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From: bjorn@alberta.UUCP (Bjorn R. Bjornsson)
Newsgroups: net.arch
Subject: Re: What I miss in micro-processors (fairly long)
Message-ID: <639@alberta.UUCP>
Date: Wed, 11-Sep-85 21:47:00 EDT
Article-I.D.: alberta.639
Posted: Wed Sep 11 21:47:00 1985
Date-Received: Thu, 12-Sep-85 02:42:36 EDT
References: <796@kuling.UUCP> <1713@orca.UUCP> <1517@umcp-cs.UUCP>
Organization: U. of Alberta, Edmonton, AB
Lines: 54

In article <1713@orca.UUCP> jans@orca.UUCP (Jan Steinman) writes
>.....................  If stack space-optimal, write a macro that
>sticks the PC in a register, then jumps.
>
>Guess what I'm really trying to say is that I don't see enough use for either
>of these things to justify the effort needed to put them in.

Gunnar's point (and rightly so) was that this takes more space and time
than a dedicated instruction.  A dedicated instruction may dramatically
shift the space/time tradeoff of inline expansion (greater space,
greater speed) versus local call (less space, less speed) in favor
of the latter.

>My favorite peeve is a rather restricted one, as yours are.  I write
>interpreters and would love a multi-way, in-line, absolute indexed branch.
>Both VAX and National came so close, but their CASE instructions are PC
>relative, and must be a constant distance from the routines (VAX) or jump
>table (National).  This means in-line coding of the routine dispatcher is not
>possible with this instruction.  It would be so nice to end each routine with
>a CASE (R7),TABLE and have control passed to the routine who's address is at
>TABLE+R7*size.

So your favourite peeve is more valid than Gunnar's, why don't you take
your own advice and:

	ashl	$2,r7,r7
	jmp	TABLE(r7)

You could even arrange things, such that the multiplication
of r7 by four is eliminated, and get away with:

	jmp	TABLE(r7)

In the VAX the location of the table is fixed with respect to the
case instruction, but the table contains displacements (ie relative
to pc/and table), whereas the the ns32000 case instruction is really
just a vanilla branch instruction that allows the full complement of
ns32000 addressing modes.

In article <1517@umcp-cs.UUCP> chris@umcp-cs.UUCP (Chris Torek) writes:
>> ...  It would be so nice to end each routine with a CASE (R7),TABLE
>> and have control passed to the routine who's address is at TABLE+R7*size.
>
>How about
>
>	movl	table[r7],pc
>
>or something like that?

"something like that", ie. register mode use of the pc, produces
unpredictable results on VAX processors.

		Bjorn R. Bjornsson
		ihnp4!alberta!bjorn