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From: josh@polaris.UUCP (Josh Knight)
Newsgroups: net.arch
Subject: Re:  Memory Rules of Thumb (really IBM 370 wait state)
Message-ID: <306@polaris.UUCP>
Date: Sun, 24-Nov-85 16:38:29 EST
Article-I.D.: polaris.306
Posted: Sun Nov 24 16:38:29 1985
Date-Received: Fri, 29-Nov-85 08:04:04 EST
References: <794@ecsvax.UUCP>
Reply-To: josh@polaris.UUCP (Josh Knight)
Distribution: net
Organization: IBM Research, Yorktown Heights, N.Y.
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In article <794@ecsvax.UUCP> hes@ecsvax.UUCP (Henry Schaffer) writes:
>
>  Note that IBM mainframes have a "wait light" which goes on when the
>cpu can't do anything because e.g., it is waiting for the completion of
>I/O.

IBM 370's have a "wait state" that the operating system can use to wait for 
interrupts rather than a "busy wait loop" if it wants to.  On the other hand
a "busy wait loop" with interrupts enabled will work just as well on a 370 as
any other architecture.  Older machines do indeed have a light or meter that
purports to measure the fraction of time that the machine is in wait state.
With a 2 MIPS system (I'm thinking of a 370/168 here) and incandescent lights,
however, exactly what it means is open to question.  Newer machines have a
"system activity display" (I think that's what it's called) that displays
the fraction of time spent in wait state, among other things.

Any opinions (expressed or implied) or errors are mine and not my employers.

-- 

		Josh Knight, IBM T.J. Watson Research
    josh at YKTVMH on BITNET, josh.yktvmh@ibm-sj on CSnet,
    ...!philabs!polaris!josh