Introduction to NASM assembly

What is assembly

Assembly language is a low-level programming language that provides a human-readable representation of machine code. Each instruction corresponds directly to an operation performed by the CPU, making it highly efficient but also more difficult to write and maintain than high-level languages like C or Python.

Assembly Pros and Cons

NASM

NASM (Netwide Assembler) is an assembler for the x86 architecture. It is widely used due to its simplicity and support for flat binary, ELF, and other output formats.

Intel vs AT&T

There are two main syntaxes for x86 assembly:

Intel syntax (used by NASM)

• Destination comes first: mov eax, 1
• Readable and intuitive for most beginners

AT&T syntax (used by GNU assembler)

• Source comes first: movl $1, %eax
• Uses % for registers and $ for constants

NASM exclusively uses Intel syntax.

NASM directives

Directives are instructions for the assembler rather than the CPU. Common NASM directives include:

• section — defines sections of code (.text, .data, .bss)
• global — exposes symbols to the linker
• db dw,dd — define bytes, words, and double words in memory
• equ — define constants

The most basic program

The most basic program we can write in assembly does absolutely nothing. It just exits. This is the code for that program:

section .text
global _start

_start:
    mov rax, 60   ; syscall number for exit
    mov rdi, 0    ; exit code
    syscall       ; make the system call

Notes

• _start is the entry point of the program.
• rax holds the syscall number (60 = exit on Linux x86-64).
• rdi holds the exit code (0 here). The value in rdi determines the exit status of the program.

To assemble and run this:

nasm -felf64 path/to/file.s && ld path/to/file.o
./path/to/file