Cyclical Redundancy Check (CRC)

The checksum that is used by Native Mode file transfer commands is a standard 16-bit Cyclical Redundancy Check (CRC).  The algorithm for calculating the CRC follows.  In this example the variable CRC is unsigned 16 bits and the input byte is 8 bits.

1. Set CRC to 0.
2. Get next byte from data stream.

3. Test the most significant bit of CRC and the input byte and do the following:

        If they are equal shift CRC left 1.

        If one bit is 0 and the other 1 then shift CRC left 1 and XOR with 4129.

4. Shift the input byte left 1.
5. If you have not shifted the input byte 8 times go to step 3.
6. If you have remaining bytes in the data stream go to step 2.

A theoretical explanation of this algorithm can be found in Numerical Recipes in C:The Art of Scientific Computing.1Press, William H., et al. Cambridge, MA: Cambridge University Press, 1992, 896-901.
A C function that calculates the CRC one bit at a time follows:

rt CalculateCRC(char* buffer, int size)

{

  unsigned short cword = 0;

  unsigned short ch;

  int i,j;

  for (i = 0; i < size; i++) {

    ch = buffer[i] << 8;

    for (j = 0; j < 8; j++) {

      if ((ch & 0x8000) ^ (cword & 0x8000)) {

         cword = (cword << 1) ^ 4129;

      }

      else {

        cword <<= 1;

      }

      ch <<= 1;

    }

  }

  return cword;

}