Program 4: Factorial Calculation

 **Program 4: Factorial Calculation**

```assembly

.model small

.stack 100h

.data

    num dw 5

    result dw 1

.code

    main proc

        mov cx, num          ; Load 'num' into CX

        mov ax, 1            ; Initialize 'result' to 1

    loop_start:

        mul cx               ; Multiply AX by CX (result *= num)

        loop loop_start      ; Decrement CX and loop if not zero

        mov result, ax       ; Store result in 'result'

        

        mov ah, 4Ch          ; Exit program

        int 21h

    main endp

end main

```

Explanation:

1. `.model small` and `.stack 100h`: Memory model and stack size definitions.

2. `.data` section: Declares the data segment.

   - `num dw 5`: Defines a word-sized variable named "num" with value 5.

   - `result dw 1`: Defines a word-sized variable named "result" with initial value 1.

3. `.code` section: Contains the main code.

   - `mov cx, num`: Move the value of "num" into the CX register.

   - `mov ax, 1`: Initialize AX to 1 (for the result).

   - `loop_start:`: Label for the loop.

   - `mul cx`: Multiply AX by CX (result *= num).

   - `loop loop_start`: Decrement CX and loop if CX is not zero.

   - `mov result, ax`: Store the result in the "result" variable.

   - `mov ah, 4Ch`: Set the exit code for DOS.

   - `int 21h`: Call DOS interrupt to exit the program.