*****Listing 6*****



     #define DOS_IMAGE 1024
     unsigned char dos_low_memory[DOS_IMAGE];

     unsigned short compute_image_checksum(start,length,copy)
          unsigned char * start;
          unsigned short length;
          unsigned char * copy;
          {
           unsigned char * end = start + length;
           unsigned short sum = 0;

           while(start < end)
             {
              if(copy)
                 *copy = *start;
              sum += *start++;
             }
           return sum;
          }

     unsigned short check_image_checksum
             (where,start,length,oldsum,copy)
         {
          unsigned short sum;
          unsigned char * end = start + length;

          sum = compute_image_checksum(start,length,NULL);
          if(sum == oldsum)
              return 1;
          fprintf(stderr,
                     "Checksum error at %s: "
                     "%p/%ud(0x%4x), "
                     "old %ux, new %ux\n",
                             where, start, length,
                             oldsum, sum);

          while(start < end)
             {
              if(*start != *copy)
                 {
                  fprintf(stderr,
                          " %p:  old: %02x, new: %02x\n",
                          start, *copy, *start);
                 }
             }
         }

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