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How to Master Memory Linking for STM32 Boards: Creating a Comprehensive Linker Script

  • Writer: Ashok Kumar Kumawat
    Ashok Kumar Kumawat
  • May 11
  • 8 min read

When delving into the intricate world of STM32 board development, one aspect that often perplexes many developers is memory linking. Properly managing memory and creating an efficient linker script is crucial for optimizing the performance of your STM32 board or any board. In this blog post, we will explore the detailed process of creating a linker script for an STM32 board and unravel the mysteries of memory linking control.


Unveiling the Magic of Memory Linking


Memory linking plays a pivotal role in the functionality and performance of embedded systems, such as the STM32 boards. Linker scripts act as the blueprint that guides the compiler in organizing the program's memory layout. By defining memory regions and sections within the linker script, developers can allocate memory efficiently, control where variables and functions are stored, and manage the program's execution flow.


Crafting Your Linker Script: A Step-by-Step Guide


Creating a comprehensive linker script for an STM32 board involves several key steps that warrant meticulous attention to detail. Let's break down the process into manageable chunks:


1. Understanding Memory Architecture


Before diving into linker script development, it's crucial to have a solid grasp of the STM32 board's memory architecture. Identify the different memory regions, such as Flash, SRAM, and memory-mapped peripherals, and their respective characteristics.

Memory Map of STM32



  1. Set Up Your Development Environment

Ensure you have the necessary tools installed:

  1. STM32CubeIDE or Keil MDK for development.

  2. STM32CubeMX for generating initialization code.


  1. Create a New Project

    1. Open STM32CubeMX.

    2. Select your STM32 microcontroller.

    3. Configure the peripherals and generate the project for STM32CubeIDE.


  2. Modify the Linker Script

  1. Locate the linker script file, usually named STM32Fxxx_FLASH.ld in your project directory.

  2. Open the linker script in a text editor.


  1. Define Memory Regions

Specify the memory regions in the linker script. For example:

MEMORY
{
  FLASH (rx)      : ORIGIN = 0x08000000, LENGTH = 512K
  RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 128K
  SDRAM (xrw)     : ORIGIN = 0xD0000000, LENGTH = 8M
}

  1. Define Sections

Define the sections where different parts of your code and data will be placed:

SECTIONS
{
  .text :
  {
    . = ALIGN(4);
    *(.text)           /* Code */
    *(.text*)          /* Code */
    . = ALIGN(4);
  } >FLASH

  .data : AT (ADDR(.text) + SIZEOF(.text))
  {
    . = ALIGN(4);
    *(.data)           /* Initialized data */
    *(.data*)
    . = ALIGN(4);
  } >RAM

  .bss :
  {
    . = ALIGN(4);
    *(.bss)            /* Uninitialized data */
    *(.bss*)
    . = ALIGN(4);
  } >RAM

  .sdram :
  {
    . = ALIGN(4);
    *(.sdram)          /* SDRAM data */
    *(.sdram*)
    . = ALIGN(4);
  } >SDRAM
}

  1. Compile and Debug

  1. Compile your project in STM32CubeIDE.

  2. Debug to ensure that the memory sections are correctly allocated.

  1. Test and Validate

Run your application and validate that the memory regions are being used as expected. You can use debugging tools to inspect memory and ensure everything is correctly mapped.


3. Declaring Sections


Next, declare sections within the linker script to categorize the program's code and data. Segregate sections based on their characteristics, such as executable code, read-only data, and initialized variables.


4. Assigning Symbols


Assign symbols to mark the beginning and end of sections, allowing the linker to resolve references across different sections. Symbols provide a reference point for the linker to connect modules and libraries seamlessly.


5. Fine-Tuning Memory Allocation


Optimize memory allocation by aligning sections to specific memory addresses, ensuring efficient access and minimizing memory fragmentation. Fine-tune the linker script to enhance program performance and resource utilization.


