What is the role of a linker script?

 A linker script plays a critical role in the process of compiling and linking code for embedded systems and other software development. It is a configuration file that defines how the linker should arrange the various sections of the program (e.g., code, data, stack) in memory. The linker script gives instructions on how to organize and allocate memory for different parts of the program and how to manage the output of the linker.

Role of a Linker Script:

1. Memory Layout Configuration:

   • The linker script defines the memory regions and how they should be used. It specifies where different sections of the program should be loaded into the target memory.

   • For example, it can define:

     • Text (Code) section: Where the executable code resides.

     • Data section: Where initialized global variables and constants are stored.

     • BSS (Block Started by Symbol) section: Where uninitialized global variables are stored.

     • Heap and Stack: Where dynamic memory (e.g., malloc) and stack memory will be located.

     • Interrupt Vector Table: Where the interrupt vector table will be placed.

2. Section Placement:

   • The linker script explicitly tells the linker where to place the various sections of the program (e.g., .text, .data, .bss, .rodata) in the physical memory.

   • This is particularly important in embedded systems, where the memory layout is often fixed, and you may need to place code and data in specific regions (e.g., placing bootloader code in a specific memory range or ensuring certain data goes into non-volatile memory).

3. Control Over Output File Format:

   • It can specify the output format of the linked binary, such as ELF (Executable and Linkable Format) or Intel HEX format.

   • The script can define how the final output file will be structured, which is essential for creating files that can be flashed to a microcontroller or executed on a specific platform.

4. Symbol Definition and Address Assignment:

   • The linker script can define symbols (addresses or variables) and assign them to specific memory locations.

   • For example, it can assign a variable to the start of the data segment or ensure that certain interrupt vector addresses are located at specific locations in memory.

5. Memory Region Definitions:

   • A linker script defines memory regions such as RAM, ROM, and FLASH. It then specifies how the program's sections (text, data, etc.) should be mapped to these regions.

   • This is crucial when dealing with embedded systems, where the memory layout can be highly constrained or non-standard.

6. Handling of Special Sections:

   • The linker script can manage sections that are used for specific purposes, like interrupt vectors, device registers, or bootloaders. It ensures that these sections are placed at the correct locations in memory to function properly.

7. Overriding Default Linker Behavior:

   • By default, the linker will try to arrange memory sections in a specific way. The linker script allows the user to override these defaults and tailor the memory layout according to the application’s needs.

Example of a Linker Script:

Here is a simple example of what a basic linker script might look like:

ld

/* Define memory regions */
MEMORY
{
  RAM (xrw) : ORIGIN = 0x20000000, LENGTH = 0x1000   /* RAM region starting at 0x20000000 with length 4KB */
  FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 0x20000  /* FLASH region starting at 0x08000000 with length 128KB */
}

/* Define sections */
SECTIONS
{
  /* Place the code section in FLASH */
  .text :
  {
    *(.text)  /* Include all code sections here */
  } > FLASH

  /* Place initialized data in the data section */
  .data :
  {
    *(.data)  /* Include all initialized data here */
  } > RAM

  /* Place uninitialized data in the BSS section */
  .bss :
  {
    *(.bss)  /* Include all uninitialized data here */
  } > RAM
}

Key Components of the Linker Script:

1. MEMORY Section:

   • Defines the available memory regions for the program (e.g., RAM, Flash).

   • Specifies the starting address and the size of each region.

2. SECTIONS Section:

   • Defines how different sections of the program (such as code, data, and BSS) should be placed into the memory regions defined earlier.

   • The syntax > REGION_NAME tells the linker which memory region to place a section in.

Conclusion:

In summary, a linker script is essential for controlling the memory layout and defining how different sections of the program are placed in memory. This is particularly important in embedded systems, where resources are limited, and precise control over memory is required. The linker script ensures that:

• The program is placed in the correct memory locations.

• The execution flow starts from the correct address.

• The system can properly initialize variables and allocate memory.