Assembly Programming: Overview & Conventions

Assembly Programming: Overview & Conventions

This page introduces x86-64 assembly programming, the syntax conventions used in this course, and the tools involved.

Introduction

Assembly language (ASM) is a low-level programming language that provides direct access to the CPU’s instruction set. Unlike high-level languages, assembly operates on registers, memory addresses, and immediate values.

In this course, we use NASM (Netwide Assembler) on GNU/Linux with Intel syntax. Intel syntax is more readable for most beginners and aligns with documentation from Intel and AMD.

Intel vs. AT&T syntax

There are two dominant syntaxes for x86 assembly:

• **Intel syntax** – used by NASM, MASM, and most documentation.
• **AT&T syntax** – used by GNU Assembler (GAS) and older Unix systems.

Main differences in AT&T syntax:

• Immediate values are prefixed with `$`, e.g. `push $4` instead of `push 4`.
• Registers are prefixed with `%`, e.g. `inc %eax` instead of `inc eax`.
• Source and destination are swapped: `add eax, 4` becomes `add $4, %eax`.
• Instruction size is suffixed to the opcode: `mov ax, bx` becomes `movw %bx, %ax`.

In this course we use **Intel syntax** exclusively.

Example comparison

Intel syntax:

mov eax, 4
add eax, 2
mov ebx, eax

AT&T syntax:

movl $4, %eax
addl $2, %eax
movl %eax, %ebx

Intel syntax is generally easier to read, especially when addressing memory:

lea eax, [rcx + rax*8 - 0x30]

vs.

lea -0x30(%rcx, %rax, 8), %eax

Why learn assembly

Learning assembly provides insight into how computers actually execute code:

• How registers, memory, and the stack interact.
• How arithmetic and logic are performed at CPU level.
• How operating system services are invoked through syscalls.

It builds a foundation for understanding compilers, reverse engineering, performance optimization, and embedded systems.