Function            CRC16_block - calculate CRC-16 for a block

  Syntax              unsigned CRC16_block(char *b, int size);

  Prototype in        comm.h

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

                      CRC16_block() will calculate the CRC-16 value for a
                      given block of bytes.

  Return value        returns a CRC-16 value for the block operated on.

  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(), CRC16tupdate(), CRC16update(),
                      CRC32tupdate()