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From: keith@MIPS.com (Keith Garrett)
Newsgroups: comp.sys.mips
Subject: Re: R2000/3000 Pipeline Questions
Message-ID: <2866@spim.mips.COM>
Date: 1 May 91 04:17:50 GMT
References: <8179@polstra.UUCP>
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In article <8179@polstra.UUCP> jdp@polstra.UUCP (John Polstra) writes:
>I have a couple of arcane questions about the integer pipeline on the
>MIPS R2000 and R3000.
>
>1. In the following sequence of instructions, is register $4 guaranteed
>to end up with the value contained in memory location "bbb"?
>
>	lw	$4, aaa
>	lw	$4, bbb
yes
>
>2. In the following sequence of instructions, is register $4, guaranteed
>to end up containing zero (regardless of what is contained in memory
>location "aaa")?
>
>	lw	$4, aaa
>	move	$4, $0
yes
>
>I notice in each case that the MIPS assembler does not insert a NOP
>after the first load instruction.  Note: these examples will seem less
>stupid if you imagine that the first instruction is in the delay slot of
>a conditional branch.
the load delay only affects the availability of the data for the following
instruction. the writes to the register file occur in order.

lw   $4, aaa      IF   RD   ALU  MEM  WB
lw   $4, bbb           IF   RD   ALU  MEM  WB
move $4, $3                 IF   RD   ALU  MEM  WB
                           ^    ^    ^ ^  ^ ^    ^
                           |    |    | |  | |    |
                           |address required for 1st load
                           |    |    |data from 1st load available
                           |    |    | |data from 1st load in register
                           |    |address required for 2nd load
                           |    |    | |  |data from 2nd load available
                           |    |    | |  | |data from 2nd load in register
                           |    |    |data required for 3rd op
                           |    |    | |  |data from 3rd op available
                           |    |    | |  | |    |data from 3rd op in register
>
>Thanks -- enquiring minds want to know . . .
you're welcome. i hope this helps
-- 
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