Path: ...!news.iecc.com!.POSTED.news.iecc.com!not-for-mail
From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: what's a register, The Design of Design
Date: Tue, 30 Apr 2024 20:22:27 -0000 (UTC)
Organization: Taughannock Networks
Message-ID: <v0rju3$1cbi$1@gal.iecc.com>
References: <v03uh5$gbd5$1@dont-email.me> <v0ekid$1p5u$1@gal.iecc.com> <v0fln1$3iapn$1@dont-email.me> <Yr8YN.7355$qdt5.5890@fx35.iad>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Injection-Date: Tue, 30 Apr 2024 20:22:27 -0000 (UTC)
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970";
	logging-data="45426"; mail-complaints-to="abuse@iecc.com"
In-Reply-To: <v03uh5$gbd5$1@dont-email.me> <v0ekid$1p5u$1@gal.iecc.com> <v0fln1$3iapn$1@dont-email.me> <Yr8YN.7355$qdt5.5890@fx35.iad>
Cleverness: some
X-Newsreader: trn 4.0-test77 (Sep 1, 2010)
Originator: johnl@iecc.com (John Levine)
Bytes: 2238
Lines: 23

According to Scott Lurndal <slp53@pacbell.net>:
>>It seems that the /360 was the first machine which put many
>>registers into a (conceptually) separate space, leaving them open
>>to implementing them either in memory or as faster logic.
>
>PDP-8 had both (auto-increment index registers in memory) and
>the separate accumulator and link registers.

The auto-increment "registers" in the PDP-8 and its 18-bit cousins
were just regular memory locations with a hack that incremented them
when you used them as an indirect address. They weren't anything like
what we call index registers, since you couldn't combine them with
anything else to get an address.  (I speak from direct experience.)

Much later the 18-bit PDP-15 added a real index register, but there is
no way you could have done that on a PDP-8 since there was no spare
place in the instruction word to put an index bit. And by that time it
was clear that the PDP-8 was headed for niche process control
applications and oblivion while the PDP-11 was the future.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly