Path: ...!news.iecc.com!.POSTED.news.iecc.com!not-for-mail
From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Upwards and downwards compatible, Computer architects leaving Intel...
Date: Tue, 17 Sep 2024 01:49:34 -0000 (UTC)
Organization: Taughannock Networks
Message-ID: <vcan7e$2i3c$1@gal.iecc.com>
References: <2024Aug30.161204@mips.complang.tuwien.ac.at> <vc8m2o$2n382$2@dont-email.me> <vc999o$2reaa$1@dont-email.me> <vca3lv$31g5l$2@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Injection-Date: Tue, 17 Sep 2024 01:49:34 -0000 (UTC)
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970";
	logging-data="84076"; mail-complaints-to="abuse@iecc.com"
In-Reply-To: <2024Aug30.161204@mips.complang.tuwien.ac.at> <vc8m2o$2n382$2@dont-email.me> <vc999o$2reaa$1@dont-email.me> <vca3lv$31g5l$2@dont-email.me>
Cleverness: some
X-Newsreader: trn 4.0-test77 (Sep 1, 2010)
Originator: johnl@iecc.com (John Levine)
Bytes: 2212
Lines: 21

According to Thomas Koenig  <tkoenig@netcologne.de>:
>> I have seen plenty of undefined behaviour in ISA's over the years.  (A 
>> very common case is that instruction encodings that are not specified 
>> are left as UB so that later extensions to the ISA can use them.)
>
>A much better idea is to raise an exception, that way you can
>be sure that nobody uses it for nefarious purposes.

That was one of the innovations of S/360.  Every unused opcode traps
with an operation exception.  This both avoided games with undocmented
features, and let them provide limited upward compatbility in software.
The 360/91 did not have the decimal instructions, but a module in OS/360
caught the traps and simulated them so programs would run, albeit slowly.

I also understand that they did a lot of development of early S/370
software on a version of CP/67 that trapped and simulated the new
instructions before the S/370 hardware was ready.

-- 
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