Nicola Perotto wrote:
> michael brown wrote:
>> wouter van ooijen wrote:
>>
>>>>> Note that this code still requires that all jump table
>>>>>
>>>> entries are in
>>>>
>>>>> the same 256-instructions page. For a jumptable that crosses a
>>>>> page boundary (hence for all >256 entries tables) you will have to
>>>>> calculate the value to load into PCLATH.
>>>>>
>>>> Unless I'm really missing something here, I don't understand
>>>> why you say
>>>> that.  His technique takes into consideration that a page is being
>>>> crossed and appropriately increments PCLATH by 1 when that is
>>>> happening.
>>>>
>>> Sorry, you are right. I did not look well enough.
>>>
> You ave anymore got a point: the jump table must be less than 256
> byte... I have not considered this...
> Bigger tables needs a little more complex code but I think that is not
> the case of the OP

I guess it goes without saying that tables greater than 8 bits in size 
will require special handling since the accessable part of PCL is only 8 
bits.

>> I don't know why Microchip doesn't tout doing it this way.  It's
>> only a
>> few instructions more and makes the table floatable.    Here is how I
>> did it for a character table on a 16F88:
>>
>>get_msg_char
>> movfw       msg_ptr             ; get desired offset from beginning 
>> of string
>> btfss       PORTA,   DIRECTION    ; Check direction
>> sublw  message_size - 1    ; When going CCW, we must fetch letters in 
>> reverse order
>> addlw       3
>> addwf      PCL, W              ; Add offset and current PCL
>> skpnc                           ; Check for page crossing
>> incf       PCLATH, F           ;  and compensate for it
>> movwf       PCL                 ; Fetch the value
>> message         dt "MESSAGE TEXT GOES HERE"
>> message_size    equ     $ - message
>>
>> I intentionally forced the text to cross a page boundary and all was
>> well.  The 3 being added is for the skpnc, incf and movwf
>> instructions.
>>
>>
> Your code works but i dont like it:
> the ADDLW 3 is in the category of dangeours tricks :-)
> just reordering the instructions you can eliminate it and have a more
> readable code

Yeah, it looks like the "ADDLW 3" and the "ADDWF PCL, W" should be 
replaced by an "ADDLW message".  Must have been tired.  BTW in case 
anyone was wondering, I set up PCLATH before calling get_msg_char. 

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