format elf executable
mov ecx, string_address                 ; address of string
mov edx, [string_address.length]        ; length of string
mov ebx, 1                              ; stdout
mov eax, 4                              ; sys_write
int $80

mov eax, 1                              ; sys_exit
int $80                                 ;

string_address db 'hello, world!'
.length dd $ - string_address    

format ELF executable
entry start

section readable executable

start:
; calculate factorial of eax
        mov     eax, 5
        call    factorial
; convert result to string
        mov     ecx, buffer
        mov     edx, buffer.size
        call    convert_uint32_to_str
; print the string
        mov     eax, 4
        mov     ebx, 1
        int     0x80
; exit
        mov     eax, ebx
        xor     ebx, ebx
        int     0x80

; input:
; eax - unsigned value whose factorial is to be calculated
; output:
; eax - factorial
factorial:
        cmp     eax, 1
        jbe     .finish
        lea     ecx, [eax-1]
.loop:
        mul     ecx
        loop    .loop
.finish:
        ret

; inputs:
; eax - unsigned value to be converted
; ecx - buffer the string is to be written to
; edx - length of buffer
; outputs:
; ecx - offset of string
; edx - length of string
convert_uint32_to_str:
        xor     edi, edi
        add     ecx, edx
        mov     ebx, 10
.loop:
        dec     ecx
        inc     edi
        xor     edx, edx
        div     ebx
        add     dl, '0'
        mov     [ecx], dl
        test    eax, eax
        jnz     .loop
.finish:
        mov     edx, edi
        ret

section writeable

buffer rb 16
.size = $ - buffer