On Sunday, March 19, 2000 Scott Dattalo wrote:
> For a single byte there is a 14-cycle solution. But since it looks like your
> having fun, I wouldn't want to spoil anything. :)

I have more fun to learn from PICLIST members :)

> BTW, John Payson posted the original solution (for reversing 7 bits). Dmitry
> optimized it slightly (again for 7 bits). John's trick is applicable here as
> well.

> Hint, for the general case, it takes 4 cycles to swap 2 bits. However, it only
> takes 2 cycles for the first two bits.

> Scott

AFAIR, Dmitry used swap to place two bits in place, and then xor'd to
swap individual pairs of bits.

How about this:
;In =   abcdefgh
;Out =  hgfedcba

        rlf In, w       ;carry = a
        swapf In, f     ;In = efghabcd
        rlf In, w       ;w  = fghabcda  (-g---c-a), carry = e

        andlw 0xEF
        skpnc
        iorlw 0x10      ;w  = fghebcda  (-g-e-c-a)

        btfsc In, 6
        xorlw 0xA0
        btfsc In, 4
        xorlw 0xA0      ;w  = hgfebcda  (hgfe-c-a)

        btfsc In, 2
        xorlw 0x0A
        btfsc In, 0
        xorlw 0x0A      ;w  = hgfedcba

;14 instructions so far
;1 more to copy w to Out
        movwf Out

>From the practical point of view, it's better to have ability to
reassign pins in any order. I use pins masks always - it's easier to
route PCB then.

 Nikolai