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From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: comp.arch
Subject: Re: Execute instructions
Message-ID: <4235@saturn.ucsc.edu>
Date: 23 Jul 88 05:58:54 GMT
References: <787@amethyst.ma.arizona.edu>
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Reply-To: haynes@ucscc.UCSC.EDU (Jim Haynes)
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Organization: California State Home for the Weird
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A number of machines have had the single execute instruction,
including some that had a conditional execute.  So far as I know
the XED is unique to the GE600 and its successors; in some sense
it's an artifact of the machine implementation.  That is, the
machine was implemented with 72-bit memory, so it always fetched
instructions in pairs, so it had a 2-instruction buffer to hold
them, so the XED could run out of that buffer.  (That was also
why there was a repeat double instruction).  Or one could say it's
a case where there isn't really an architecture because it is
intertwined with the implementation.

I've always wondered who was (were) the architects on that machine.


haynes@ucscc.ucsc.edu
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