Date: Mon, 17 Feb 1997 15:35:37 +0200 (IST)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: Ian Miller <itmiller AT dra DOT hmg DOT gb>
cc: DJGPP workers <djgpp-workers AT delorie DOT com>
Subject: Re: Exact timming II
In-Reply-To: <330822FF.41C67EA6@dra.hmg.gb>
Message-ID: <Pine.SUN.3.91.970217153452.18258A-100000@is>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On Mon, 17 Feb 1997, Ian Miller wrote:

> come up with this evening. (I'm hoping that there is a
> get_timer_mode() library function because I probably don't know
> enough about programming the hardware otherwise.)

No, there's no way to read the timer mode (AFAIK), so you will need to
use indirect methods.  One way is to write a tight loop that reads the
BIOS tick count and the timer counter values, and stores them in an
array.  Let the program run for a few seconds (more than that will
require a huge buffer), then print the values and see whether the
timer counter goes through 0 once or twice for a single increment in
the BIOS tick count.  Call `uclock' only once (to put the timer into
mode 2), then use this fragment from its source to read the counters:

		 tics = _farpeekl(_dos_ds, 0x46c);
		 outportb(0x43, 0x00);
		 lsb = inportb(0x40);
		 msb = inportb(0x40);

Another possibility is to call the BIOS Event Wait service
(Int 15H/AH=83h) to see how much does the timer counter change for a
given period of time (timer in mode 3 changes twice as fast as mode
2).  The BIOS Event Wait service (described in the Interrupt List) can
be set to wait for an integer multiple of 976 microseconds; timer in
mode 2 gets decremented by 1 every 838 nanoseconds, while mode 3
decrements it by 2 every 838 nanoseconds.  So you should see the timer
counter change either by 1165 (mode 2) or by 2329 (mode 3) every 976
microseconds.