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FrEqDe
Asked: 2020-07-04 15:14:52 +0800 CST 2020-07-04 15:14:52 +0800 CST

Why is an adjustment made when doing arithmetic operations in assembler?

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I have a code to make a sum in assembler and it works fine, but I don't understand why it is necessary to make an adjustment to the characters, I have only been able to observe that if I don't make the adjustment when printing the result, a character is displayed that does not correspond to a number.

When I read a character I subtract 30H.

    ;read number1
    MOV AH, 01H
    INT 21H

    ;adjustment
    SUB AL, 30H
    MOV number1, AL


After doing the sum, add 30H.

    ADD AL, number1

    ;Adjustment
    ADD AL, 30H
    MOV result, AL
ensamblador

2 Answers

  1. Best Answer

    What you are reading is not an integer, it is a character. In ASCII, the character '0'is at position 48, or 30H. So to convert a character corresponding to a decimal digit (characters between 48 and 57) to a decimal digit, 48 is subtracted from it.

    As you can imagine, this returns garbage if you type a letter instead of a number.

    • 2

  2. You add 0x30it because you set the integer value to be displayed on the screen as a character value.

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