PIC Microcontroller Bit Math Method

Testing bits in W

Dmitry Kiryashov says:

You can't apply btfss/btfsc operations to [the W register]. You need to put W value to some temp location first and finally check temp with btfss/btfsc commands.

HINTS:

1.  ANDLW   0x80 ; will test bit .7 in W, other bits are cleared ;-)
    ANDLW   0x40 ; test bit .6
   ;...
   

   result of operation in _Z flag (=0 means bit=1 otherwise bit=0 result to _Z=1)

   ANDLW is destructive command. You cannot restore W if you didn't save it before.

2.  ADDLW   0x88 ; will test bits .7 and .3
   

   results of operation in _C and _DC flags. To restore initial W just ADDLW -0x88 ;-)

3. You can also apply XORLW value command to check identity W to some value. It is useful command (sometimes) 'cause you can do comparison for alot of values and restore W after that to initial value.

   XORLW   A_CONST
   SKPNZ
   GOTO    A_CONST_MATCH
   
   XORLW   B_CONST^A_CONST ;compare with B_CONST
   SKPNZ
   GOTO    B_CONST_MATCH
   
   XORLW   C_CONST^B_CONST ;compare with C_CONST
   ;....
   
   XORLW   LAST_CONST ;last used const value -> restore initial W
   ;.....