; Compilation
; nasm -f elf thisfile.asm
; ld thisfile.o
; ./a.out

section .data
    buflen:     dw  8192 ; 4 byte integer with value 8192
    error:      db  "Error opening the file, does the file exist?", 10 ; Error-string + \n
    errorlen:   equ $-error ; Length of the error string
section .bss
    buf resb    8192        ; 8192 bytes of unallocated space
section .text
    global _start           ; Tell the linker where the program starts

_start:
    pop     ebx         ; Pop first argument from stack to register ebx (argc)
    pop     ebx         ; Pop second (argv[0])
    pop     ebx         ; Third argument is argv[1], store to ebx

    mov     eax, 5 ; sys_open   ; Store kernel call code of open() to eax
                        ; ebx has the filename (second argument)
    mov     ecx, 0 ; O_RDONLY   ; Third argument is O_RDONLY, it is declared as 0
    int     0x80            ; Call the kernel (open()) (stores file descriptor to eax)

    test    eax, eax        ; Test the return value
    jg  print           ; If return value > 0, jump to print

    push    eax         ; If not, store the error value
    mov eax, 4          ; write()
    mov ebx, 1          ; stdout
    mov ecx, error      ; Address of error string
    mov edx, errorlen       ; Error string length
    int 0x80            ; Call kernel (write())

    pop eax         ; Pop error value from stack
    mov ebx, eax        ; Set error value to first argument
    mov eax, 1          ; Exit()
    int 0x80            ; Call kernel (exit())
    ; Program end (if wrong arguments)

print:
    mov ebx, eax        ; Get file descriptor from eax
    mov eax, 3          ; read()
                    ; ebx already has the file descriptor (first argument)
    mov ecx, buf        ; Address of buffer as a second argument
    mov edx, buflen     ; Buffer length. Third argument
    int 0x80            ; Call kernel (read())

    mov edx, eax        ; Store eax value (read bytes) to edx
    mov eax, 4          ; write() again
    mov ebx, 1          ; stdout (to screen)
    ; ecx is already buffer
    ; edx is the read bytes (assigned at line 48)
    int 0x80            ; Call kernel (write)

    mov eax, 1          ; exit()
    mov ebx, 0          ; With value 0
    int 0x80            ; Call kernel (exit(0))
    ; Program end