This blog post will guide you through a C++ program that performs the addition of two numbers using inline assembly. While modern compilers provide high-level arithmetic operations, understanding inline assembly can help in optimizing performance and understanding low-level interactions with the CPU. Let’s explore this step-by-step!

#include<iostream.h>
#include<conio.h>
#include<stdio.h>

void main()
{
    clrscr();
    short int a, b, c;
    
    cout << "Enter First Number: ";
    cin >> a;
    cout << "Enter Second Number: ";
    cin >> b;
    
    asm mov ax, a
    asm mov ah, 00h
    asm mov bx, b
    asm mov bh, 00h
    asm add al, bl
    asm mov c, ax
    
    cout << "Result: ";
    cout << c;
    
    getch();
}

Understanding the Code

Variable Declarations

• short int a, b, c; → Declares three short integers to store input values and the result.

User Input

• The program takes two user inputs and stores them in a and b.

Inline Assembly Operations

• asm mov ax, a → Moves the first number into register AX.
• asm mov ah, 00h → Clears the upper byte of AX to ensure correct 16-bit arithmetic.
• asm mov bx, b → Moves the second number into register BX.
• asm mov bh, 00h → Clears the upper byte of BX.
• asm add al, bl → Adds the lower bytes (8-bit addition) and stores the result in AL.
• asm mov c, ax → Stores the final result back into the variable c.

Output Display

• The computed sum is displayed using cout.

Output

When the program runs, it follows these steps:

1. User Input Stage
   • The program prompts the user to enter two numbers.
   • For example, the user inputs 12 and 51.
2. Processing with Inline Assembly
   • The values 12 and 51 are stored in CPU registers.
   • The assembly instructions perform an addition operation.
   • The sum of 12 and 51 is computed as 63.
   • The result is then stored in a variable.
3. Displaying the Output
   • The computed sum is displayed to the user.
   • The final output appears as:

Enter First Number: 12
Enter Second Number: 51
Result: 63