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From: keith@mips.com (Keith Garrett)
Newsgroups: comp.arch
Subject: addressing on Mips (Was: 64 Bit addressing on R4000?)
Message-ID: <5240@spim.mips.COM>
Date: 29 Jun 91 18:26:51 GMT
References: <14900025@hpdmd48.boi.hp.com> <880@taniwha.UUCP> <MEISSNER.91Jun28105156@tiktok.osf.org>
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[discussion of possible differences between 32 & 64 bit instructions deleted]
In article <MEISSNER.91Jun28105156@tiktok.osf.org> meissner@osf.org (Michael Meissner) writes:
>The other thing I've thought about is how do you do generalized
>external references.  On the current R[236]000's a normal (non-GP)
>reference is two instructions:
>
>	lui	$at,addr>>16
>	<lw/sw>	<reg>,addr&0xffff($at)
this won't work in the general case. the offset is sign-extended before
it is added to the base register contents. here is the general solution:
	lui	$at,(addr>>16)&0xffff
	ori	$at,addr&0xffff
	<lw/sw>	<reg>,0($at)
>
>I wonder whether we will need four instructions in the 64 bit
>universe.
how about:
	lui	$at,(addr>>48)&0xffff
	ori	$at,(addr>>32)&0xffff
	sll	$at,16
	ori	$at,(addr>>16)&0xffff
	sll	$at,16
	ori	$at,addr&0xffff
	<lw/sw>	<reg>,0($at)
there may be a better solution for this, that i haven't considered.
an alternative for either case:
	<lw/ld>	$at,offset($gp)
	<lw/sw>	<reg>,0($at)
this requires an extra memory reference, but is probably an overall win for
64 bits, and is OK for 32 bits.
-- 
Keith Garrett      "This is *MY* opinion, OBVIOUSLY"         (408) 524-8110
   Mips Computer Systems, 930 Arques Ave MS 1-02, Sunnyvale, Ca. 94088-3650
   "Beware of the half-truth - you may have gotten hold of the wrong half."
   keith@mips.com -or- {ames,decwrl,prls}!mips!keith     -Rabbi Seymour Essrog