Redwood Shores, 6 januari 2009.

Dear Harry,

They are using the ^-XOR to undo the modification they made to the source
variable in the previous xorlw. So the value in W is modified by the check,
and by adding the previous value to the next check using ^, it undoes this
damage while doing the next check at the same time.

I can imagine writing code like this, although with this amount of values
I'd probably opt for a jump table instead (if the memory was available). I
certainly would've added more comments about what was happening especially
in the beginning.

Greetings,
Maarten Hofman.

2009/1/6 Harry H. Arends <h.arends@home.nl>

> Part of code:
> ===============================================
> FindCV:
>    bcf     NOCV
>    bcf     RDONLY
>    xorlw   0x00                ; CV513
>    btfsc   STATUS,Z
>    retlw   PRIM_ADDR0
>    xorlw   (0x00 ^ 0x08)       ; CV521
>    btfsc   STATUS,Z
>    retlw   PRIM_ADDR1
>    xorlw   (0x08 ^ 0x1C)       ; CV541
>    btfsc   STATUS,Z
>    retlw   CVAR_29
>    xorlw   (0x1C ^ 0x02)       ; CV515
>    btfsc   STATUS,Z
>    retlw   MAX_BRIGHT
>    xorlw       (0x02 ^ 0x03)           ; CV516
>    btfsc       STATUS,Z
>    retlw       E_CV516
>    xorlw       (0x03 ^ 0x04)                   ; CV517
>    btfsc       STATUS,Z
>    retlw       E_CV517
>    xorlw       (0x04 ^ 0x05)                   ; CV518
>    btfsc       STATUS,Z
>    retlw       E_CV518
>    xorlw       (0x05 ^ 0x20)                   ; CV545
>    btfsc       STATUS,Z
>    retlw       E_CV545
>    xorlw       (0x20 ^ 0x21)                   ; CV546
>    btfsc       STATUS,Z
>    retlw       E_CV546
>    bsf         RDONLY
>    xorlw       (0x21 ^ 0x06)                   ; CV519
>    btfsc       STATUS,Z
>    retlw       MFG_VR_CODE
> ===================================================
> This routine is called with the actual CV value in w eg CV513=0x00 and
> CV519=0x06
>
> > -----Oorspronkelijk bericht-----
> > Van: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
> > Namens Olin Lathrop
> > Verzonden: dinsdag 6 januari 2009 22:07
> > Aan: Microcontroller discussion list - Public.
> > Onderwerp: Re: [PIC] xorlw question
>  >
> > Harry H. Arends wrote:
> > > Could someone explain this construction to me.
> > > I looked in the MPASM help file but cant find a description.
> > >
> > >     xorlw  (0x05 ^ 0x20)    ; CV545
> >
> > The "^" is documented in the manual, so I guess you're asking
> > why someone would do this?  To answer that, we need to see a
> > few more lines around this one.
> >
> >
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