Message-ID: <0eec01c2e70c$1f78cef0$0600000a@broadpark.no>
From: "Gisle Vanem" <giva AT bgnett DOT no>
To: <djgpp-workers AT delorie DOT com>
References: <10303100251 DOT AA12110 AT clio DOT rice DOT edu>
Subject: Re: Example uclock() code
Date: Mon, 10 Mar 2003 14:51:17 +0100
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Reply-To: djgpp-workers AT delorie DOT com

"Charles Sandmann" <sandmann AT clio DOT rice DOT edu> said:
...
> +    #define CALIBRATION_TICS 5 /* Testing shows 1.7% std error */
> +    static long divider;
> +    static unsigned long long otsc;
> +    if (uclock_bss != __bss_count) {
> +      otics = _farnspeekl(0x46c);
> +      while ( (tics = _farnspeekl(0x46c)) == otics);

A debugger will propably hang indefinitely here. Maybe add 
a __asm__("sti").

> +      otsc = _rdtsc();
> +      otics = tics;
> +      while ( (tics = _farnspeekl(0x46c)) <= otics+CALIBRATION_TICS);
> +      divider = (_rdtsc() - otsc) / ((tics - otics)*65536);
> +      if (divider) uclock_bss = __bss_count;
> +    }

Instead of counting the TSC over CALIBRATION_TICS, it's 
better to *average* the TSC over CALIBRATION_TICS samples.
This because of the jitter in the while-wait loops. It doesn't
(as you said in original message) really wait N*ticks.

BTW. the accuracy will depend on DOS-box setting of
"Program | Advanced | Emulation of timekeeper" (or what
ever it's called in English Windows). Reading the PIT is 
not very accurate even when set on.

--gv