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Re: [avr-gcc-list] swap bits

From: Theodore A. Roth
Subject: Re: [avr-gcc-list] swap bits
Date: Tue, 21 Jan 2003 10:00:00 -0800 (PST)

On Tue, 21 Jan 2003, Torsten Hahn wrote:

:) Hi,
:) iam looking for a way, to efficiently swap all bits in an register. Can
:) somebody help me with a little asm-macro or a pice of efficient c-code ?
:) It schould take an 8bit value as input an give back an value, where bi7
:) of the input is now the bit 0, bit6 is bit1 etc.

How about this? (input in r1):

        ldi     r0, 8
        ror     r1
        rol     r2
        dec     r0
        brne    loop
        mov     r1, r2    ; optional

The trick here is that the ror shifts the bits from the input into the
carry and the rol shifts the carry into output.

I don't think you can do this in a single register. If it can, I'd like to
hear from some assembly guru out there.

Ted Roth

avr-gcc-list at http://avr1.org

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