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From: gah4 <gah4@u.washington.edu>
Newsgroups: comp.compilers
Subject: Re: Looking for a garbage collection paper
Date: Thu, 29 Sep 2022 21:10:30 -0700 (PDT)
Organization: Compilers Central
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On Thursday, September 29, 2022 at 10:15:49 AM UTC-7, Thomas Koenig wrote:

(snip)

>> > It also
> >> says "a single instruction". I don't think that
> >> cur_col = (cur_col + 1) mod 3
> >> can be implemented in a single instruction in common hardware.

(snip, I wrote)

> > It would seem more likely on a machine with a word size a
> > multiple of 3, with 36 bit words not so rare 50 years ago.

(snip)

> What about
>
> int a[] = {1, 2, 0};
>
> cur_col = a[cur_col];
>
> That would qualify as a single indexed load, provided cur_col
> started out with a value between 0 and 2.
> [Duh, of course that will work on any word addressed machine. -John]

A word addressed machine with an indexed load.

Or a machine with indexed load that scales for the size, like VAX.

Or a table of bytes, so the index unit is 1.

But not if index registers are different from other registers, like
(if I remember) they are on the 7090.
[Yes, the 704 series had separate index registers.  It occurs to me that
another way to do this is to use the rotate instructions the 70x and PDP-6/10
had.  Since the word is 36 bits, you rotate by 12 each time and you'll have
three bit patterns. -John]