13.5.3 Flag Clearing
A flag is cleared by writing a one to the flag bit. Always use store or move instructions to write a one in
certain bit positions. Do not use the BSET instructions. Do not use any C-constructs that compile to BSET
instructions. “BSET flag_register, #mask” must not be used for flag clearing because BSET is a
read-modify-write instruction which writes back the “bit-wise or” of the flag_register and the mask into
the flag_register. BSET would clear all flag bits that were set, independent from the mask.
For example, to clear flag bit 0 use: MOVB #$01,PITTF.