PIC Microcontoller Bit Math Method

Which bit is set?

Dmitry Kiryashov [zews at AHA.RU] says:

...to convert byte mask back into number. I suggest that somebody else probably has successfully researched this before. This is what I've discovered... It takes 12 clocks/words

;       W before -> W after
;       abcdefgh -> 00000ABC
;       10000000 -> 7 = 4+2+1
;       01000000 -> 6 = 4+2+0
;       00100000 -> 5 = 4+0+1
;       00010000 -> 4 = 4+0+0
;       00001000 -> 3 = 0+2+1
;       00000100 -> 2 = 0+2+0
;       00000010 -> 1 = 0+0+1
;       00000001 -> 0 = 0+0+0

        movwf   temp    ;save abcdefgh
        swapf   temp,W  ;efghabcd
        xorwf   temp,F  ;........
        andlw   0xF     ;0000abcd
        skpz
        movlw   4       ;load A=0/4

        btfss   temp,3
        btfsc   temp,2
        iorlw   2       ;add B=0/2

        btfss   temp,3
        btfsc   temp,1
        iorlw   1       ;add C=0/1
                        ;W holds 7..0 of result

Marc [marc at AARGH.FRANKEN.DE] says:

You can make it a table-read, and fill the gaps with other sub functions. Under the presumption that you can make use of any area of 3+ words in size, this method uses 6 cycles (incl call/ret) and 11 words.

        addwf   pc,f
        (1 wasted word)
        retlw   0
        retlw   1
        (1 wasted word)
        retlw   2
        (3 spare words)
        retlw   3
        (7 spare words)
        retlw   4
        (15 spare words)
        retlw   5
        ...

Code:

• Gonzalo Damian Barco shares this code:

  ;Gonzalo Barco
  ;A looped approach (a bit smaller and lot slower)
  ;In case you are sure there's at least one bit set you can loose the first bsf
  
  COMPACT4BIT	MACRO	FILE
  ;Returns bit 3 set on error (no bits set)
  	local	loop, tail
  	bsf	STATUS, C	;To be sure we don't looooooooooooop forever
  	clrw			;Clear counter
  
  loop:
  	rrf	FILE, F		;Send bits off the edge...
  	btfsc	STATUS, C	;...on to STATUS, C
  	goto 	tail		;If we hit the bit... stop
  	addlw	1		;Otherwise add to the counter...
  	goto	loop		;...and keep looping
  tail:
  	movwf	FILE		;Overwrite source with result
  	ENDM
  

  +

Comments:

• It's worth noting that these rquire that only one bit be set. If you have more than one bit set, the operation (value & -value) removes all bits except the lowest set bit - essentially a complement, addition of one, (two's complement) and then an and operation with the original value.
  
  -Adam Davis+

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