task3.asm

section .data
    prompt          db 'Enter a number (0-12): ', 0
    result_msg      db 'Factorial is: ', 0
    newline         db 10, 0
    input_buffer    db 10 dup(0)         ; Buffer for user input
    result_buffer   db 20 dup(0)         ; Buffer for result output

section .bss
    ; Empty for this program

section .text
global _start

_start:
    ; Display prompt
    mov     rax, 1                  ; sys_write
    mov     rdi, 1                  ; stdout
    mov     rsi, prompt
    mov     rdx, 22                 ; Length of the prompt
    syscall

    ; Read user input
    mov     rax, 0                  ; sys_read
    mov     rdi, 0                  ; stdin
    mov     rsi, input_buffer
    mov     rdx, 10                 ; Max input size
    syscall

    ; Convert input to integer
    mov     rsi, input_buffer       ; Pass input buffer
    call    atoi                    ; Result in RAX

    ; Validate input (number between 0 and 12)
    cmp     rax, 12
    ja      invalid_input
    cmp     rax, 0
    jl      invalid_input

    ; Calculate factorial
    push    rax                     ; Push input number onto stack
    call    factorial
    add     rsp, 8                  ; Clean up stack

    ; RAX now contains the factorial result

    ; Convert result to string
    mov     rsi, result_buffer      ; Buffer for the result
    call    itoa

    ; Display result message
    mov     rax, 1                  ; sys_write
    mov     rdi, 1                  ; stdout
    mov     rsi, result_msg
    mov     rdx, 14                 ; Length of the message
    syscall

    ; Display result
    mov     rax, 1                  ; sys_write
    mov     rdi, 1                  ; stdout
    mov     rsi, result_buffer
    mov     rdx, 20                 ; Assume max length
    syscall

    ; Newline
    mov     rax, 1                  ; sys_write
    mov     rdi, 1                  ; stdout
    mov     rsi, newline
    mov     rdx, 1
    syscall

    ; Exit program
    mov     rax, 60                 ; sys_exit
    xor     rdi, rdi
    syscall

invalid_input:
    ; Print an error message and exit
    mov     rax, 1                  ; sys_write
    mov     rdi, 1                  ; stdout
    mov     rsi, newline
    mov     rdx, 22                 ; Error message
    syscall
    mov     rax, 60                 ; sys_exit
    xor     rdi, rdi
    syscall

; Subroutine: Factorial Calculation
factorial:
    mov     rbx, 1                  ; Initialize result in RBX
    cmp     rax, 0                  ; If input is 0, return 1
    je      factorial_end
factorial_loop:
    imul    rbx, rax                ; RBX = RBX * RAX
    dec     rax                     ; Decrement RAX
    jnz     factorial_loop
factorial_end:
    mov     rax, rbx                ; Return result in RAX
    ret

; Subroutine: ASCII to Integer Conversion (atoi)
atoi:
    xor     rax, rax                ; Clear RAX
    xor     rcx, rcx                ; RCX for multiplier (10^n)
    mov     rcx, 10

atoi_loop:
    movzx   rdx, byte [rsi]         ; Load next character
    cmp     rdx, 10                 ; Check for newline
    je      atoi_done
    sub     rdx, '0'                ; Convert ASCII to digit
    imul    rax, rcx                ; Multiply current result by 10
    add     rax, rdx                ; Add digit to result
    inc     rsi
    jmp     atoi_loop

atoi_done:
    ret

; Subroutine: Integer to ASCII Conversion (itoa)
itoa:
    xor     rcx, rcx                ; Counter for digits
itoa_loop:
    xor     rdx, rdx                ; Clear RDX
    mov     rbx, 10
    div     rbx                     ; Divide RAX by 10
    add     dl, '0'                 ; Convert remainder to ASCII
    push    rdx                     ; Store digit on stack
    inc     rcx
    test    rax, rax                ; Check if RAX is 0
    jnz     itoa_loop

itoa_pop:
    pop     rdx                     ; Get digit from stack
    mov     [rsi], dl               ; Store it in buffer
    inc     rsi
    loop    itoa_pop

    mov     byte [rsi], 0           ; Null-terminate the string
    ret