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From: robertsw@gtephx.UUCP (Wild Rider)
Newsgroups: comp.arch
Subject: Re: Compilers and efficiency
Message-ID: <336@scorpio.gtephx.UUCP>
Date: 17 Apr 91 20:32:48 GMT
References: <7117@auspex.auspex.com> <1406@ncis.tis.llnl.gov> <1991Apr12.001324.5@ux1.cso.uiuc.edu>
Reply-To: robertsw@scorpio.UUCP (Wild Rider)
Organization: AG (formerly GTE) Comm. Sys., Phx, AZ
Lines: 32


	In article <1991Apr12.001324.5@ux1.cso.uiuc.edu>
	msp33327@uxa.cso.uiuc.edu (Michael S. Pereckas) writes:

>The problem is that you will have to include those instructions in the
>next version of your processor in order to remain binary compatible,
>and if things work out differently, as they probably will, they may be
>hard to implement.  That happens in microcoded machines: we have some
>extra microcode store space, lets add some instructions.  Next
>version, you have to work like hell to cram all those instructions in
>because things worked out differently, and probably almost no one uses
>those instructions anyway.

	or, you could do like motorola did when they designed the 030:
	dump a couple of unused 020 instructions, i.e., the "callm"
	("call module") and the corresponding "rtm" ("return from
	module") instructions.  yes, that's correct, you can have an
	executable which runs _only_ on the 020, if it uses those
	instructions.  i guess motorola called up all their major 020
	customers and asked them: "do you use the callm/rtm
	instructions?"  apparently, the answer was "no" from every
	customer so motorola dumped the 2 unused instructions.  i just
	dug out my "mc68020 user's manual" and looked up the "callm"
	and "rtm" instructions... yuk.  no wonder they dumped them.
	talk about "complex instructions" ... anyway, i'll bet moto's
	designers freed up quite a bit of microcode real estate when
	they released those 2 losers.
-- 
Wallace Roberts, AG (formerly GTE) Communication Systems, Phoenix, AZ
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