Eric Sosman wrote:[color=blue]
>
slebetman@yahoo.com wrote:[color=green]
>>
>> Personally I don't know of any computer ever constructed in the
>> history of computing that have an instruction that reverses the
>> bits within a byte/word like what you want. Also I don't know of
>> any programming language in the history of computing that has
>> built in functions to do what you want. So you'll have to do it
>> yourself regardless of what programming language or CPU you use.[/color]
>
> Machines with special features to help with Fourier
> transforms (digital signal processors and the like) have,
> I'm told, addressing modes where array indices are bit-
> reversed. Thus, a loop that runs from "0 to 7" would
> access elements 0, 4, 2, 6, 1, 5, 3, and 7, in that order.
> I don't know whether such machines can deliver the bit-
> reversed index as an explicit value, or whether the reversal
> remains hidden within the addressing circuitry. Nor do I
> know how the capability is usually exposed to programmers.
> Nonetheless, the bit-reversal action is certainly present.
>
> The only circumstance I've seen where bit-reversal is
> called for is when you're rearranging the transformed values
> after the Fast Fourier Transform's "butterfly" steps have
> scrambled them (or pre-scrambling the inputs beforehand,
> depending on how the FFT is set up). So when someone asks
> about bit-reversal I immediately suspect he's doing FFT's --
> and if he's doing FFT's, there's at least a chance he's
> doing them on a machine with special capabilities, and those
> capabilities may even be exposed to C. Not as an integral
> part of the language, of course, but as an extension or a
> set of extra functions or something of the kind. If it's
> there, the O.P. will need to search the documentation for it.[/color]
Way back when my 8080 based embedded machines provided for it with
a parallel input/output port. All it took was a jumper plug with 8
wires, and some byte swapping software. Turned out I never needed
to do a FFT, and the provision never was used for that.
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