Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Sat, 12 Oct 2024 08:45:57 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
Lines: 47
Message-ID: <2024Oct12.104557@mips.complang.tuwien.ac.at>
References: <vbd6b9$g147$1@dont-email.me> <09ce1622b872f0b0fa944e868a8c97be@www.novabbs.org> <vbnisc$2hb59$1@dont-email.me> <2024Sep10.094353@mips.complang.tuwien.ac.at> <vckf9d$178f2$1@dont-email.me> <O2DHO.184073$kxD8.113118@fx11.iad> <e290e18a59651f93e4b46f4839713b1c@www.novabbs.org> <dfXIO.169387$1m96.107616@fx15.iad>
Injection-Date: Sat, 12 Oct 2024 11:12:29 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="29d5d06e203f002c66a9502ffb11d396";
	logging-data="109941"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18HtZgECMl7/UmasXUUnFo/"
Cancel-Lock: sha1:WZvTBdkvlq3h3gPdgyWIcyhKykk=
X-newsreader: xrn 10.11
Bytes: 3497

EricP <ThatWouldBeTelling@thevillage.com> writes:
>But then, risc processors mostly, started using exceptions for housekeeping
>- SPARC for register window sliding, Alpha for byte, word and misaligned
>memory access

On Alpha the assembler expands byte, word and unaligned access
mnemonics into sequences of machine instructions; if you compile for
BWX extensions, byte and word mnemonics get compiled into BWX
instructions.  If the machine does not have the BWX extensions and it
encounters a BWX instruction, the result is an illegal instruction
signal at least on Linux.  This terminates your typical program, so
it's not at all frequent.

Concerning unaligned accesses, if you use a load or store that
requires alignment, Digital OSF/1 (and the later versions with various
names) by default produced a signal rather than fixing it up, so again
programs are typically terminated, and the exception is not at all
frequent.  There is a system call and a tool (uac) that allows telling
the OS to fix up unaligned accesses, but it played no role in my
experience while I was still using Digital OSF/1 (including it's
successors).

On Linux the default behaviour was to fix up the unaligned accesses
and to log that in the system log.  There were a few such messages in
the log per day, so that obviously was not a frequent occurence,
either.  I wrote a program that allowed me to change the behaviour
<https://www.complang.tuwien.ac.at/anton/uace.c>, mainly because I
wanted to get a signal when an unaligned access happens.

As for the unaligned-access mnemonics, these were obviously barely
used: I found that gas generates wrong code for ustq several years
after Alpha was introduced, so obviously no software running under
Linux has used this mnemonic.

>The solution for Alpha was to add back the byte and word instructions,
>and add misaligned access support to all memory ops.

Alpha added BWX instructions, but not because it had used trapping to
emulate them earlier; Old or portable binaries continued to use
instruction sequences.  Alpha traps when you do, e.g., an unaligned
ldq in all Alpha implementations I have had contact with (up to a
800MHz 21264B).

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>