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Re: Need XOR solution
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Re: Need XOR solution


  • Subject: Re: Need XOR solution
  • From: email@hidden (Michael Sullivan)
  • Date: Thu, 8 Aug 2002 12:06:43 -0400
  • Organization: Society for the Incurably Pompous

Marc K. Myers writes:

> That's perfectly clear, with just the needed depth of explanation. I
> understood the comparison of the modulated numbers as the actual
> extraction of the XORed result, but the addition of 256 when it was true
> threw me. Your explanation of the use of the extra left-hand bit and
> how it rotates back to the rightmost position cleared that up for me.
> You think like an assembly language programmer. Thanks, Nigel.

Heh. As someone who's done some assembly a very long time ago, it's
absolutely galling that this requires 25 AS numeric instructions (17
with the optimization Nigel mentions later) and a loop construct to do
something that works out to only 1/4 of a processor instruction. (1 XOR
and 3 overhead instructions to get the info in and out of the registers
-- since a G4 has 128 bit words you could be doing 16 bytes at a time
that way, so a single 8-bit byte takes only 1/4 of a processor
instruction).

But given AS limitations (no bitwise operations), it's definitely the
way an assembly programmer would solve the problem. I would've used
pretty much the same algorithm, but N.G. beat me to it.


Michael

--
Michael Sullivan
Business Card Express of CT Thermographers to the Trade
Cheshire, CT email@hidden
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 >Re: Need XOR solution (From: "Marc K. Myers" <email@hidden>)

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