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Editorial Team
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Asked: 2026-06-08T06:23:42+00:00

.MODEL SMALL .STACK 1000 .DATA MSGA DB 13,10,Input first number: ,$ MSGB DB 13,10,Input

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.MODEL SMALL
.STACK 1000
.DATA

MSGA DB 13,10,"Input first number: ","$"
MSGB DB 13,10,"Input second number:","$"
MSGC DB 13,10,"The sum is: ","$"

NUM1 db ?
NUM2 db ?
NUM3 db ?

.CODE

MAIN PROC NEAR

MOV AX, @DATA
MOV DS, AX

; get first number
LEA DX, MSGA
MOV AH, 09h
INT 21h

MOV AH, 01
INT 21H
SUB AL, '0'

MOV BL, AL

MOV AH, 01
INT 21H
SUB AL, '0'

MOV CL, AL

; get second number
LEA DX, MSGB
MOV AH, 09h
INT 21h

MOV AH, 01
INT 21H
SUB AL, '0'

MOV DL, AL

MOV AH, 01
INT 21H
SUB AL, '0'

MOV DH, AL

; add
ADD CL, DH 
ADC BL, DL
MOV NUM1, CL
ADD NUM1, '0'
MOV NUM2, BL
ADD NUM2, '0'

; output sum
LEA DX, MSGC
MOV AH, 09h
INT 21h

MOV DL, NUM2
MOV AH, 02H
INT 21h

MOV DL, NUM1
MOV AH, 02H
INT 21h

MOV AH, 4Ch
INT 21h

MAIN ENDP
END MAIN

Above is my code for adding 2 two-digit numbers in assembly. I wonder why the ADC doesn’t work. If the ones digits don’t obtain a carry when added, my code works. But not otherwise. Did I misunderstand what the ADC really does? What should I do with my code?

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1 Answer

  1. Editorial Team

    You appear to be working with decimal math, but you’re not using AAA. ADC does what you’d expect, but with that code there is never an actual carry from the first addition (9+9 is not bigger than 255, after all).

    So the solution, of course, is to use AAA, like this (not tested):

    add al,dl
aaa        ; takes no arguments and works on al
add ah,dh  ; adc not necessary, aaa already incremented ah if there was a carry

    AAA (ASCII Adjust for Addition) tests if al is bigger than 9, and if so:

    • adjusts al,
    • sets the carry flag, and
    • increments ah
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