This C++ program calculates the factorial of a number between 0 and 8 using inline 8086 assembly instructions. The multiplication is handled within an assembly loop, showcasing a basic yet insightful use of mul, dec, and jnz instructions.

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

void main() {
    clrscr();
    short a;
    unsigned int c;

    cout << "\n Enter Number between 0 to 8:";
    cin >> a;

    asm mov ax, 0000h
    asm mov al, 01h     // Initialize AX to 1
    asm mov cx, 0000h
    asm mov cx, a       // Set CX loop counter to input value

bac:
    asm mul cx          // Multiply AX by CX
    asm dec cx          // Decrement CX
    asm jnz bac         // Loop until CX reaches zero

    asm mov c, ax       // Move result from AX to variable c

    cout << "\n Factorial of " << a << " is " << c;
    getch();
}

Understanding the Code

Variable Declarations

• a → Input value for which factorial is to be calculated.
• c → To store the final result.
• Registers AX, CX are used in the assembly block for arithmetic.

Assembly Instructions

• mov ax, 0000h and mov al, 01h initialize the result in AX to 1.
• mov cx, a sets the loop counter.
• Inside the loop:
  • mul cx multiplies AX with CX.
  • dec cx decrements CX.
  • jnz bac jumps back to label bac if CX is not zero.
• After the loop, mov c, ax stores the result into the C++ variable.

Result

• Final factorial result is printed using the value of c.

Output

Enter Number between 0 to 8:6
Factorial of 6 is 720

Output Explanation

The user is prompted to enter a number between 0 and 8. When 6 is entered:

• The loop begins with AX = 1.
• Then AX is multiplied successively by 6, 5, 4, 3, 2, and 1 using the mul instruction.
• The result is stored in variable c which holds 720.
• The final output confirms the computed factorial: “Factorial of 6 is 720.”