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Multi-tasking in C

Posted on 1998-12-24
Medium Priority
Last Modified: 2010-04-02
How can I program a timer in C? I would like to have a timer which will be activated say every 2 micro-second or so and
perform a particular task, say TASK 1. So this timer will keep on running in the background and whatever code that has been executed in "main()" will not slow down TASK 1 or has no effect on it.
Question by:thankyouall
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Author Comment

ID: 1180983
I'm using Turbo C++ v3.0 for DOS

Expert Comment

ID: 1180984
You might hook your routine into INT 0x1c (the ticker interrupt), but you cant call any DOS functions from an interrupt routine...

Expert Comment

ID: 1180985
the code in main will always note a small slowdown. This is however transparent to main, and main does not have to think about it (interrupts take over control of the cpu for a small amount of time).

The 0x1c interrupt is to slow to do a 2ms timer. For this you might want to hook to the timer interrupt (9 or 8, one of them is the timer, the other the keyboard.. i always mix those.. ;-)

Once you have grabbed this irq, you can then increase the timer-intervall time, and call the old irc-handler at 18.2 times/second. it's pretty straightforward
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Author Comment

ID: 1180986
I still dun get it...
LVL 11

Expert Comment

ID: 1180987
There is absolutely no built-in multitasking support in the C language, you have to use operating system support for that.
In your case it is a bit difficult because DOS is not a multitasking OS but you can fake it using Interrupt Service Routines.  You can hook the timer interrupt and provide your own service routine for it.  Beware of reentrancy issues.

Expert Comment

ID: 1180988
Every 2 MICROseconds???  Wow, you'll need a super-efficient timer for that ;o)
You'll be lucky if your main loop is even processed nearly as often as that.  ;o)

Accepted Solution

_Canidae_ earned 200 total points
ID: 1180989
Just ignore that last comment, 'twas a slip o' the mind.  Anyway, here's an example of how to use a simple timer.  This sure won't help if you really did want a timer with microsecond-precision, but otherwise perhaps it will.  BTW:  This compiled fine under TC3 :o)      Also:  sorry if the indentation is a little strange here, I tried to fix it.

#include <stdio.h>
#include <conio.h>

void main()
      unsigned long far *clock;
      clock = (unsigned long far *) 0x0040006cL;
      int go=1, running=0;
      unsigned long start, now;
      float sofar=0.0;
      printf("Press G to run, S to stop, C to clear or ESC to quit\n\n");
      while (go)
            if (kbhit())
                  int c=getch();
                        /*if ESC pressed*/
                        case 27:      go=0;
                        case 'g':
                        case 'G':      if (!running)
                        case 's':
                        case 'S':      running=0;
                        case 'c':
                        case 'C':      start=*clock;
                        default:      putchar(7); //invalid key, so beep
            if (running)
                  unsigned long elapsed=now-start;
            printf("\r%8.2f Seconds", sofar);

Author Comment

ID: 1180990

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