ADM instruction in asm

Question

In the following code

MOV AL,NUMBER1
ADD AL,NUMBER2
MOV AH, 00H
ADC AH, 00H

for which lines 3 and 4? What are they doing?

Also, why does the code clear AH? (I assume that the AL "ADD" operation may result in a carryover.)

Answer 1

To understand this, start by looking at what each instruction does:

• MOV AH, 00H

  This command will set the register to 0 without affecting the flags. MOVAH

• ADC AH, 00H

  This command will add the source operand (0), the carry flag (CF), and the destination operand ( ), storing the result in the destination operand ( ). ADCAHAH

  It is symbolic that then: AH = AH + 0 + CF

  , MOV , CF, ADC, ADD ( 2).

  , AH 0 , : AH = CF.

, :

• NUMBER1 AL: AL = NUMBER1

• NUMBER2 AL: AL = NUMBER1 + NUMBER2

• AH: AH = 0

• AH CF, NUMBER1 NUMBER2. , AH 1, , 0 . (AH = CF)

, 16- 8- . -C :

BYTE NUMBER1;
BYTE NUMBER2;
WORD RESULT = (WORD)NUMBER1 + (WORD)NUMBER2;

BYTE WORD . ? , . 8- , , 8 .

, AL AH , , AX. , AX. 16- NUMBER1 NUMBER2.