playground:reversing_bits_in_a_byte
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If you quickly need to flip the bits in a byte in reverse (turning bits from 01234567 to 76543210) you can use this unrolled loop.
ldx #0 .for(var i=0;i<8;i++) { lsr // shift A down, bit 0 to C tay // copy to Y doesn't change C txa // pull x to a, doesn't change C rol // shift left, C to bit 0 tax // stash a in x tya // get start a back from y } txa
(kickassembler loop syntax)
This takes a byte in A, reverses the bits and exits with the reversed bits in A again, using X and Y for temporary storage and only short 2-cycle instructions in the loop. Of course a table lookup will be faster if you do this a lot in your code.
– this would be equally fast, at 100cycl +rts (enter with the value in .A, result in .A, .X will be 0)
ldx #8 loop asl ror $2 dex bne loop lda $2 rts
or unrolled (56 cycl +rts):
asl ror $2 asl ror $2 asl ror $2 asl ror $2 asl ror $2 asl ror $2 asl ror $2 asl lda $2 ror rts
playground/reversing_bits_in_a_byte.1582393213.txt.gz · Last modified: 2020-02-22 18:40 by verz