Hi,
If datalines is configured as an output Port which I assume it is, then what
you write to the port is not necessarily what you read from the port. There
are different set of latches at the port pins. This probably justifies the
comment

>I have this (probably silly) "thing" about writing to a register and
>reading from it on the very next instruction.

I agree using the xorlw 0xFF will remove at least one source of error

ie

>     call    mess2    ;get the character
>     movwf    datalines    ;send it to lcd
>     ;         housekeeping for LCD snipped
>     comf    datalines,0    ;end of table has 0xff marker, so I complement
it
>     btfsc status, z    ;complemented character = 0?

becomes

        call    mess2                   ;get the character
        movwf    datalines      ;send it to lcd
        ;         housekeeping for LCD snipped
        xorlw     0xFF                  ;complement the character using xorlw
        btfsc status, z         ;complemented character = 0?

Regards,
James Caska
caska@virtualbreadboard.com
ujVM - 'The worlds smallest java virtual machine'





-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Tony Nixon
Sent: Monday, 8 October 2001 9:31 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: zero flag after comf


Doug Hemingway wrote:
>
> I am creating a general macro for sending messages to an LCD display, and
> have run into a problem.
>
> Here's my code:
>
>     clrf    counter
> disloop    incf    counter,1    ;get next character position from table
>     movf    counter,0    ;put char position into W
>     call    mess2    ;get the character
>     movwf    datalines    ;send it to lcd
>     ;         housekeeping for LCD snipped
>     comf    datalines,0    ;end of table has 0xff marker, so I complement
it
> and check for zero
>     btfsc status, z    ;complemented character = 0?
>     goto    exitaddr    ;yes, done
>     goto    disloop    ;no, not done, get next character.
> exitaddr                ;continue on
> mess2    addwf    pc,1
>     retlw    'H'
>     retlw    'I'
>     retlw    0xff
>
> The code does not take the branch to exit.  As I read the spec sheet, the
> comf command should set the z flag if the result is zero (ie, datalines
> contained 0xff before the comf command).
>
> What am I doing wrong?


Even though it would make no difference, I would change...

  comf datalines,0

to..

  xorlw 0xFF

I have this (probably silly) "thing" about writing to a register and
reading from it on the very next instruction.

Question Time :-)
Where is the "mess2" jump table located in memory??
Is the PCLATH set up to handle it?
Is it crossing a page boundary?
Why do you want this as a MACRO? Wouldn't it be better as a subroutine?
Can you simulate it in MPLAB and see if the subroutine does in fact
return with 0xFF?

--
Best regards

Tony

mICros
http://www.bubblesoftonline.com
mailto:sales@bubblesoftonline.com

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