Doesn't it take _no_ gates to reverse a word, no matter how long it is? Just a bunch of wires. On Sun, Apr 20, 2014 at 10:38 AM, Joerg Arndt <arndt@jjj.de> wrote:
* Gareth McCaughan <gareth.mccaughan@pobox.com> [Apr 20. 2014 16:25]:
[...]
(Perhaps it might, for FFT-related reasons. But if no major CPU architecture has ever had a bit-reversal operation -- is that true? -- then it seems pretty likely that at least once someone's given careful thought to whether it would be worth including and decided not.)
I do not know how many gates it takes for revbin, but speculate it would be very cheap (certainly cheaper than shift/rotate by [arg] positions), as you'd need just N/2 "swap bit pair" mechanisms (where N = word length).
There is another _very_ good reason for revbin: quite a few tricks in use the fact that the adder propagates "instantaneously" through arbitrarily man positions. I'll show a very neat trick in another message that works in _one_ direction.
Best, jj
-- g
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