A minimal assembly program

This is the second post about x86 assembly. In the previous post I described how to compile a hello world program. In this post i will go through and even smaller program.

Minimal example

          global _start

          section .text
_start: mov rax, 60 ; system call for exit
          mov rdi, 0 ; exit code 0
          syscall ; invoke operating system to exit

This programs doesn’t print any output and only sets an exit code. If the program doesn’t set an exit code the program will crash with a Segmentation fault error.

Sections

There is only one section in this program called .text. Here you put your source code which is readonly when the program is ran. Sections is used by the assembler to group data and code in different locations in memory. For example in the hello world example we had a section called .data which we initialized a constant sequence of bytes.

Labels

There is also only one label in this program called _start. A label can be seen as pointer to an instruction in memory. A label can then be used as a reference in code to run a specific block of code(almost like a function call). The label _start has a special meaning for the linker ld and that is where an executable will start executing instructions.

Global keyword

The global keyword tells the assembler that this label should be able to be referenced outside of this assembly file. It’s mostly like exporting a function in high level languages.

Instructions

The syntax for operations is opcode <operand>, <operand>. We first have an opcode which tells the cpu what we want to do and with which operands.

The instruction mov rax, 60 tells the CPU to move the decimal constant value 60 to a CPU general-purpose register called rax. When an operand is constant value like above it’s called immediate addressing. CPU registers are small high-speed memory locations in the CPU.

Syscall

The syscall is a special operation that tells the CPU that the program wants to talk to the operating system. The program halts and the operating system takes over and reads the rax register for the syscall number which in linux 64bit is the syscall for exit. The rdi register where we put 0 holds the value for the exit code. When a syscall is done the program usually resumes executing but not in this case as we told the operating system that we wanted to exit.