#15 Mastering GAS: A Complete Guide to the GNU Assembler

Series for explaining and teaching GNU GAS Assembler using AT&T syntax – all codes are reviewed and tested daily on

Fedora Linux 42

GNU Assembler version 2.44-6

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Memory Sections in GAS

Memory Allocation and Management in GAS

Memory allocation and management are crucial in low-level programming and form the backbone of writing optimized, efficient, and reliable assembly programs. Understanding how memory is allocated, how data is stored, and how different sections of a program are laid out in memory is essential for any assembly language programmer.

In GAS (GNU Assembler), memory management is primarily concerned with how different segments of the program, such as executable instructions, data, constants, and variables, are stored and accessed. By controlling memory allocation at such a low level, programmers can optimize memory usage and avoid common issues like memory corruption or inefficient program execution.

In this section, we’ll delve deeper into memory allocation and management techniques used in GAS, covering how memory sections work, the different types of memory regions, and how GAS handles memory in various contexts.

Memory Layout Overview

Before we dive into specifics, it’s important to first understand the general layout of a program’s memory. A typical process’s memory is divided into several key sections that hold different types of data and instructions. These sections are arranged in a defined order, and each one has its own purpose and specific directives in GAS to manage it.

Here is an overview of the typical memory sections:

1. Text Section (.text): This section holds the executable machine instructions (the actual code) that the CPU will run. The text section is typically marked as read-only and executable, meaning the instructions can be executed but cannot be modified at runtime.

2. Data Section (.data): This section contains initialized data, which are global or static variables defined by the programmer. These variables have values set before the program runs.

3. BSS Section (.bss): The BSS section holds uninitialized global or static variables that will be initialized to zero by default when the program starts. These variables are not included in the binary directly and are allocated space at runtime.

4. Read-Only Data Section (.rodata): This section is dedicated to storing data that is not meant to be modified during execution, such as constant values, strings, and lookup tables.

5. Heap: The heap is used for dynamic memory allocation during program execution. It is managed at runtime and grows and shrinks dynamically as memory is allocated and freed.

6. Stack: The stack stores local variables, function parameters, and the return address for function calls. It is a region of memory that grows and shrinks as functions are called and return.

Each of these sections serves an important purpose, and GAS provides directives to place data and instructions into the correct sections.

Memory Allocation in GAS

In GAS, memory allocation is controlled by specific assembler directives that define where various parts of the program’s data and instructions will reside. GAS uses these directives to map program data to specific locations in memory, ensuring efficient and orderly management of program resources.

a. Allocating Memory for Code (Text Section)

The .text section is where the actual machine code instructions are placed. This section is typically read-only, which ensures that the program's code cannot be accidentally modified during execution. The .text section is set up using the .section directive in GAS.

Example:

.section .text
.global _start
_start:
    # Program instructions
    movl $1, %eax       # Load the immediate value 1 into %eax
    int  $0x80          # Invoke a system call

This example creates a basic program in the .text section with a single system call to terminate the program. The .text section contains the executable code, which the CPU can execute directly when the program is loaded into memory.

b. Allocating Memory for Initialized Data (Data Section)

The .data section holds variables that are initialized before the program starts running. These variables have predefined values, which are stored in the memory by the assembler. The .data section is useful for storing global variables and constants that will be accessed or modified throughout the program.

Example:

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.section .data
my_int:     .long 42                # Initialize a 32-bit integer with value 42
my_string:  .asciz "Hello, World!"  # Null-terminated string

In this case, my_int is a 4-byte integer initialized to 42, and my_string is a string initialized to "Hello, World!". These variables will be placed in the .data section and can be read or modified during the execution of the program.

c. Allocating Memory for Uninitialized Data (BSS Section)

The .bss section is used for uninitialized variables, such as global or static variables that are not assigned any value when the program starts. The space for these variables is reserved in memory, but their values are initialized to zero automatically at runtime. The .bss section does not occupy space in the binary file, which helps save space in the program’s disk storage.

Example:

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.section .bss
buffer: .skip 100    # Reserve 100 bytes for a buffer (uninitialized data)

Here, we reserve 100 bytes of uninitialized memory for the buffer. The contents of buffer will be zeroed out when the program starts. The .bss section is typically used for large arrays, buffers, and variables that don’t need to be initialized explicitly in the source code.

d. Allocating Read-Only Data (RODATA Section)

The .rodata section is used for data that will not be modified during execution, such as string literals, constant values, or lookup tables. Storing such data in the .rodata section ensures that the data remains immutable and can be accessed efficiently.

