Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!vette!brooks
From: brooks@vette.llnl.gov (Eugene Brooks)
Newsgroups: comp.arch
Subject: Re: Multi-Processor Serializability
Keywords: data ordering, coherence, shared memory multiprocessing
Message-ID: <19635@lll-winken.LLNL.GOV>
Date: 1 Feb 89 06:22:39 GMT
References: <3492@cloud9.Stratus.COM>
Sender: usenet@lll-winken.LLNL.GOV
Reply-To: brooks@maddog.llnl.gov.UUCP (Eugene Brooks)
Organization: Lawrence Livermore National Laboratory
Lines: 14

In article <3492@cloud9.Stratus.COM> tomc@cloud9.Stratus.COM (Tom Clark) writes:
>In  any computer system, the programmer expects operations in the source
>code to be carried out in the order specified.  This is especially  true
>in  a  multi-processor  system  where multiple processes may share data.

In ANSI C the tool to use to enforce reference order for reads and writes
is the keyword volatile, although I suppose that the purveyors of this keyword
did not have shared memory multiprocessors in mind when they inserted it into
the ANSI C definition.   If instruction reordering can be performed invisibly
by the system at lower levels than the compiler, the effect of these keywords
must trickle down into the hardware properly.  We use the volatile keyword
support in GNU C on our shared memory multiprocessors routinely in codes which
require certainly references to actually go out to main memory for
synchronization purposes.