Path: utzoo!mnetor!uunet!husc6!bu-cs!bzs
From: bzs@bu-cs.BU.EDU (Barry Shein)
Newsgroups: comp.arch
Subject: Re: More than 32 bits needed where?
Message-ID: <20076@bu-cs.BU.EDU>
Date: 21 Feb 88 19:05:01 GMT
References: <235@unicom.UUCP> <28200089@ccvaxa> <3104@watcgl.waterloo.edu> <9618@steinmetz.steinmetz.UUCP> <173@babbage.acc.virginia.edu>
Organization: Boston U. Comp. Sci.
Lines: 38
In-reply-to: mac3n@babbage.acc.virginia.edu's message of 21 Feb 88 00:12:39 GMT



> >Other uses of more that 32 bits is for social security numbers.
>
>Note that you don't really integers for this, unless you plan to do arithmetic
>on SSNs.

Comparison is "arithmetic". I believe most machines will perform
load register, compare registers faster than memory-memory compares,
as in:

	LOAD	R1,WANTED
	LOAD	R2,ARRAY
LOOP:
	CMP	R1,@R2
	BEQ	FOUND
	INCRL	R2
	BR	LOOP

vs.
	LOAD	R1,WANTEDP
	LOAD	R2,ARRAY
LOOP:
	CMPLC	LENGTH,R1,R2	# compare logical character
	BEQ	FOUND
	BR LOOP

(obviously a test for end is missing.)

It's no accident that most of the various flavors of identifiers in
OS/370 are limited to 8 bytes (see compare double), or similarly the 6
SIXBIT chars (36 bits) in PDP10 systems.

I will agree that in many cases such concerns for speed at the cost
of human interface (upper case only, six character file names) was
not a very good trade-off, but large databases (which SSN's imply)
are a different matter.

	-Barry Shein, Boston University