Re: Does anyone understand this...

[wsquires@lonestar.utsa.edu (William H. Squires)]

In article <1992Nov24.172459.41341@slate.mines.colorado.edu> gdesroch@slate.mines.colorado.edu (DESROCHERS GARY FREDERIC) writes:
>
>     I need to know what a
>
>BIT $CFFF
>
>does.  I don't need breif answers like 'It clears all I/O ROMS'
>or 'It switches out C8 ROMS.".  I can get these out of the IIc rom listing
>and II plus listing that I have.  What does it do?!?  The best answer would
>be as technical as you can give me or where I can get the answers.
>
>     I appreciate anything you can give.
>
>Gary
>gdesroch@slate.mines.colorado.edu
>
>--
>Gary F. Desrochers
>gdesroch@slate.mines.colorado.edu

  The BIT <operand> instruction tests bits in acc. against bits in <operand>.
If any set bits in <operand> match set bits in acc., then a BNE <somewhere>
will be taken, and a BEQ will not. This instruction is also used as a fake
LDA/STA instruction where you don't want to affect a memory location or the
accumulator. For example:

GETKEY  LDA     $C000
        BPL     GETKEY
        BIT     $C010
        ...

is sometimes found because the programmer simply wanted to address $C010,
but didn't want to store anything there. As for what $CFFF does, well, you
got me!
  Hope this helps!
                                wsquires@lonestar.utsa.edu