help with my calculator project

I am currently working on a calculator project using the PIC16F84 and I need help with the overflow error detection when adding two 16 bit numbers

Also I am facing problems with the multiplying of two 16 bit numbers

To determine if the addition has overflowed... If the result is smaller than either of the original numbers, then it overflowed.

Code:

---

V1      VAR  WORD
V2      VAR  WORD
Result  VAR  WORD

Result = V1 + V2
if Result < V1 then OverFlowed

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What problem are you having with Multiplying???

Darrel