# PICMicrocontoller Bit MathMethod

## 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

btfss   temp,3
btfsc   temp,2

btfss   temp,3
btfsc   temp,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
goto	loop		;...and keep looping
tail:
movwf	FILE		;Overwrite source with result
ENDM
```
+

• 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.

 file: /Techref/microchip/math/bit/whatbit.htm, 3KB, , updated: 2009/4/16 13:48, local time: 2024/2/21 13:05, TOP NEW HELP FIND:  34.236.134.129:LOG IN

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