;adding two 16-bit signed numbers, the first is stored at $10 and $11, the second at $12 and $13.
;The result will be stored at $14 and $15.

;add the low bytes

LDA $10              ;low byte of first operand
CLC
ADC $12              ;low byte of second operand
STA $14              ;low byte of sum

;add the high bytes

LDA $11              ;high byte of first operand
ADC $13              ;high byte of second operand
STA $15              ;high byte of result
BVS HandleOverflow   ;only check for overflow when adding the most significant bytes.