Hello world in Linux

This is a minimal hello world program in Linux. It doesn’t need to link any external libraries.

SECTION .text

global _start ; "global" means that the symbol can be accessed in other modules. In order to refer to a global symbol from another module, you must use the "extern" keyboard
_start:
    mov eax, 4 ; syscall: write
    mov ebx, 1 ; stdout
    mov ecx, msg
    mov edx, msgLen
    syscall

    mov eax, 1 ; syscal: exit
    mov ebx, 0 ; return code
    syscall

msg: db "hello",10
msgLen: equ $ - msg ; the $ sign means the current byte address. That means the address where the next byte would go

I compile it with

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

Since this program doesn’t use any external functions, the executable is very small, and it doesn’t link any dynamic libraries (not even libc).

$ ldd hello
    not a dynamic executable
$ wc -c hello
792 hello

So it says that the executable is 792 bytes but we can easily make it smaller by passing the -s flag to ld. Or we could also use the strip command.

$ nasm -f elf64 hello.asm && ld -s -nostartfiles hello.o -o hello
$ wc -c hello
384 hello

$ nasm -f elf64 hello.asm && ld -nostartfiles hello.o -o hello
$ strip hello
$ wc -c hello
384 hello

For more info about making small executable read this article (although it’s a bit old and uses 32-bit instructions).