Message-Id:	<1.5.4.32.19970514125351.00315e3c@ubeclu.unibe.ch>
Mime-Version:	1.0
Date:	Wed, 14 May 1997 14:53:51 +0200
To:	djgpp AT delorie DOT com
From:	Roger Noss <noss AT pupk DOT unibe DOT ch>
Subject:	Allegro example skips vertical retraces

Forget Win95/NT -- I scrounged a 486 with DOS 6.22 and decided to let
Allegro do the timing, but something is amiss.  I started with Allegro
example #7 (ex7.c), configured the 1st timer for 1 ms and removed the other
two.  In a loop I wait for the vertical retrace using Allegro function
_vga_vsync() and quickly store the timer's counter.  Every 100 samples I
print the differences.  Aside from removing a 5-second counting demo, those
are the only other changes, honest.

Many vertical retraces are missed.  What am I doing wrong?  I doubt it is
"void main()" despite the raging and vital discussion over main's return type.

The code follows.  I compiled it with "gcc myex7.c -lalleg" and ran a.exe.

Roger Noss

/* 
 *    Example program for the Allegro library, by Shawn Hargreaves.
 *
 *    This program demonstrates how to use the timer routines.
 *    These can be a bit of a pain, because you have to be sure you lock 
 *    all the memory that is used inside your interrupt handlers.
 */

/* modified to measure vertical retrace May 14, 1997, by Roger Noss */

#include <stdlib.h>
#include <stdio.h>

#include "allegro.h"

/* these must be declared volatile so the optimiser doesn't mess up */
volatile int x = 0;

/* timer interrupt handler */
void inc_x()
{
   x++;
}

END_OF_FUNCTION(inc_x);


void main()
{
   int i;
   int xvec[100];

   allegro_init();
   install_keyboard(); 
   install_timer();

   printf("\nPress a key to set up interrupt handlers\n");
   readkey();

   /* all variables and code used inside interrupt handlers must be locked */
   LOCK_VARIABLE(x);
   LOCK_FUNCTION(inc_x);

   /* the speed can be specified in milliseconds (this is once a ms) */
   install_int(inc_x, 1);

   /* the interrupts are now active... */
   i=0;
   while (!keypressed())
   {
      _vga_vsync();
      xvec[i++] = x;
      if (i == 100)
      {
 	for ( i=1; i<100; i++ ) printf("%d ", xvec[i]-xvec[i-1] );
	printf("\n");
        i=0;
      }
   }

   exit(0);
}

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