MWAIT	V1.02

	Nick de Smith, Applied Telematics Group Ltd.,
	Telematics House, Vale Avenue,
	Tunbridge Wells, Kent TN1 1DJ
	England
	+44 892 511000 (Fax: +44 892 515402)
	PSI%234213300154::NICK
	NICK@NCDLAB.ULCC.AC.UK

The code in this directory provides replacements for LIB$CRC_TABLE and the VAX
CRC instruction that give performance increases of up to 300% and more for short
buffers and "unknown" polynomials (not one of the set (FAL:0000E905,
AUTODIN-II:EDB88320 and CRC-16:0000A001).

CRC.MAR		Source for VAX$CRC, VAX$FAST_CRC and VAX$FAST_CRC_TABLE

TEST.C		Timing comparisons for various uses of the above routines.

FAST_CRC_TABLE.C A 'C' version of VAX$FAST_CRC_TABLE for comparison.


For short buffers, a direct replacement for LIB$CRC, or the VAX CRC instruction
is given by the routine VAX$CRC:

crc = VAX$CRC( table, inicrc, length, address )

	table	address of a 16 entry table generated by LIB$CRC_TABLE
		Input CRC table

	inicrc	longword, by value
		Initial CRC value

	length	longword, by value
		Length of input data stream

	address	address of "length" bytes of data
		Data on which to perform the checksum

For really fast CRCs, use VAX$FAST_CRC. uses the same parameters as VAX$CRC,
except that the CRC table parameter is 256 longwords generated by
VAX$FAST_CRC_TABLE. If the stream length is less than 4 bytes, VAX$FAST_CRC will
generate an SS$_BADPARAM signal. To be even faster, align the data buffer on a
longword boundary, and use buffers that are a multiple of 4 bytes long.

crc = VAX$FAST_CRC( table, inicrc, length, address )

	table	address of a 256 entry table generated by VAX$FAST_CRC_TABLE
		Input CRC table (see VAX$FAST_CRC_TABLE).

	inicrc	longword, by value
		Initial CRC value

	length	longword, by value
		Length of input data stream

	address	address of "length" bytes of data
		Data on which to perform the checksum

To generate the 256 entry (8 bit) shift tables used by VAX$FAST_CRC, use

void VAX$FAST_CRC_TABLE( polynomial, table )

	polynomial	longword, by reference
		initial polynomial value, eg. 00008408 for CCITT

	table	address of a 256 entry longword table
		8 bit shift table for use by VAX$FAST_CRC.


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