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Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!tektronix!reed!omen!caf
From: caf@omen.UUCP (Chuck Forsberg WA7KGX)
Newsgroups: net.micro,net.micro.pc,net.lang.c,net.unix-wizards
Subject: Re: Help with C-kermit on SCO Xenix V
Message-ID: <329@omen.UUCP>
Date: Wed, 30-Apr-86 07:52:42 EDT
Article-I.D.: omen.329
Posted: Wed Apr 30 07:52:42 1986
Date-Received: Sat, 3-May-86 17:44:54 EDT
References: <294@catnip.UUCP>
Reply-To: caf@omen.UUCP (Chuck Forsberg WA7KGX)
Organization: Omen Technology, Portland
Lines: 74
Keywords: It isn't working
Xref: watmath net.micro:14484 net.micro.pc:8023 net.lang.c:8793 net.unix-wizards:17885

In article <294@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes:
>I recent obtained a copy of C-kermit 4.2(030) PRERELEASE #2, 5 March 85.
>We have been using this version at work on a Perkin Elmer 3250XP and an
>AT&T 3B5, and so I wasn't expecting any problem bringing it up on Xenix.
>Well the package compiles without error, and appears to work okay, until
>you attempt to do a transfer.  Then it can't even seem to get past the
>header packet, and keeps printing SS%S%S%S%S% and the like on the screen.
>Looking at the debugging output from both ends show that the Xenix machine
>is computing the checksum incorrectly.  Please, can anyone help???
>
The Microsoft C compiler has a few problems with right shifts such as used
in the Kermit CRC calculations.  Here is something that should work better.
(From Professional-YAM, the most powerful COMM program for the PC)


/*  C H K 1  --  Compute a type-1 Kermit 6-bit checksum.  */

chk1(pkt)
char *pkt;
{
	register chk;

	chk = chk2(pkt);
	return((((chk & 0300) >> 6) + chk) & 077);
}


/*  C H K 2  --  Compute the numeric sum of all the bytes in the packet.  */

chk2(pkt)
register char *pkt;
{
	register chk;

	for (chk = 0; *pkt; ++pkt)
		chk += (kparity ? (*pkt & 0177) : (*pkt & 0377));
	return chk;
}


/*  C H K 3  --  Compute a type-3 Kermit block check.
 *
 * Calculate the 16-bit CRC of a null-terminated string using a byte-oriented
 * tableless algorithm invented by Andy Lowry (Columbia University).  The
 * magic number 010201 is derived from the CRC-CCITT polynomial x^16+x^12+x^5+1.
 * Note - this function could be adapted for strings containing imbedded 0's
 * by including a length argument.
*/
chk3(s)
char *s;
{
	register unsigned int c, q;
	LONG crc = 0;

	while ((c = *s++) != '\0') 
	{
		if (kparity)
			c &= 0177;
		else
			c &= 0377;
		q = (crc ^ c) & 017;		/* Low-order nibble */
		crc = (crc >> 4) ^ (q * 010201);
		q = (crc ^ (c >> 4)) & 017;	/* High order nibble */
		crc = (crc >> 4) ^ (q * 010201);
	}
	return(crc);
}

   Chuck Forsberg WA7KGX  ...!tektronix!reed!omen!caf   CIS:70715,131
   Author of Professional-YAM communications Tools for PCDOS and Unix
 Omen Technology Inc     17505-V NW Sauvie Island Road Portland OR 97231
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