Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: negative addresses (really unsigned arithmetic)
Message-ID: <1988May22.020336.17472@utzoo.uucp>
Organization: U of Toronto Zoology
References: <2393@uvacs.CS.VIRGINIA.EDU> <9485@apple.Apple.Com> 
Date: Sun, 22 May 88 02:03:36 GMT

> ... what are the "nasty surprises"
> hiding in unsigned arithmetic that do not also exist for signed
> arithmetic AS IT IS COMMONLY IMPLEMENTED IN HARDWARE?

Well, for example, consider that a+b>c does not imply a>c-b in unsigned
arithmetic.  (To make this more obvious, consider that b>c does not imply
c-b<0, since no unsigned number is less than zero.)  Remember too that one
unsigned number in a calculation tends to make the whole calculation be
done unsigned, by C rules, sometimes unexpectedly.

> Why should signed arithmetic be more efficient than unsigned?

Because the hardware sometimes supports it rather better.  On the machine
I'm typing this on, for example, unsigned multiplication or division is
significantly slower than the signed forms, because the hardware multiply
and divide instructions are signed-only.