Path: csiph.com!weretis.net!feeder6.news.weretis.net!news.misty.com!news.iecc.com!.POSTED.news.iecc.com!nerds-end
From: drb@ihatespam.msu.edu (Dennis Boone)
Newsgroups: comp.compilers
Subject: Re: C arithmetic, was Software proofs, was Are there different
Date: Sun, 12 Feb 2023 04:37:07 +0000
Organization: Compilers Central
Sender: johnl@iecc.com
Approved: comp.compilers@iecc.com
Message-ID: <23-02-041@comp.compilers>
References: <23-01-092@comp.compilers> <23-02-003@comp.compilers> <23-02-019@comp.compilers> <23-02-025@comp.compilers> <23-02-026@comp.compilers> <23-02-029@comp.compilers> <23-02-032@comp.compilers> <23-02-035@comp.compilers> <23-02-039@comp.compilers>
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="61234"; mail-complaints-to="abuse@iecc.com"
Keywords: architecture, history, comment
Posted-Date: 12 Feb 2023 12:18:23 EST
X-submission-address: compilers@iecc.com
X-moderator-address: compilers-request@iecc.com
X-FAQ-and-archives: http://compilers.iecc.com
Xref: csiph.com comp.compilers:3382

 > The three index registers were 1,2, and 4, and if you specified more
 > than one, it OR'ed them together and subtracted the result, which was
 > really strange.  There were a lot of other machines with index registers
 > but none I know of that subtracted or OR'ed.  I have also been unable to
 > tell whether the OR'ing was deliberate or just a result of minimizing
 > the number of tubes that they then documented.

Not having to have the circuitry to make sure multiple registers
were never gated onto the internal bus seems like a likely reason.

But one _could_ also conceivably use multiple index registers for
multiple dimensions, with careful allocation.

De
[As I said, I've found lots of guesses but no documents. Multiple
dimensions only work if each dimension is a power of two which,
considering the 704's tiny memory, seems unlikely. My guess is the
same as yours, fewer tubes, but it's just a guess. -John]