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From: dick@venera.isi.edu (Richard Gillmann)
Newsgroups: alt.bbs,comp.sys.mac.programmer,comp.sys.mac,comp.sources.wanted
Subject: Re: Wanted: CRC-16 Routines in Pascal
Summary: MS Pascal version
Keywords: crc xmodem bbs help wanted pascal checksum
Message-ID: <6672@venera.isi.edu>
Date: 1 Nov 88 22:02:44 GMT
References: <1971@uokmax.UUCP>
Reply-To: dick@venera.isi.edu.UUCP (Richard Gillmann)
Distribution: usa
Organization: Information Sciences Institute, Univ. of So. California
Lines: 16


Here is a version of the CRC calculation as used by XMODEM, YMODEM, etc.
It is written in Microsoft Pascal.  The umulok routine is a library call
that does an unsigned multiply and allows overflow.  It's used to
simulate a shift and check of the carry bit.  Of course, this could be
much speeded up by writing in assembler or using tables.

{Computes a 16-bit circular redundancy check.  Initialize your variable for
  crc_value to 0, then call this routine for each byte.} 
procedure crc_16(c : char; var crc_value : word);
begin
  crc_value := crc_value xor (wrd(c)*256);
  for var i := 1 to 8 do
    if not umulok(crc_value,2,crc_value) then {hi bit was a 1}
      crc_value := crc_value xor 16#1021; {X^15+X^12+X^5+1}
end {crc_16};