Compiling your first assembly program

Background

Lately I’ve been studying assembly language for the x86/x86_64 architecture. Assembly language is a human readable form of machine code and is as low level as it gets. Today there aren’t many use cases for it as other low level languages like c and rust is fast enough. But I’ve always wanted to know more about what happens at the lowest level and I think it’s good to have knowledge how the most primitives of the CPU and operating system works.

I will be writing a series of post of what I’ve learned.

Assembler

When you have written an assembly program you need to translate it to machine code. This is done with an assembler. There are several different assemblers but I will be using NASM. The Netwide assembler(NASM) is an x86 assembler that works on both on linux and windows and can output object format files like elf32/elf64 and win32/win64.

Linker

When the assembler has generated the object files we need to use a linker to generate and executeable file. The linker combines object files, relocates their data and resolves symbol references. I will be using ld which is the GNU linker.

Hello world

This is a simple hello world example for 64-bit linux. I will explain what each row does in the next post.

;
; Example code is taken from the docs of NASM
;
          global _start

          section .text
_start: mov rax, 1 ; system call for write
          mov rdi, 1 ; file handle 1 is stdout
          mov rsi, message ; address of string to output
          mov rdx, 13 ; number of bytes
          syscall ; invoke operating system to do the write
          mov rax, 60 ; system call for exit
          xor rdi, rdi ; exit code 0
          syscall ; invoke operating system to exit

          section .data
message: db "Hello, World", 10 ; note the newline at the end

Compile and run

First we need to turn our assembly file to an object file

$ nasm -felf64 hello.asm

Nasm will by default call the object file hello.o. Then we need to use the linker to create and executable.

$ ld hello.o

The default output filename of ld is a.out and can be run by typing.

$ ./a.out
Hello, World

You should get the output Hello World