# SXMicrocontroller 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

mov	temp, W	;save abcdefgh
mov	W, <>temp	;efghabcd
xor	temp, W	;........
and	W, #\$F		;0000abcd
sb	Z

sb	temp.3
snb	temp.2

sb	temp.3
snb	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.
```	add	pc, W
(1 wasted word)
retw	#0
retw	#1
(1 wasted word)
retw	#2
(3 spare words)
retw	#3
(7 spare words)
retw	#4
(15 spare words)
retw	#5
...
```

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