Path: utzoo!utgpu!news-server.csri.toronto.edu!torsqnt!lethe!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Newsgroups: comp.os.msdos.programmer
Subject: Re: How can I get a timed delay of 1 millisecond ?
Message-ID: <1990Sep14.151951.18485@druid.uucp>
Date: 14 Sep 90 15:19:51 GMT
References: <6250@castle.ed.ac.uk>
Organization: D'Arcy Cain Consulting, West Hill, Ontario
Lines: 44

In article <6250@castle.ed.ac.uk> elee24@castle.ed.ac.uk (H Bruce) writes:
>What is the best way to get a program to wait for one milliosecond ?
>(I am using Microsoft C V5.1).
>
Unfortunately the best granularity is about 10/182 of a second since the
system clock ticks at about 18.2 times per second.  It's even worse because
you could be of by almost 10/182 of a second because you don't know if you
are sampling at the start or the end of a tick.  If you know what the speed
of your CPU is you can perform instructions that take the required amount of
time to complete.  There are two disadvantages to this.  You have to resort
to assembler which may be a problem for some, and more importantly, events
asynchronous to your program (interupts) may distort the timing.

If you can live with about 1/10 sec granularity here is a simple timing
construct.  First set up a pointer to a far time_t (long if your compiler
doesn't understand time_t) and initialize it to location 0x046c.  This is
a memory location which is updated at every clock tick.  Then read the value
at that location and add the wait value and keep checking the location.  Here
is a simple function to do this.

void	pause(int tenths)
{
	time_t	far *cur_time = (time_t far *)(0x046c);
	time_t	end_time = *cur_time;

	/* note that we get the current time as soon as possible in the fewest */
	/* number of instructions in order to get the most accurate start time */
	/* now we will add the number of ticks to get the real end time */
	end_time += (tenths * 182)/100;	/* each tenths is 1.82 ticks */

	while (end_time > *cur_time)	/* wait till time is up.
		;
}

You can get fancier if you want but this is the basic idea.  Note that
this routine will fail at midnight when the clock rolls over but that's
a really small window.  If it is really important you can modify the code
to account for this.

-- 
D'Arcy J.M. Cain (darcy@druid)     |
D'Arcy Cain Consulting             |   MS-DOS:  The Andrew Dice Clay
West Hill, Ontario, Canada         |   of operating systems.
+ 416 281 6094                     |