frame including sync)
14.318Mhz/12=1.93182Mhz
1.93182Mhz/65536=18.2065Hz

> # I had hoped someone much more familiar with the issues you bring up
would
> # respond to you first...
>
> No worries.  I've also posted some actual code over on DJGPP-Workers,
> since this didn't seem like the appropriate place for it... I'll link
> to it though:
>
http://www.delorie.com/djgpp/mail-archives/browse.cgi?p=djgpp-workers/2007/01/05/17:36:42
>

Yes, thanks for that too.  I usually only look at mail archives if GMANE
NNTP-izes it.


Rod Pemberton