simple avr using a rotory encoder

Development Boards MicroMod
1 
#include<90s2313.h>

#define enc_a PIND.3
#define enc_b PIND.4

bit a, b;

void main(void)
{

  signed char d;

  PORTB=0xFF;
  DDRB=0xFF;

  PORTD=0x18;
  DDRD=0x07;

  while (1)
    {
      if(a != enc_a || b != enc_b)
        {
          if(enc_a ^ b)
            d++;
          else
            d--;
          a = enc_a;
          b = enc_b;
        }

    }
}