Introduction to x64 assembly

x64 assembly language is a low level programming language which is used to write software that runs directly on 64 bit x86 processors. It consist a series of instruction that are executed by the cpu to perform specific tasks.

(x64 == AMD64 == x86-64)

• Table of contents

Prerequisite - basic CPU-architecture , basic programming concepts

Now we know that x64 assembly is a language based on instruction sets of a 64 bit extention of x86 cpu architecture. Lets make a dive in , setting up environment, understanding code and doing some practicals.

Tools for x64 assembly

1. Assembler : Assembler is used to convert assembly code into binary (machine code). ex: NASM (Netwide Assembler) or GAS (GNU Assembler)
2. IDE/Editer : A text editor utility for writing code. ex: VSCode , Vim , nano etc.
3. Debugger : To analyze the code/errors. ex: GDB (GNU Debugger) or LLDB.

Structure of a x64 assembly code

A x64 assembly code generally have 3 sections -

1. .data section
2. .bss section
3. .text section

(You can find other sections also like: .comment , .rodata , .note and you can create your own custom section.)

.data section

The .data section in x64 assembly is used to declare and initialize global and static variables . These variables are stored in memory and are writable during the program's execution.

section .data
    message db "Hello, World!", 0 ; message→variable_name, db→size_type , "Enter your name: "→value
    message_len equ $ - message           ; message_len→var_name, $→current_address, message→starting_address_of_message

.bss section

bss section defines the uninitialized variables or buffers that will be allocated memory at runtime.

section .bss
    name resb 32                  ; Reserve 32 bytes for user input (used here only for showcase)

.text section

text section is the main section in which all the struction and program logics are defined. Learn about → Registers and Instructions

section .text
    global _start                 ; making _start label globally accessable

_start:
    ; Write message to stdout
    mov rax, 1                       ; syscall: write
    mov rdi, 1                       ; File descriptor: stdout
    lea rsi, [message]               ; Address of message (use lea!)
    mov rdx, len                     ; Length of the message
    syscall

    ; Exit program
    mov rax, 60                      ; syscall: exit
    xor rdi, rdi                     ; Exit code 0
    syscall

Execute x64 assembly code

1. Step 1 : Save the assembly code into a file and name it with a .asm extention. like: helloWorld.asm
2. Step 2 : Assemble the asm file into a object file using a assembler, like: NASM.
   command → nasm -f elf64 -o helloWorld.o helloWorld.asm -f flag is used to select the format of output. like: win64, elf64
3. Step 3 : Create an executable from the object file. example: ld linker.
   command → ld -o helloWorld helloWorld.o this will create a executable file helloWorld that can be run into linux using ./helloWorld.