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= Labels and Entry Point (_start) =
This page explains how labels work in assembly programming and how the program’s entry point is defined.

== What are labels ==
A label is a symbolic name for a memory address.  
It marks a location in code or data that can be referenced later.

Syntax:
 LabelName:

Example:
 SECTION .data
 EatMsg: db "Eat at Joe’s"

 SECTION .text
 mov rcx, EatMsg   ; rcx ← address of the string
 mov rdx, [EatMsg] ; rdx ← first 8 bytes of the string

Here:
* EatMsg refers to the address of the string.
* [EatMsg] refers to the contents at that address.

== Why labels are needed ==
Labels replace hardcoded addresses and make programs relocatable:
* You don’t know where in memory your program will be loaded.
* The linker can adjust label references automatically.
* The code becomes modular and position-independent.

== Code labels ==
Labels can also be attached to code lines.  
They mark destinations for jumps and branches, forming the basis for control flow.

Example:
 mov eax, 5
loop_start:
 dec eax
 jnz loop_start   ; jump back if eax ≠ 0

Here, loop_start marks the instruction to jump to.  
Such code labels are essential for loops, branches, and function calls.

== The _start label ==
_start: defines the entry point of an assembly program on Linux.  
It is the first instruction executed when the program runs.

Example:
 global _start
_start:
 mov rax, 60      ; system call for exit
 xor rdi, rdi     ; return code 0
 syscall

* The keyword global makes _start visible to the linker.
* The operating system looks for this symbol to know where to begin execution.

Every NASM program for Linux must define exactly one entry point, usually called _start.

== Summary ==
* Labels are symbolic names for addresses.
* Data labels point to memory locations.
* Code labels mark positions for jumps.
* The _start label defines where execution begins.

[[Category:Assembly Programming]]