h4ck0lympus/x86

x86

Hello World

section .text 
    global _start 
_start : 
        mov rax, 1 
        mov rdi, 1 
        mov rsi, _str 
        mov rdx, 14 
        syscall 

        mov rax, 60 
        mov rdi, 0
        syscall
section .data
    _str db "Hello, world",0xa

to run :

$ nasm -f elf64 hello.asm -o hello.o 
$ ld hello.o -o hello 
$ ./hello 
hello

Principal of everything we are doing is same as ARM ASM

Calling conventions

• .text section

  • We are using syscall code for write (1) to print on screen. So we set rax to 1 to set syscall code
  • set arg0 i.e fd using rdi register - 1 (stdout)
  • set arg1 i.e buf (what to write) using rsi register- _str
  • set arg2 i.e size_t count (count of characters) - 14 using rdx register
  • then syscall using syscall

  thats it for write() but what about exit

  • set rax register to 60 (syscall code for exit)
  • set rdi register to 0 (return value)
  • syscall

• '.data' section

  • we use _str is the label name db which stands for define bytes, 0xa is for a newline char

mov_add

section .text 

global _start 
_start : 
    mov rax, 60 
    mov rbx, 5
    mov rdi, 10
    add rdi, rbx 
    syscall  ; this should output 15

output :

$ ./bin/mov_add 
$ echo $?
15

• syscall mode - exit (syscall code = 60)
• we can use any register because we are not using anything like write() where reg. matters
• rbx = 5
• rdi (return code) = 10
• rdi = rdi + rbx = 10 + 5 = 15
• syscall