Embracing the Power of Control


Mastering memory linking for STM32 boards empowers developers to exert precise control over memory utilization and program execution. By crafting a well-structured linker script, you can unlock the full potential of your STM32 board and optimize its performance for diverse embedded applications.


Remember, the devil is in the detail when it comes to memory linking. Invest time in understanding the intricacies of linker script development, and your STM32 board projects will thrive with efficiency and reliability.


So, roll up your sleeves, dive into the depths of memory linking, and unleash the true potential of your STM32 boards through the art of linker script mastery!


let’s dive deeper into Step 6, where we define sections and symbols in the linker script. This step is crucial as it determines how different parts of your program are placed in memory.


Sections and Symbols in the Linker Script

1. .text Section

The .text section is where your code (executable instructions) is stored. This section is typically placed in Flash memory because it is read-only and non-volatile.

.text :
{
  . = ALIGN(4);       /* Align the current location counter to a 4-byte boundary */
  *(.text)            /* Include all .text sections from input files */
  *(.text*)           /* Include all .text* sections from input files */
  . = ALIGN(4);       /* Align the location counter again */
} >FLASH              /* Place this section in the FLASH memory region */

  • . = ALIGN(4);: This aligns the current location counter to a 4-byte boundary, ensuring proper alignment for the code.

  • *(.text): This includes all sections named .text from the input files.

  • (.text): This includes all sections that start with .text from the input files.

  • >FLASH: This specifies that the .text section should be placed in the FLASH memory region.

2. .data Section

The .data section contains initialized data. This data is stored in Flash but copied to RAM at runtime.

.data : AT (ADDR(.text) + SIZEOF(.text))
{
  . = ALIGN(4);
  *(.data)            /* Include all .data sections from input files */
  *(.data*)
  . = ALIGN(4);
} >RAM                /* Place this section in the RAM memory region */

  • AT (ADDR(.text) + SIZEOF(.text)): This specifies the load address of the .data section. It is placed right after the .text section in Flash.

  • >RAM: This specifies that the .data section should be placed in the RAM memory region.

3. .bss Section

The .bss section contains uninitialized data. This section is placed in RAM and is zero-initialized at runtime.

.bss :
{
  . = ALIGN(4);
  *(.bss)             /* Include all .bss sections from input files */
  *(.bss*)
  . = ALIGN(4);
} >RAM                /* Place this section in the RAM memory region */

  • *(.bss): This includes all sections named .bss from the input files.

  • (.bss): This includes all sections that start with .bss from the input files.

  • >RAM: This specifies that the .bss section should be placed in the RAM memory region.

4. .sdram Section

The .sdram section is used for data that needs to be placed in external SDRAM.

.sdram :
{
  . = ALIGN(4);
  *(.sdram)           /* Include all .sdram sections from input files */
  *(.sdram*)
  . = ALIGN(4);
} >SDRAM              /* Place this section in the SDRAM memory region */

  • *(.sdram): This includes all sections named .sdram from the input files.

  • (.sdram): This includes all sections that start with .sdram from the input files.

  • >SDRAM: This specifies that the .sdram section should be placed in the SDRAM memory region.

Symbols

Symbols in the linker script are used to mark specific addresses or sizes. For example:

  • ADDR(section): Returns the address of the specified section.

  • SIZEOF(section): Returns the size of the specified section.

  • ALIGN(n): Aligns the location counter to the next multiple of n.

These symbols help in defining the layout and ensuring proper alignment and placement of sections in memory.


What is the purpose of ALIGN(n)?

The ALIGN(n) directive in a linker script is used to align the location counter to the next multiple of n bytes. This ensures that the memory address where the next section or data will be placed is aligned to a specific boundary, which can be crucial for performance and correctness in embedded systems.

Purpose of ALIGN(n)

  1. Performance Optimization: Many processors perform better when data is aligned to certain boundaries. For example, accessing a 32-bit word that is aligned to a 4-byte boundary is typically faster than accessing one that is not aligned.

