N STEENKAMP [M.ING E&E] <PICLIST@MITVMA.MIT.EDU> wrote:

> I have been stuck now for at least 1/2 hour trying to figure out a
> way to conditionally complement the carry flag on an '84.  The best
> I could come up with was:
>
>    comf  STATUS, W
>    movwf Temp
>    btfsc My_Flags, Complement_Carry
>     rrf  Temp, W
>
> This just doesn't feel like the right way!
>
> I also tried:
>    movlw 01h
>    btfsc MyFlags, Complement_Carry
>     xorwf STATUS, F
>
> but it didn't work.  I was hoping that the xorwf instruction would
> operate as normal and change the carry flag and afterwards set the Z
> flag if the result was 0, but C is totally protected from being
> modified.

    Niki:

    Yes, you're right... It works like this:

    If you use an instruction whose destination is the status
    register, and if that instruction can modify the Carry, Zero, OR
    the Decimal-Carry flags, then the values that your instruction
    explicitly writes to ALL THREE of those bits are ignored and the
    Carry, Zero, and/or Decimal-Carry flags are set by  the
    processor as appropriate.

    When the PIC executes the "XORWF STATUS" instruction in your
    example code, the Zero, Carry, and Decimal-Carry flags are
    unchanged, then the Zero flag is adjusted.

    This behavior is fully documented in the data books.

> Has anyone got a better approach (preferably one that does not need
> a file register)?

    Yes:

        MOVF    STATUS,W
        BTFSC   Complement_Carry
        XORLW   1
        MOVWF   STATUS

    -Andy

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