Listing 2:
   
   #include <stdio.h>
   #define IOBYTE_ADDRESS 0X03
   #define IOBYTE_SET_FOR_PARALLEL 0X00
   #define IOBYTE_SET_FOR_SERIAL 0X80

   main()
   /* Toggles the IOBYTE */ 
       {
       unsigned char *pc = (char *) IOBYTE_ADDRESS;
       if (*pc !== IOBYTE_SET_FOR_PARALLEL)
           {
           printf("\n Changing from Serial to Parallel \n");
           *pc = IOBYTE_SET_FOR_PARALLEL;
           }
       else
           {
           printf("\n Changing from Parallel to Serial \n");
           *pc = IOBYTE_SET_FOR_SERIAL;
           }
       exit(0);
       }