HOME                                    ELECTRONICS                                8051

MULTIBYTE ADDITION / SUBTRACTION:

                .ORG 0H
                MOV R0,#30H
                MOV @R0,#0FFH
                INC R0
                MOV @R0,#0FFH
                INC R0
                MOV @R0,#0FFH
                INC R0
                MOV @R0,#0FFH

                MOV R1,#40H
                MOV @R1,#0FFH
                INC R1
                MOV @R1,#0FFH
                INC R1
                MOV @R1,#0FFH
                INC R1
                MOV @R1,#0FFH
                MOV R3,#04H
                CLR C
                MOV SP,#10H
BACK:       MOV A,@R0
                ADDC A,@R1
                PUSH 0E0H
                DEC R0
                DEC R1
                DJNZ R3,BACK
                JNC SKIP
                MOV A,#01H
                PUSH 0E0H
SKIP:       SJMP SKIP                ;result in memory location 15h-11h.

For subtraction replace ADD & ADDC by SUBB.