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Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!ima!johnl
From: johnl@ima.UUCP (John R. Levine)
Newsgroups: net.arch
Subject: Re: ORed indexes
Message-ID: <106@ima.UUCP>
Date: Fri, 2-May-86 18:30:59 EDT
Article-I.D.: ima.106
Posted: Fri May  2 18:30:59 1986
Date-Received: Sun, 4-May-86 04:49:08 EDT
References: <5100066@ccvaxa> <353@astroatc.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Javelin Software Corporation
Lines: 24
Summary: IBM had or'ed indexes in 1958

In article <5100066@ccvaxa> aglew@ccvaxa.UUCP writes:
>How would you react to an instruction set that had no indexed addressing
>mode, but did have a mode where a register was ORed into the address?

OR'ed indexes, eh?  Back in olden days, the IBM 704, 709 (tubes), and 7040
and 7090 (transistors) each had 3 index registers, numbered 1, 2, and 4.
Normally, if you specified one index register, it was subtracted in twos
complement from the base address in the instruction.  (Why subtracted rather
than added is something I never understood, particularly since they were
subtracted twos-comp and the machine was otherwise sign-magnitude.  But I
digress.)  If you set more than one of the three bits in the index register
field, the corresponding registers were or'ed together before they were
subtracted from the base address.  Sounds handy, no?  Well, no.  It was
completely useless and probably due to a random wiring peculiarity in the
704.  In the 7094, there were 7 different index registers and specifying
register 6 got you register 6 rather than registers 2 and 4 or'ed.  I gather
that almost every 7090 program worked on the 7094, which means that nobody
used the or'ing.  Pfui.
-- 
John R. Levine, {ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl
Levine@YALE.EDU

The opinions expressed above are those of a 12-year-old hacker who has
broken into my account and not those of any person or organization.