# SXMicrocontroller Bit MathMethod

## Converting a 3-bit number to a 8-bit mask

From: Mike Keitz. There is a trick method that works for 4 bits. It replaces a 2-bit number in a variable with a 4-bit mask:

```        mov     W, ++BitP       ;W =    0001  0010  0011 0100
snb     BitP.1          ;If 0 or 1, result is almost correct now.
or      BitP, W         ;BitP =[0000][0001] 0011 0111
inc     BitP            ;BitP = 0001  0010  0100 1000
```

It could be the core of an 8-bit routine thus:

```; Convert 3-bit number (0-7) in INDEX to a 8-bit mask (00000001 ...
; 10000000) in BitP.
mov	W, INDEX
mov	BitP, W
mov	W, ++BitP	;The 4-bit converter
snb	BitP.1
or	BitP, W
inc	BitP
snb	INDEX.2	;Is it high 4 bits?
swap	BitP
```

#### via lookup table

7 cycles including the call. 10 words.
```two_to_n:
and	W, #\$7
retw	\$1
retw	\$2
retw	\$4
retw	\$8
retw	\$10
retw	\$20
retw	\$40
retw	\$80
```

#### Tricky

UNTESTED! 8 cycles, 8 words. Iso.
```	;build MASK from INDEX.1-3

mov	w,#1
sb	INDEX.2
mov	w,#4
sb	INDEX.3
sb	INDEX.1
```

Interested:

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