From mboxrd@z Thu Jan  1 00:00:00 1970
From: random832@fastmail.com (Random832)
Date: Fri, 11 Dec 2015 20:46:28 -0500
Subject: [TUHS] why is sum reporting different checksum's between v6 and
	v7
References: <20151212012257.3740418C0AA@mercury.lcs.mit.edu>
Message-ID: <m2r3is5thn.fsf@fastmail.com>

Noel Chiappa writes:
> No, I don't think so, depending on the exact detals of the implementation. As
> long as when folding the two halves together, you add any carry into the sum,
> you get the same result as doing it into a 16-bit sum.

The issue I was suggesting comes if you've lost carry bits
_before_ folding the two halves together, when you were working
in 32-bit arithmetic.

> (If my memory of how
> this all works is correct - the neurons aren't what they used to be,
> especially late in the day... :-)
>
>     > Also, if this sign extends, then its behavior on "negative" (high bit
>     > set) bytes is likely to be very different from the SysIII one, which
>     > uses getc.
>
> I have this bit set that in C, 'char' is defined to be signed

The SysIII sum.c file uses getc and stores the result in an int,
not a char.

I *think* the definition of getc returns positive values the
same as modern systems do, despite the manpage's caution to
check feof because EOF is a "valid integer value":

#define	getc(p)		(--(p)->_cnt>=0? *(p)->_ptr++&0377:_filbuf(p))

_filbuf also has & 0377 in the relevant place.

If getc returns negative values for high-bit characters, on the
other hand, then they would sign-extend to 32 bits when the long
math is done, still yielding different results.