Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!baum
From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment important?
Message-ID: <43342@apple.Apple.COM>
Date: 26 Jul 90 00:22:27 GMT
References: <104037@convex.convex.com> <8840016@hpfcso.HP.COM> <2370@crdos1.crd.ge.COM> <1990Jul25.223437.15301@mozart.amd.com>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 28

[]
>In article <1990Jul25.223437.15301@mozart.amd.com> davec@nucleus.amd.com (Dave Christie) writes:
>Note that there are two degrees of misalignment: 
>	1) within a word, and
>	2) crossing a word (& possible page) boundary. 
>
>For 1):
>If the realignment hardware is not in your main fetch path because it
>would impact your cycle time, then it will likely mean an extra stage
>of processing for instructions which use it, which can add various bits
>of complexity.  Considering that, plus
>	1) a 4-way mux isn't a serious time sink, and
>	2) how much, or even whether, it influences the cycle time is 
>	   technology and implementation dependent
...
>"standard" operating frequencies these days (e.g. 16,20,25,30,40,50);
>The difference that a 4-way mux might make would tend to be taken care

I've wondered about that. Getting naturally aligned operands requires a
4:1 mux on the low byte, a 2:1 on the next byte, and nothing on the
upper bytes. To get all alignments requires a 4:1 mux on all bytes. It
merely makes all bytes be just as bad as the low byte. In theory.
In practice, loading and wire delays probably have as much impact as logic.


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