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From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: CRCs
Message-ID: <1991Apr22.002327.5546@nntp-server.caltech.edu>
Date: 22 Apr 91 00:23:27 GMT
References: <8720@crash.cts.com>
Organization: California Institute of Technology, Pasadena
Lines: 28

Morgan, if you can live with 512 bytes of CRC tables then you can do
block CRC's in something like 25 cycles per byte CRC'd...

	lda	CRC
	xba
	sep	#$30

crclp	eor	[data],y
	tax
	lda	tblL,x
	xba
	eor	tblH,x
	iny
	bne	crclp
	
	rep	#$30
	xba
	sta	CRC

will do 256 bytes at a time, with a best case of 24 cycles/byte and a worst of
27. (Aligning the DP saves 1, aligning tblH and tblL guarantees 4 cycles each.)
I know the table is generated by something simple like (sombody check me):
	tbl[byte] = updcrc(byte, 0);
where updcrc(byte, oldcrc) does not use the table, and tbl is split into high
and low byte arrays for the assembly fragment above.

Todd Whitesel
toddpw @ tybalt.caltech.edu