Message-Id: <199706061959.PAA27952@delorie.com>
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Date: Fri, 06 Jun 1997 16:01:07 -0500
To: djgpp AT delorie DOT com
From: Conrad Wei-Li Song <conradsong AT mail DOT utexas DOT edu>
Subject: PIT timer alert under Win95
Precedence: bulk

Win95 and Win95 DOS mode uses 8253/8254 PIT counting mode 3 instead of the
default mode 2.  What this means:
o The timer counts down by 2 instead to 1 but has to hit zero twice before
  generating an interrupt.
o The interrupt rate doesn't change, but the counter is now unusuable.
o Real-time programming needs to be accomplished by using the RTC instead
  of the PIT.  As an alternative, the PIT can be reprogrammed to mode 2
  with no apparant side-effects.  <-- NO GUARANTEE, take this up with
  Microsoft.
  OO Using the RTC sucks because of its fixed resolution.  The PIT counter
     gives extremely high-resolution with little overhead by using the PIT
     counter instead of rate generators.
  OO The PIT counter refers to the internal counters on the PIT, not the
     one maintained by software which services the timer interrupt.  The
     PIT issues an interrupt everytime the internal counter reaches 0,
     and the counter is then reset (usually to 65536).

Note: Increasing the interrupt rate of the PIT is a poor solution due to
      the increased overhead cost.
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