I am trying to find out how long this code has been this way.  According
to cvsweb, these assembler calls have "always" been like that (that is,
since 2 april 1996). NetBSD's version is apparently derived from 
"@(#)in_cksum.c  8.1 (Berkeley) 6/10/93".

So I started looking through my copy of the PUPS archive, and I found
(strangely enough) the file
PUPS/Archive/Trees/2.11BSD/usr/src/usr.sbin/timed/vax/cksum.c with SCCS
tag "@(#)cksum.c     1.2 (Berkeley) 9/18/85";

It has not been gcc-ified yet because it just contains asm("adwc
(r9)+,r8;"); for the addition (thereby strongly depending on the
compiler specifics), and also other things clearly show it as a an
earlier version of in_cksum.c. 

I found "@(#)in_cksum.c  7.6 (Berkeley) 12/16/90" in
/PUPS/Archive/Distributions/4bsd/4.4BSD-Alpha/src.tar.gz's file
usr/src/sys/vax/vax/in_cksum.c and it is more or less the same as the
one from 2.11BSD.

I recommend adding some of the original comments back into the source:

                /*
                 * Do as much of the checksum as possible 32 bits at at time.
                 * In fact, this loop is unrolled to make overhead from
                 * branches &c small.
                 *
                 * We can do a 16 bit ones complement sum 32 bits at a time
                 * because the 32 bit register is acting as two 16 bit
                 * registers for adding, with carries from the low added
                 * into the high (by normal carry-chaining) and carries
                 * from the high carried into the low on the next word
                 * by use of the adwc instruction.  This lets us run
                 * this loop at almost memory speed.
                 *
                 * Here there is the danger of high order carry out, and
                 * we carefully use adwc.
                 */

The Net2 release also contains version 7.6 of 12/16/90. I haven't been
able to find a newer version. I wanted to look at the Quasiarus sources
but the .Z files were apparently not .Z files and therefore unreadable.

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert - rhialto@polder --They that can give up essential 
\X/ land.nl --liberty to purchase a little temporary safety
--------------deserve neither liberty or safety. - Benjamin Franklin, 1759
---People will accept your ideas much more readily if you tell them that
---Benjamin Franklin said it first. - Unknown