Article by Ayman Alheraki on January 11 2026 10:37 AM
In assembly language programming, the .text section plays a crucial role in defining where the executable code resides in memory. It is where all the machine instructions generated from the assembly code are stored, and it is critical to understand how this section works to produce executable programs.
The .text section is a standardized area in the program's memory that holds the instructions which the CPU will later execute. This section is one of the most important parts of a program since it contains the raw operations, computations, and logic that the program performs. In modern systems, understanding how to properly define and use the .text section is a key component of mastering low-level programming and using tools like GAS (GNU Assembler).
In this section, we will dive deep into the .text section, how it fits into the larger memory structure of a program, how to define and work with it in GAS, its unique characteristics, and the best practices for using it effectively.
The .text section is one of the segments in a program's memory that is reserved for storing executable code, which consists of the machine-level instructions that the CPU will interpret and execute.
Executable Instructions: The .text section stores the raw instructions that are assembled from the source assembly code. These instructions represent the operations the program will perform when it is run. Each instruction corresponds to a specific task for the CPU to execute, whether it is a simple arithmetic operation or a more complex I/O operation.
Memory Segmentation: When a program is loaded into memory, it is segmented into different sections, each serving a specific purpose. The .text section is dedicated solely to the program's executable code. Other sections, such as .data and .bss, contain variables and data that the program will manipulate, but it is the .text section that contains the logic that drives the behavior of the program.
Protection and Access: In most modern operating systems, the .text section is given read-only and execute-only access permissions. This means the instructions can be executed but cannot be modified during runtime. This protection mechanism is designed to prevent malicious code from modifying its own instructions (e.g., in buffer overflow attacks). This principle is part of security mechanisms such as WˆX (Write XOR Execute), which helps prevent certain types of attacks.
Position in Memory: When a program is compiled and linked, the .text section is placed in a specific location in the program’s memory address space. The operating system determines where the code is loaded in memory, but the programmer does not typically control this placement. What’s important is that the .text section is accessible to the CPU to fetch and execute the program's instructions.
In GAS (GNU Assembler), the .text section is defined using the .section directive or simply by specifying .text to indicate that the following code belongs to this section. In the context of GAS, the .text section is typically where the actual program logic is written in assembly code, and it is where the instructions that will be executed are placed.
The basic syntax to define and use the .text section in GAS is as follows:
.section .text.global _start_start: ; Your executable code goes here mov $1, %eax ; Load the value 1 into eax register int $0x80 ; Trigger system call interrupt.section .text: This directive tells the assembler that the following code will be placed in the .text section. This is crucial as it helps the assembler correctly categorize the code. In GAS, the .text section is often the default section, but it is good practice to explicitly declare it.
.global _start: The .global directive makes the label start accessible outside of the current assembly file. It is commonly used to mark the entry point of
the program. When linked, the operating system or runtime environment knows that the execution of the program starts at this label.
_start: This label marks the entry point of the program. Execution of the program begins here. In the example, we load the value 1 into the eax register and trigger the interrupt 0x80, which corresponds to a system call on Linux. This results in executing the sys exit system call.
The .text section is a distinct and essential segment of memory in a program, designed to store executable instructions that the CPU will run. There are several characteristics that define the .text section:
Executable: The .text section contains the actual machine code instructions that perform the actions of the program. The CPU reads and executes these instructions directly.
Read-Only: In most modern operating systems, the .text section is marked as read-only. This means that while the CPU can read and execute the instructions, the instructions cannot be modified during execution. This helps prevent vulnerabilities such as code injection attacks.
Alignment: Most architectures require that instructions be aligned to specific boundaries. For example, x86 processors often expect instructions to be aligned
to 4-byte boundaries. This alignment improves CPU performance by ensuring that instructions can be fetched efficiently from memory. The assembler usually ensures that instructions are aligned appropriately by default, but developers must be aware of the alignment requirements for their target architecture.
Memory Protection: The .text section is typically marked with execute permissions in memory, allowing the processor to run the instructions. The section is also protected against writes to prevent modifications to the code, enforcing security practices to avoid scenarios like buffer overflow attacks.
The .text section is one of the first things loaded into memory when a program starts, and it is the part that is directly executed by the CPU. Understanding its importance in the execution
of a program is critical for anyone working with low-level languages such as assembly.
Program Control: The .text section is the core of program execution. Whenever the CPU executes the program, it fetches instructions from the .text section. These instructions define the logic of the program, whether it involves performing arithmetic operations, making system calls, interacting with other software components, or managing memory.
Linking and Loading: During the linking process, the .text section is combined with other sections of the program (such as the .data and .bss sections) to create an executable file. The linker ensures that the .text section is placed at the appropriate location in memory when the program is loaded.
Security Considerations: Security features such as Data Execution Prevention (DEP) or Non-Executable Pages (NX) rely on the .text section being properly marked as executable and read-only. This is important for defending against exploits that try to modify the program’s instructions or inject malicious code into the program’s execution flow.
When writing assembly code using GAS, it's essential to correctly organize and structure the code within the .text section. Here are a few key points to consider:
Efficient Code Organization: Structure your code logically, using labels and functions, so that it is easier to navigate and maintain. The .text section should primarily contain the instructions for the program’s operation, but it is important to avoid making the code too convoluted. Use simple, well-defined functions and labels to keep the program clear and efficient.
Calling Functions: Use the .text section to define both the main program logic and any functions that the program will call. Each function should begin with a label, and control can be transferred between functions using instructions such as call and ret.
System Calls: A significant feature of the .text section is its ability to make system calls to the operating system. For example, in a Linux system, you can use the int 0x80 instruction to make a system call. This enables your program to interact with the OS for various tasks like input/output operations, file handling, and memory management.
Here’s a more elaborate example of using the .text section to write an assembly program with GAS. This example demonstrates a simple program that outputs a string to the terminal using Linux system calls.
x .section .text .global _start
_start: # Write the message to stdout mov $4, %eax # syscall number for sys_write mov $1, %ebx # file descriptor for stdout (1) mov $message, %ecx # pointer to the message mov $13, %edx # length of the message int $0x80 # make the syscall
# Exit the program mov $1, %eax # syscall number for sys_exit xor %ebx, %ebx # return code 0 int $0x80 # make the syscall
.section .datamessage: .asciz "Hello, World!\n" # Define the message
Write the message:
The program writes the string "Hello, World!**n" to standard output (stdout).
The instruction mov eax, 4 loads the system call number for the write syscall into the eax register.
mov ebx, 1 sets the file descriptor to 1 (stdout).
mov ecx, message loads the address of the message into the ecx register.
mov edx, 13 indicates the length of the message, which is 13 bytes.
The program then exits using the sys exit system call. mov eax, 1 sets the syscall number for exit, and xor ebx, ebx sets the return code to 0.
This example demonstrates the basic structure of an assembly program, showing how to utilize the .text section to store the executable instructions and use system calls to interact with the operating system.
The .text section is the heart of an assembly program, containing all the executable instructions that define the program's behavior. Understanding its role and how to define and
organize code within it is essential for mastering low-level programming with GAS. The
.text section helps structure your code, ensures proper memory protection, and allows you to interact with the system via system calls. By following best practices for structuring your program and using the .text section effectively, you can create efficient and secure programs that perform low-level tasks on a variety of architectures.