  2. Hardware Requirements: Some hardware peripherals or memory types require data to be aligned to specific boundaries. For instance, certain DMA controllers might require data buffers to be aligned to 4-byte or 8-byte boundaries.

  3. Correctness: Misaligned data can cause exceptions or faults on some processors. Ensuring proper alignment helps avoid such issues.

Example Usage

Here’s an example of how ALIGN(4) is used in a linker script:

.text :
{
  . = ALIGN(4);       /* Align the current location counter to a 4-byte boundary */
  *(.text)            /* Include all .text sections from input files */
  *(.text*)           /* Include all .text* sections from input files */
  . = ALIGN(4);       /* Align the location counter again */
} >FLASH              /* Place this section in the FLASH memory region */

In this example:

  • The first ALIGN(4) ensures that the start of the .text section is aligned to a 4-byte boundary.

  • The second ALIGN(4) ensures that the end of the .text section is also aligned to a 4-byte boundary, which can be useful for the placement of subsequent sections.

Practical Considerations

  • Alignment Size: The value of n in ALIGN(n) should be chosen based on the requirements of your processor and peripherals. Common values are 2, 4, 8, or 16 bytes.

  • Memory Waste: Aligning data can sometimes lead to unused memory spaces (padding). This is a trade-off between alignment benefits and memory efficiency.


How do I fix misaligned data in my code?

Copilot

Sent by Copilot:

Fixing misaligned data in your code involves ensuring that data structures and variables are properly aligned to the boundaries required by your processor. Here are some strategies to help you achieve this:

1. Use Compiler Directives

Most compilers provide directives to specify alignment for variables and structures. For example, in GCC, you can use the attribute((aligned(n))) directive:

// Align a variable to a 4-byte boundaryint __attribute__((aligned(4))) myVar;

// Align a structure to a 4-byte boundarystruct __attribute__((aligned(4))) MyStruct {
    int a;
    char b;
};

2. Use Standard Library Functions

Some standard libraries provide functions to allocate aligned memory. For example, in C11, you can use aligned_alloc:

#include <stdlib.h>

void* ptr = aligned_alloc(4, sizeof(int) * 10); // Allocate memory aligned to 4 bytes

3. Align Data in Linker Script

Ensure that sections in your linker script are aligned properly using the ALIGN(n) directive:

.data : AT (ADDR(.text) + SIZEOF(.text))
{
  . = ALIGN(4);       /* Ensure the start of .data section is aligned to 4 bytes */
  *(.data)
  *(.data*)
  . = ALIGN(4);       /* Ensure the end of .data section is aligned to 4 bytes */
} >RAM

4. Align Data in Assembly Code

If you are writing assembly code, you can use alignment directives provided by the assembler. For example, in ARM assembly, you can use the .align directive:

.section .data
.align 4
myVar: .word 0x12345678

5. Use Padding in Structures

Sometimes, adding padding bytes to structures can help ensure proper alignment:

struct MyStruct {
    int a;
    char b;
    char padding[3]; // Add padding to align the next member
    int c;
};

6. Check Compiler Settings

Ensure that your compiler settings are configured to enforce alignment. Some compilers have options to enable or disable strict alignment checks.

Example: Fixing Misaligned Structure

Let’s say you have a structure that is misaligned:

struct Misaligned {
    char a;
    int b;
};

To fix this, you can add padding or use compiler directives:

struct Aligned {
    char a;
    char padding[3]; // Add padding to align the next member
    int b;
};

// Or use compiler directivestruct __attribute__((aligned(4))) AlignedDirective {
    char a;
    int b;
};

Debugging Alignment Issues

  • Use Debugger: Inspect memory addresses of variables to ensure they are aligned.

  • Compiler Warnings: Enable compiler warnings for alignment issues.

  • Static Analysis Tools: Use static analysis tools to detect alignment problems.

By following these strategies, you can ensure that your data is properly aligned, which can improve performance and prevent hardware faults.

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