Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!agate!ucbvax!hplabs!hpda!hpcupt1!dclaar
From: dclaar@hpcupt1.HP.COM (Doug Claar)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Precise loop timing on PC?
Message-ID: <5930025@hpcupt1.HP.COM>
Date: 9 Feb 89 18:36:51 GMT
References: <5930024@hpcupt1.HP.COM>
Organization: Hewlett Packard, Cupertino
Lines: 26

I received mail that indicated that DRAM refresh and the pre-fetch 
queue make this difficult to do. Another reply suggested polling the
timer, but that would seem to slow down the loop. One variation on
this that I thought of (well, actually, basic does something like 
this) is to grab the timer interrupt and boost the timer rate. I 
would then call the real timer interrupt every N times the timer
ticks, where N is how much faster my new timer is going.

old timer: _________|_________|_________|_________|_________|_________|
 
new timer: _|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
 
counter:            5 4 3 2 1 0
 
timer code:
   do my stuff;
   decrement counter;
   if counter = 0
      call old timer
      set counter to 5
   end

Doug Claar
HP Computer Systems Division
UUCP: mcvax!decvax!hplabs!hpda!dclaar -or- ucbvax!hpda!dclaar
ARPA: dclaar%hpda@hplabs.HP.COM