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From: mohta@necom830.cc.titech.ac.jp (Masataka Ohta)
Newsgroups: comp.arch
Subject: Re: endian etc
Message-ID: <186@titccy.cc.titech.ac.jp>
Date: 14 May 91 14:16:59 GMT
References: <3407@spim.mips.COM> <166@armltd.uucp>
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Distribution: comp
Organization: Tokyo Institute of Technology
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In article <166@armltd.uucp> abaum (Allen Baum) writes:

>I've thought about this a bit, and I'm not sure its true. All the byte lane
>switching hardware already exists for the low order byte;

Yes.

>The upper halfword
>doesn't get mucked with at all, except for getting sign extended or cleared.

And, thus, multiplexers already exist for the high order bytes.

>So, the critical path already exists. The muxing isn't terribly symmetrical-
>all the work goes into the LS byte, almost none into the MS byte. That means
>the layout probably has holes in it, which could be filled with the rest of
>the byte lane logic, at (here I'm speculating some) no extra cost in space, or
>time.

The possible problem is that, if things going to 64 bit, it will become a
little more complex.

>By the way, note that while the buffering, etc. can go on in parallel, if
>sign extension is required, it can't - you can have muxes set up for sign
>selection, but you have to wait until it gets their, buffer it so it can
>drive 24 loads, and then stick it into all those upper bit positions. 

With 32 bit word, 4 to 1 MUX is enough for the upper bit posisions, including
sign-extended/non-sign-extended half-word/byte loading, which is not so
different from the current 3 to 1 MUXing.

>As a sweeping generalization, the path from cache to registers/forwarding
>path is THE critical path (if it isn't, you've probably done the design
>wrong, or have a CISC architecture)

No. THE critical path is on register/ALU loop, which determines the maximuum
clock speed (see Jouppi). Transfer between cache and a register involves
TLB look up and cache access requiring a little more time. Thus, if it
requires 2.5 (or 1.5 or 2.3 or...) clock cycles, it in not on the critical
path.

							Masataka Ohta