What Is Linker Script?

Definition: Linker Script

A linker script is a file used by a linker to control the memory layout of an executable or other output file. It provides detailed instructions on how to map the sections of a program into the target system’s memory, defining where code, data, stack, heap, and other segments should be placed.

Overview of Linker Script

A linker script plays a crucial role in the process of linking, which is the final step in the compilation of a program. The linker combines object files generated by the compiler, resolves symbol references, and generates the final executable. The linker script gives the developer precise control over the placement and organization of the program in memory.

Purpose and Importance

Linker scripts are essential for embedded systems and other environments with specific memory requirements. They allow developers to:

• Control Memory Layout: Define the exact placement of code and data in memory, which is critical for systems with limited or specific memory configurations.
• Optimize Performance: Place frequently used code and data in faster memory regions to improve performance.
• Manage Special Sections: Handle sections like bootloaders, interrupt vectors, and peripheral registers, ensuring they are placed at specific memory addresses.
• Resolve Address Conflicts: Prevent overlaps and conflicts between different parts of the program by explicitly defining memory regions and their usage.

Key Components of a Linker Script

1. Memory Regions: Define the different memory areas available in the system, such as RAM, ROM, and flash memory. Each region has a name, starting address, and size.plaintextCopy codeMEMORY { FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 256K SRAM (rwx) : ORIGIN = 0x20000000, LENGTH = 64K }
2. Sections: Specify how different sections of the program (e.g., .text, .data, .bss) should be mapped to the memory regions defined earlier.plaintextCopy codeSECTIONS { .text : { *(.text) *(.text*) } > FLASH .data : { *(.data) *(.data*) } > SRAM AT > FLASH .bss : { *(.bss) *(.bss*) } > SRAM }
3. Entry Point: Define the entry point of the program, which is the address where execution begins.plaintextCopy codeENTRY(_start)
4. Symbols: Allow the definition of symbols to mark specific addresses or values within the memory layout.plaintextCopy code_stack_top = ORIGIN(SRAM) + LENGTH(SRAM);

Benefits of Using Linker Scripts

1. Customization: Tailor the memory layout to fit the specific needs of your application and hardware.
2. Optimization: Improve the performance and efficiency of the program by strategically placing code and data.
3. Flexibility: Handle various memory configurations and special requirements, such as bootloaders or memory-mapped peripherals.
4. Debugging: Facilitate debugging by clearly defining memory regions and sections, making it easier to understand and track memory usage.

Use Cases for Linker Scripts

1. Embedded Systems: Define precise memory layouts for microcontrollers and other embedded devices with limited memory resources.
2. Real-Time Operating Systems (RTOS): Ensure that critical sections of code and data are placed in fast memory regions to meet real-time performance requirements.
3. Bootloaders: Specify the placement of bootloader code in non-volatile memory regions.
4. Safety-Critical Systems: Control memory layout to ensure the deterministic behavior of software in automotive, aerospace, and medical applications.

How to Write a Linker Script

Creating a linker script involves understanding the memory architecture of the target system and the requirements of your program. Here are the steps to write a basic linker script:

1. Identify Memory Regions: Determine the memory regions available in your system, such as ROM, RAM, and any special memory areas.
2. Define Memory Regions: Use the MEMORY command to define each memory region with its name, permissions, starting address, and size.plaintextCopy codeMEMORY { FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 256K SRAM (rwx) : ORIGIN = 0x20000000, LENGTH = 64K }
3. Map Sections to Memory: Use the SECTIONS command to specify how each section of the program should be placed in the memory regions.plaintextCopy codeSECTIONS { .text : { *(.text) *(.text*) } > FLASH .data : { *(.data) *(.data*) } > SRAM AT > FLASH .bss : { *(.bss) *(.bss*) } > SRAM }
4. Set the Entry Point: Define the entry point of the program using the ENTRY command.plaintextCopy codeENTRY(_start)
5. Define Symbols: Create symbols to mark specific addresses or values.plaintextCopy code_stack_top = ORIGIN(SRAM) + LENGTH(SRAM);

Advanced Features of Linker Scripts

1. Overlay Sections: Define overlay sections to manage different sections sharing the same memory space, useful in scenarios like banking or paging.plaintextCopy codeOVERLAY { .overlay1 : { *(.overlay1) } > SRAM .overlay2 : { *(.overlay2) } > SRAM }
2. Output Sections: Customize the output format of sections, such as alignment and padding.plaintextCopy code.text ALIGN(4) : { *(.text) } > FLASH
3. Output Filenames: Specify the output file names for different sections or regions.plaintextCopy codeOUTPUT_ARCH(arm)
4. Additional Commands: Use commands like ASSERT to enforce conditions and INCLUDE to incorporate other linker scripts.plaintextCopy codeASSERT(SIZEOF(.data) <= LENGTH(SRAM), "Data section too big!") INCLUDE "common.ld"

Frequently Asked Questions Related to Linker Script