> I currently have an application where in the comms receive
> handler, I test the received command byte and branch off
> to other places depending on the value. As the values are
> not consecutive, is there a better way to test them than just
> doing the following time after time?
>
> ...
> movf    rx_cmd,w        ;get the received command byte
> xorlw   msg_eg1 ;compare with msg_eg1 (a constant)
> btfsc   zero            ;is it msg_eg1 ?
> goto    rx_eg1          ;yes, jump to that handler
> ...
>
> Block repeats for as many test cases as I need.
> It just seems ugly - is there a better/shorter way?
> The msg constants are all over the place value wise.
> I can't arrange them to be in continuous blocks.

Do you have some program memory to spare?  If so, a jump table is probably
the simplest and also the clearest and easiest to maintain.  Warning, this
code was just typed in without being checked:

     movlw   high table   ;get high byte of table start address
     movwf   pclath       ;init jump address high byte
     movlw   low table    ;get low byte of table start address
     addwf   opcode, w    ;make low byte of table entry address
     skip_ncarr           ;no carry into upper address byte ?
     incf    pclath       ;propagate the carry
     movwf   pcl          ;jump to the selected table entry
;
;   Opcodes table.  There is one table entry for each of the 256
;   possible opcodes.  Each table entry jumps to the handler code
;   for that opcode.
;
;   NOTE:  All handler routines must be in the same code page as
;     this table.
;
table
     goto    handler0     ;opcode 0
     goto    handler1     ;opcode 1
     .
     .
     .


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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