0x06 Yasm: Hello World! ELF64

First, try to write some x64 assembler program. I will use yasm and ld. I have a few books about assembler, but not for 64-bit architecture. Lucky for me, I found the "Introduction to 64 Bit Assembly Language Programming for Linux and OS X" by Ray Seyfarth.

What I remember from old asm, are sections: .text section for code and .data for data.

Let us start with 'the simplest' example:

;asm00.s

section .data
    msg       db     'Hello world!'
    nl        db     0x0a
    msgLen    equ    $-msg
section .text
global _start
_start:
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    mov rdx, msgLen
    syscall

    mov rax, 60
    mov rdi, 0
    syscall

To build this code one must run yasm assembler and ld linker:

$ yasm -f elf64 asm00.s
$ ld -o asm00 asm00.o

And the effect:

$ ./asm00
Hello world!
$ 

Ok, so what happens here? We have here 2 calls to the syscall. The first one is calling to sys_write (mov rax, 1) to print to stdout (file descriptor 1 mov rdi, 1). In rsi and rdx we pass address of text to print and its length. The second call is to sys_exit (mov rax, 60). I want to exit with 0, so I pass it to rdi.

The list of available syscalls I found here.

The fun begins...

...SQUEAK!