Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!hplabs!pyramid!prls!mips!hansen
From: hansen@mips.COM (Craig Hansen)
Newsgroups: comp.arch
Subject: Re: conditional branches
Message-ID: <1689@mips.mips.COM>
Date: 26 Feb 88 23:36:58 GMT
References: <191@telesoft.UUCP> <9782@tekecs.TEK.COM> <1042@edge.UUCP>
Lines: 19

]I've seen this assertion a couple of times, but it ain't so.  You need
]to worry about carry when doing a subtract, but not when just doing a
]compare.  If both operands are positive, a simple bit-for-bit
]comparison is sufficient, analogous to the way that C programs compare
]strings.  When the operands can be signed, a little extra logic is
]required, but nothing as fancy as a full adder/subtractor.

However, to do the bit-for-bit comparison in parallel (most C programs
compare strings one byte at a time), you need a structure that looks
_remarkably_ like a carry chain. In fact, the structure's just as
"fancy" as the carry chain in a subtractor, because it is functionally
identical to the carry chain in a subtractor. Remember that the
comparison of the high-order bits has priority over the comparison of
low-order bits....

-- 
Craig Hansen
Manager, Architecture Development
MIPS Computer Systems, Inc.
...{ames,decwrl,prls}!mips!hansen or hansen@mips.com   408-991-0234