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I think the problem is with the instruction "ADDWF sum, sum".
ADDWF adds the value in the instruction to its existing value and either stores the result in W or back in the source register. The source register can be defined as '0' to store the value in W or '1' to store it in the source register but not as 'sum' which is an address value.
Try changing it to "ADDWF sum,1" or "ADDWF sum,f" and see if that fixes it.
Brian.