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Re: [avr-gcc-list] swap bits

From: Klaus Rudolph
Subject: Re: [avr-gcc-list] swap bits
Date: Tue, 21 Jan 2003 18:17:50 +0100

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.
> I have no idea how to to this very well.


        push r17                2 cycles (internal ram)
        push r18                2 cycles (internal ram)

R16 value to transfer
R17 result
R18 counter

        ldi r18, 0x08           1 cycle
        ror r16                 8 cycles
        rol r17                 8 cycles
        dec r18                 8 cycles
        brne loop               17 cycles

        mov r16, r17            1 cycle
        pop r18                 2 cycles (internal ram)
        pop r17                 2 cycles (internal ram)

51 cycles total without wasting any register.
10 words flash.
51 cycles is very expensive. Any better ideas here :-)

Best asm contest is open now....

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