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

#define IOBASE		0x300
#define AD_CHNL		IOBASE
#define AD_HIGH		IOBASE+1
#define AD_LOW		IOBASE+2

#define dac_chnnl(i)	outportb(AD_CHNL,(unsigned char)i);

void dac_value(int i)
{
  unsigned char low,high;

  high = (i >> 4) & 0xff;
  low  = (i << 4) & 0xf0;
  outportb(AD_HIGH,high);
  outportb(AD_LOW,low);
}

void main(void)
{
  int c;

  printf("\n Input the channel:");
  scanf("%d",&c);
  dac_chnnl(c);					/* select the DAC channel */
  printf(" Press any key to continue ..");	/* wait key */
  getch();
  c = 0;
  do {
     dac_value(c++);
     c = c & 0xfff;
  } while(kbhit()==0);
}