section .data
    prompt      db "Hexdecimal value : ", 0
    hex_digits  db "0123456789ABCDEF"

section .bss
    input       resb 4        ; To store 2 digits + newline + null
    outbuf      resb 3        ; 2 hex digits + newline

section .text
    global _start

_start:
    ; -------- Prompt user --------
    mov rsi, prompt
    call print_string

    ; -------- Read input --------
    mov rax, 0          ; sys_read
    mov rdi, 0          ; stdin
    mov rsi, input
    mov rdx, 4
    syscall

    ; -------- Convert ASCII to integer --------
    ; input = "42\n"
    movzx rax, byte [input]        ; first digit
    sub rax, '0'
    imul rax, 10

    movzx rcx, byte [input + 1]    ; second digit
    sub rcx, '0'
    add rax, rcx                   ; rax now has decimal number

    ; -------- Convert to Hexadecimal --------

    ; High nibble
    mov rcx, rax
    shr rcx, 4
    and rcx, 0xF
    movzx rdx, byte [hex_digits + rcx]
    mov [outbuf], dl

    ; Low nibble
    mov rcx, rax
    and rcx, 0xF
    movzx rdx, byte [hex_digits + rcx]
    mov [outbuf+1], dl

    ; Newline
    mov byte [outbuf+2], 10

    ; -------- Print Hex --------
    mov rax, 1          ; sys_write
    mov rdi, 1          ; stdout
    mov rsi, outbuf
    mov rdx, 3
    syscall

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

; ------------------------------------------------
; print_string: prints null-terminated string at RSI
; ------------------------------------------------
print_string:
    push rsi
    xor rcx, rcx
.count:
    cmp byte [rsi + rcx], 0
    je .done
    inc rcx
    jmp .count
.done:
    mov rax, 1          ; sys_write
    mov rdi, 1          ; stdout
    pop rsi
    mov rdx, rcx
    syscall
    ret