Function            CRC16update - update a CRC-16 value

  Syntax              unsigned CRC16update(char c, unsigned crc);

  Prototype in        comm.h

  Remarks             updates a CRC-16 value based on the polynomial
                      X16 + X15 + X2 + 1 given c and the original crc
                      value. The CRC-16 value is commonly used by
                      transmission protocols such as Xmodem and Ymodem.

                      This function does all calculations on the fly.

  Return value        returns an updated CRC-16 value.

  Note                There are two common flavors of 16 bit CRCs, the
                      CRC-16 used by transmission protocols and a
                      CRC-CCIT based on a different polynomial. The two
                      are NOT compatible. I believe the CRC used in .ARC
                      files is of the CRC-16 variety, but will not swear
                      by it until I have verification. The CRC-16 value
                      calculated by TCHK is in proper bit order for use
                      in transfer protocols.

  See also            Checksum_block(), CRC16_block(), CRC16tupdate(),
                      CRC32tupdate()