Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!apple!mips!rogerk
From: rogerk@mips.COM (Roger B.A. Klorese)
Newsgroups: comp.arch
Subject: Re: test and set facility
Keywords: MIPS
Message-ID: <29566@servitude.mips.COM>
Date: 16 Oct 89 20:26:57 GMT
References: <200@rangkom.MY> <29463@obiwan.mips.COM> <18381@pasteur.Berkeley.EDU>
Reply-To: rogerk@mips.COM (Roger B.A. Klorese)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 29

In article <18381@pasteur.Berkeley.EDU> jas@postgres.berkeley.edu (Jim Shankland) writes:
>How do you do atomic test-and-set on a uniprocessor MIPS
>system, without atomicity being broken by preemption?  In kernel mode,
>presumably you can always disable interrupts, do the test, do the set,
>and re-enable interrupts.  But what about in user mode?  It's awfully
>handy if user processes sharing memory can do atomic t&s without a
>kernel call ....

In our current release, you don't (despite the following entry in our
documentation, which was entered too early).  But in our forthcoming
release, you would use the sysmips(MIPS_ATOMIC_SET,...) system call:


SYSMIPS(2-SysV)     RISC/os Reference Manual      SYSMIPS(2-SysV)
     
     When cmd is MIPS_ATOMIC_SET, two arguments are expected.
     The first argument should be the address of an int variable,
     and the second argument should be an int value.  This com-
     mand atomically sets the variable to the value supplied and
     returns the old value of the variable.  Note that if the old
     value of the variable may be -1, it is not possible to dis-
     tinguish that return from an error return, except by setting
     the errno variable to 0 before calling sysmips() and testing
     whether errno is non-zero on return.
-- 
ROGER B.A. KLORESE      MIPS Computer Systems, Inc.      phone: +1 408 720-2939
928 E. Arques Ave.  Sunnyvale, CA  94086                        rogerk@mips.COM
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"I want to live where it's always Saturday."  -- Guadalcanal Diary