Example:

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.section .rodata
msg: .asciz "This is read-only data"   # Null-terminated read-only string

The string "This is read-only data" is placed in the .rodata section and marked as immutable, meaning it cannot be modified during execution. This helps protect the program from accidental data corruption and allows the system to optimize access to this read-only data.

3. Dynamic Memory Allocation

While GAS does not directly handle dynamic memory allocation (as it is a low-level tool), dynamic memory allocation can still be managed through system calls or external libraries. Dynamic memory allocation allows programs to allocate memory at runtime, based on the needs of the application. This is especially useful for handling data structures whose sizes are not known at compile-time, such as linked lists or dynamic arrays.

In operating systems like Linux, dynamic memory allocation is typically handled by the heap, which grows and shrinks as memory is allocated and freed during the program's execution. In assembly, you can request dynamic memory using system calls like mmap() or library functions like malloc() (from the C standard library). However, GAS does not directly provide functions for allocating heap memory.

For example, using a system call to request dynamic memory with mmap() in Linux:

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    movl $90, %eax     # mmap system call number (Linux)
    movl $0,  %ebx     # Address (0 means system chooses the address)
    movl $1024, %ecx   # Size of memory to allocate (1024 bytes)
    movl $3,  %edx     # Protection flags (read/write)
    movl $34, %esi     # Flags (anonymous mapping)
    movl $-1, %edi     # File descriptor (-1 for anonymous)
    movl $0,  %ebp     # Offset (0 for anonymous)
    int  $0x80         # Trigger the system call

This code snippet allocates 1024 bytes of memory in the heap using the mmap() system call in Linux. The allocated memory can then be used for runtime operations, such as storing user input or temporary data.

Memory Management Best Practices in GAS

Proper memory management is essential in assembly programming to ensure efficient execution and prevent issues like memory leaks, segmentation faults, or inefficient memory usage. Here are some best practices to follow when working with memory in GAS:

a. Minimizing Memory Fragmentation

Memory fragmentation occurs when free memory is scattered in small chunks, preventing large contiguous blocks of memory from being allocated when needed. To minimize fragmentation, it's important to allocate memory in larger blocks, especially for large arrays or buffers. It’s also helpful to reuse memory when possible to reduce the number of allocations and deallocations.

b. Proper Use of the Stack

The stack is a crucial part of memory management, especially for function calls and local variables. When calling a function, the stack is used to store return addresses, function parameters, and local variables. The stack grows and shrinks automatically as functions are called and return, but excessive recursion or allocation of large local variables can overflow the stack. To prevent stack overflow, carefully manage the size of local variables and avoid deep recursion unless necessary.

c. Memory Alignment

Memory alignment refers to storing data at specific memory addresses that are divisible by the size of the data type. Proper alignment improves memory access performance, as misaligned memory access can be slower or cause hardware exceptions. In GAS, you can ensure proper alignment using the .align directive.

Example:

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.section .data
    .align 4          # Align the following data on a 4-byte boundary
var: .long 10         # 4-byte integer variable

This ensures that var is aligned on a 4-byte boundary in memory, improving the access speed for this variable.

d. Freeing Dynamically Allocated Memory

In GAS, dynamic memory allocation is typically handled through external system calls or libraries. When you allocate memory dynamically (for example, with mmap() or malloc()), it is crucial to free that memory once it is no longer needed. This helps prevent memory leaks, which can lead to excessive memory consumption and performance degradation.

In C, you would use the free() function to release allocated memory. While GAS does not provide built-in mechanisms for freeing memory, when writing assembly programs that interact with C libraries, it is your responsibility to ensure that any dynamically allocated memory is properly freed.

Conclusion

Memory allocation and management in GAS involve careful use of the program's memory sections. The .text, .data, .bss, and .rodata sections each serve a unique purpose in organizing the program's code and data. Through careful management, you can ensure that your program runs efficiently and makes optimal use of available resources.

While GAS doesn’t directly manage memory allocation for dynamic data structures like the heap, you can still request memory dynamically using system calls or external libraries. By following best practices for memory management, such as minimizing fragmentation, using the stack effectively, aligning data properly, and freeing dynamically allocated memory, you can ensure that your assembly programs are efficient, reliable, and robust.