# PICMicrocontoller 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:

```        incf    BitP, W  ;W = 0001 0010 0011 0100
btfsc   BitP, 1  ;If 0 or 1, result is almost correct now.
iorwf   BitP, F  ;BitP = [0000] [0001] 0011 0111
incf    BitP, F  ;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.
movfw   INDEX
movwf   BitP
incf    BitP, W         ;The 4-bit converter
btfsc   BitP, 1
iorwf   BitP, F
incf    BitP, F
btfsc   INDEX, 2        ;Is it high 4 bits?
swapf   BitP, F
```

Code:

• mark_jeronimus-gmail- shares this code:
```Still, a simple table lookup or 8-bits is less instruction cycles and doesn't need a temp register.

And it can do a little math too (set bit 7-WREG instead of bit WREG)

GET_BIT
RETLW	B'00000001'
RETLW	B'00000010'
RETLW	B'00000100'
RETLW	B'00001000'
RETLW	B'00010000'
RETLW	B'00100000'
RETLW	B'01000000'
RETLW	B'10000000'

GET_BIT2
RETLW	B'10000000'
RETLW	B'01000000'
RETLW	B'00100000'
RETLW	B'00010000'
RETLW	B'00001000'
RETLW	B'00000100'
RETLW	B'00000010'
RETLW	B'00000001'
```
+

 file: /Techref/microchip/math/bit/mask.htm, 2KB, , updated: 2008/10/8 04:23, local time: 2022/1/18 11:56, TOP NEW HELP FIND:  54.91.62.236:LOG IN

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