Path: utzoo!attcan!uunet!lll-winken!ames!pasteur!ucbvax!decwrl!pyramid!oracle!hqpyr1!csimmons
From: csimmons@hqpyr1.oracle.UUCP (Charles Simmons)
Newsgroups: comp.arch
Subject: MIPS Architechture Question
Keywords: read-modify-write?  What's that?
Message-ID: <557@oracle.UUCP>
Date: 13 Jan 89 04:03:13 GMT
Sender: news@oracle.uucp
Lines: 38

So, I'm sitting around one day porting some code to a MIPS R2000
processor, and suddenly I realize that I need a "test-and-set"
instruction.  Soon thereafter, I realize that the MIPS R2000
doesn't seem to have anything remotely resembling a read-modify-write
instruction.  (For my application, I can't inhibit interrupts,
because I'm running in user mode.)

So...  a few questions arise.  Some people around here find
it hard to believe that a recently designed architechture doesn't
have instructions for multiprocessor synchronization.  Of course,
these same people refuse to use any hardware that doesn't have
at least 16 processors.

For myself, I kind of started wondering what would happen
if I started up a multiply instruction, and then did a
load/nop/add/store squence of instructions while waiting for
the multiply to complete.

I looked into my reference manual (_MIPS R2000 RISC Architechture_
by Gerry Kane), and noticed that this small area wasn't talked
about real well.  So...  What happens when the R2000 is executing
a multiple cycle instruction (i.e. multiply or divide), and an
interrupt, system call, or page fault occurs?   Hmmm...  I suppose
the simplest implementation would be to do nothing special, and
let the multiply or divide complete while the current context is
being saved.  Most likely, the multiply or divide will complete
before getting around to saving HI and LO.

So, I guess my questions are:

1)  How much did the MIPS architechts worry about multiprocessor
configurations, and why did they decide not to implement some
sort of synchronization primitive?

2)  Is there any real cute kludge anyone can suggest for faking
something that would act like an interlocked instruction?

Thanks, Chuck