Date: Wed, 22 Mar 1995 16:58:48 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, phk@ref.tfs.com Cc: CVS-commiters@freefall.cdrom.com, cvs-sys@freefall.cdrom.com, davidg@freefall.cdrom.com, rgrimes@gndrsh.aac.dev.com Subject: Re: cvs commit: src/sys/i386/isa wd.c wdreg.h Message-ID: <199503220658.QAA05594@godzilla.zeta.org.au>
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>Couldn't we just make a spin for a number of updates to the 1.19MHz
>counter (#2 ?) for this kind of delays ? For 1.25 uS we want to
>see it change twice:
> n=2;
> a= inb(TIMER);
> while (n--) {
> while (a == (b = inb(TIMER)))
> ;
> a = b;
> }
>Wouldn't that be a sensible addition to DELAY (microDELAY ?)
No, it is essentially a broken form of DELAY(). First, it always does
2 inb()'s, so it always takes > 2.5uS on some buses, so it is unsuitable
for use in wdwait(), where the point is that a 450nS delay is wanted and
DELAY(5) was very wasteful. DELAY(n) is equivalent to DELAY(20) for
n <= 20, so DELAY(1) would be no better. All these DELAYS()s do 3 inb()'s
plus some bookkeeping which is fast on fast machines, so they are not
much different from the above. inb(0x84) is better, but it is more
likely to be too short (DELAY(1) is too long on almost all machines while
DELAY(20) is too short on almost all modern machines).
I couldn't get the free-running counter to work right when I rewrote
DELAY(). I didn't have adequate documentation at hand. If I remember
right, 8254's have a nice free-running mode but 8253's don't.
I didn't calibrate DELAY() properly because it was too hard to do before
clock interrupts are working and it isn't easy to handle turbo mode
switches and cache effects...
Bruce
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