pete286/timer.c

#include <stdio.h>
#include <dos.h>
#include "timer.h"

#define TIMER_INTERRUPT 0x1c
#define REG_8253_CTL 0x43
#define REG_COUNTER0 0x40

volatile unsigned int timer_counter = 0;

static void interrupt (*oldTimerISR)() = NULL;
static void (*callback)() = NULL;

static void interrupt timer_isr() {
  disable();
  timer_counter ++;
  if (callback) callback();
  enable();
  oldTimerISR();
}

void timer_setcallback(void (*cb)()) {
  callback = cb;
}

void timer_setrate(unsigned int rate) {
  outp(REG_8253_CTL, 0x3c);
  outp(REG_COUNTER0, rate & 0xff);
  outp(REG_COUNTER0, (rate >> 8) & 0xff);
}

static void timer_cleanup() {
  if (oldTimerISR != NULL) {
    setvect(TIMER_INTERRUPT, oldTimerISR);
    timer_setrate(TIMER_18HZ);
    oldTimerISR = NULL;
  }
}
void timer_init(unsigned int rate) {
  timer_setrate(rate);
  oldTimerISR = getvect(TIMER_INTERRUPT);
  setvect(TIMER_INTERRUPT, timer_isr);
  atexit(timer_cleanup);
}