Introduction#

Note

This page will probably be moved to the guide section when it is done.

About NASM & x86-64#

NASM (Netwide Assembler) is a popular, open-source assembler for the x86 and x86-64 architectures, supporting multiple output formats and platforms.

Key Features:

  • Intel-style syntax (destination first: mov dest, src)

  • Multi-Platform Support:

    • Runs on Windows, Linux, macOS, and other Unix-like systems

    • Supports various output formats: ELF, COFF, Mach-O, Win32/64, raw binary

  • Powerful macro system with conditional assembly support

x86-64 Specifics:

  • 64-bit registers (RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP, R8-R15)

  • 64-bit memory addressing

  • SSE/AVX extensions available

Example: Hello World in NASM#

 1section .data
 2   message db 'Hello, World!', 0xA
 3   length  equ $ - message
 4
 5section .text
 6   global _start    ; Entry point for Linux
 7
 8_start:
 9   ; Write to stdout
10   mov rax, 1        ; sys_write
11   mov rdi, 1        ; stdout
12   mov rsi, message
13   mov rdx, length
14   syscall
15
16   ; Exit
17   mov rax, 60       ; sys_exit
18   xor rdi, rdi
19   syscall

Data Directives#

db    ; Define byte (8-bit)
dw    ; Define word (16-bit)
dd    ; Define doubleword (32-bit)
dq    ; Define quadword (64-bit)
resb  ; Reserve bytes
equ   ; Define constant

Sections in Assembler#

section .data      ; Initialized global/static data
section .bss       ; Uninitialized data (Zeroed)
section .text      ; Code
section .rodata    ; Read-only data

1 .text Section (Code)#

  • Contains executable instructions

  • Read-only and executable

section .text          ; Start of code section
global _start          ; Make _start visible to linker
_start:                ; Entry point
    mov rax, 1         ; Instructions go here
    ret

2 .data Section (Initialized Data)#

  • Read-write memory

  • Variables with initial values

  • Loaded from executable file

section .data
number dd 42       ; Initialized to 42
msg db 'Hello', 0  ; String with null terminator
array times 10 db 0 ; Array of 10 zeros

3 .bss Section (Uninitialized Data)#

  • Read-write memory

  • Variables without initial values

  • Not stored in executable (saves space)

  • Automatically zeroed by OS

section .bss
buffer resb 1024   ; Reserve 1024 bytes (uninitialized)
count resd 1       ; Reserve 1 dword

4 .rodata Section (Read-Only Data)#

  • Read-only memory

  • Constants, strings, lookup tables

  • Cannot be modified (segfault if you try)

section .rodata
pi dq 3.1415926535
const_string db "Constant!", 0

Multiple Sections#

Yes! You can have multiple sections of the same type:

; Multiple .data sections
section .data
   var1 dd 10

section .text
   mov rax, [var1]

section .data         ; Another .data section
   var2 dd 20        ; This continues where previous .data left off

section .text         ; Another .text section
   add rax, [var2]

Building with NASM#

Basic compilation and linking for 64-bit systems:

Linux#

# Basic assembly and linking
nasm -f elf64 source.s -o source.o  # Compile source.s into source.o
ld source.o -o output               # Link source.o and create binary

# With gcc (recommended for C interoperability)
nasm -f elf64 program.asm -o program.o
gcc program.o -o program

Windows#

# 64-bit Windows
nasm -f win64 program.asm -o program.obj
golink /entry:_start program.obj kernel32.dll

macOS#

# 64-bit macOS
nasm -f macho64 program.asm -o program.o
ld program.o -o program -macosx_version_min 10.15 -lSystem
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