Hi guys,

Firstly, don't scream.. I HAVE read the datasheets!. My table reading
subroutine is based on the final example in AN556 "Implementing a Table
Read" and can be used across page boundaries and be located anywhere in
memory... BUT!... will not work if the table itself is longer than 256
bytes. This is because the table element offset is passed in w (8 bits) and
then used in the calculation for the program counter. Trying to access
elements that start beyond an offset of 256 bytes won't work.

I could make a new separate table, but would prefer to keep it all together.
Based on the app note from Microchip, I can't see a simple way to do this
with one table. This particular example (example 5 in AN556) is already
complicated, and would be a mess if I try to make it cater for such things.

Has anyone posted a lookup table reader that can access defined elements in
a table longer than 256 bytes?. Weird thing is that AN556 does not
specifically warn of this. Maybe I'm doing something wrong... Hopefully one
of you can help.

My application is listed in part below. Firstly, the top of the table looks
like this. I have the modem strings arranged with the string length first,
then the string. This is the best way for this particular application.

;**********************************************************
;* Data tables
;**********************************************************
command
        movwf   PCL                                     ;Update PCL with computed value
table_base                                              ;Table start reference
off_hook
        dt      0x07,"AATDT;",0x0d
hang_up
        dt      0x05,"AATH",0x0d
modem_reset
        dt      0x05,"AATZ",0x0d
dummy_dial
        dt      0x17,"AATM2S07=48DT,xxxxxxxx",0x0d
;
You can call up one of these modem strings by simply doing this...

        movlw   modem_reset-table_base          ;Calculate table offset for reset string
        call    send_command
;
And the big daddy of a table reader/modem string sender...

;*********************************************************
;* Send modem command string - called with table offset in w
;*********************************************************
send_command
        movwf   table_offset            ;Save table offset
        movlw   LOW table_base          ;Get low 8 bits of table address
        addwf   table_offset,f          ;8-bit add to get abs location of string, could
overflow...
        movlw   HIGH table_base         ;Get high 5 bits of table address
        btfsc   STATUS,C                        ;Did that last add cross page boundary?
        addlw   1                               ;Yes, roll over into next page
        movwf   PCLATH                  ;Preload high address
        movf    table_offset,w          ;Update calculated offset
get_length
        call    command                 ;Get first byte (string length)
        movwf   msg_length                      ;Save message length
send_next
        movlw   1                               ;Increment table offset... carefully
        addwf   table_offset,f
        movlw   HIGH table_base
        btfsc   STATUS,C                        ;Crossed page boundary when incremented?
        addlw   1                               ;Roll over into next page
        movwf   PCLATH                  ;Preload high address
        movf    table_offset,w          ;Update offset
        call    command                 ;Jump to table with offset in w...
loop_com
        btfss   PIR1,TXIF                       ;Check if txreg empty
        goto    loop_com                        ;Loop until free
;
wait_CTS                                        ;Hardware flow control. CTS is normally LOW
        btfsc   PORTB,CTS                       ;Are we clear to send?
        goto    wait_CTS                        ;If CTS = HIGH, modem is busy
;
gie_tx
        bcf     INTCON,GIE                      ;Disable interrupts, dammit!
        btfsc   INTCON,GIE
        goto    gie_tx

        movwf   TXREG                           ;Send byte to modem
        bsf     INTCON,GIE                      ;Re-enable interrupts
        decfsz  msg_length,f    ;Decrement length, if not zero
        goto    send_next                       ;Keep going...
        return