[dcbrown]

I think for the 12F, the code could be done as follows. There isn't a ADDWFC (add w/carry) as there is on the 18F so I suppose you just test the Carry bit in the status reg directly?). Anyway, I might have flipped the value of the carry bit but I believe it is asserted to 1 upon carry out and asserted to 0 for a borrow - but I might have them mixed up so make sure you test this before using it. Just one way, I'm sure there are lots of others.

Where a is at address 0x20/0x21
      b               0x22/0x23
      c               0x24/0x25

:                   c = a+b;
     MOVF   0x20, W           ; a.lo              -> W
     ADDWF  0x22, W           ; W + b.lo          -> W, set carry bit status
     MOVWF  0x24              ;                      W -> c.lo
     MOVF   0x21, W           ; a.hi              -> W
     BTFSC  STATUS,C          ; test carry bit in STATUS[0x03].<0>
     ADDLW  1,    W           ; W + Carry in bit  -> W
     ADDWF  0x23, W           ; W + b.hi          -> W
     MOVWF  0x25              ;                      W -> c.hi

:                  c = a-b;
     MOVF   0x20, W           ; a.lo              -> W
     SUBWF  0x22, W           ; W - b.lo          -> W, set borrow bit status
     MOVWF  0x24              ;                      W -> c.lo
     MOVF   0x21, W           ; a.hi              -> W
     BTFSS  STATUS,C          ; test carry bit in STATUS[0x03].<0>
     SUBLW  1,    W           ; W - Borrow        -> W
     SUBWF  0x23, W           ; b.hi - W          -> W
     MOVWF  0x25              ;                      W -> c.hi