CRC16 CRC16


NAME

crc16 - calculate and display CRC checksum for file(s)


SYNOPSIS

crc16 [-c][-s] file(s)

wildcards are permitted
default polynomial is CRC-16, as used in ARC, PKPAK, etc..
specify -c for CCITT, as used in the XMODEM-CRC protocol
specify -s for the IBM SDLC algorithm


DESCRIPTION

This algorithm is adapted from the classic paper "Byte-wise CRC Calculations",
Aram Perez in IEEE Micro June 1983 pp 40-50.

The default table is that for CRC-16 (x16 + x15 + x2 + 1). It is derived from
combinations of the bits of the low order byte shifted eight times and
combined according to the polynomial. Other polynomials may be derived, see
the article by Perez.

x8 x7 x8 x7 x6 x5 x4 x3 x2 x1 0 0 0 0 0 x8
x7 x6 x7 x6 x5 x4 x3 x2 x1 x7
x6 x5 x6
x5 x4 x5
x4 x3 x4
x3 x2 x3
x2 x1 x2
x1 x1

Entry 1 (00000001) is therefore
1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 1 = C0C1
2 (00000010) is
1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 1 = C181
3 (00000011) is their binary sum
0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 = 0140

This program will calculate the CRC used by the popular ARC archiving program.
The program can be used to check that a file has been archived properly. The
program will also calculate the crc used by XMODEM which is the CCITT standard
polynomial (x16 + x12 + x5 + 1), this polynomial is also used by IBM corp. in
their SDLC synchronous transfer protocol.

To cause the CCITT polynomial to be used use the -c option, for the SDLC
modification where the crc is initialized to all ones use the -s option.



AUTHOR

Rob Duff, 2255 Cambridge St., Apt 301, Vancouver, BC V5L 1E5 Canada
BBS: The DuffleBoard, (604) 251-1816 2400/N/8/1

This program is not copyrighted.