Behind the scenes of a Map file

As you might already know that map files are generated by linker but have you ever wondered where the linker gets the information to create this map file? Please chcekout my previous blog on map file if you havent already where i explained what is map file and its role in embedded system. In this blog, we’ll explore the various sources that the linker uses to determine memory addresses and sizes, which are critical for generating an accurate and useful map file.

Sources of Information for Linkers

1. Linker Script

The linker script is a primary source of information that dictates how different sections of code and data are laid out in memory. It is a configuration file that provides:

• Memory Regions: Defines available memory regions and their starting addresses and sizes.
• Section Placement: Specifies how different sections (e.g., .text, .data, .bss) should be placed within these memory regions.

Example Linker Script Segment:

• MEMORY: Defines the starting addresses and sizes of memory regions.
• SECTIONS: Specifies how different sections are placed in memory.

2. Object Files

Object files are produced by the compiler and contain:

• Section Headers: Describe the size and alignment requirements of each section.
• Symbol Tables: Include information about symbols (functions, variables) and their relative addresses.

When the linker combines object files, it uses this information to determine the final memory layout.

3. Linker Options and Flags

Linker options and flags can influence how memory addresses and sizes are assigned:

• -T: Specifies the linker script to use.
• -Map=<file>: Generates a map file with detailed memory layout.
• -e <entry>: Sets the entry point address for the program.

These options allow developers to control and customize the linking process, affecting how addresses and sizes are determined.

4. Compiler Specifications

Compilers can provide additional directives that affect memory layout:

• __attribute__((section("name"))): Places variables or functions into specific sections.
• #pragma directives: Control section placement or alignment in some compilers.

These specifications ensure that specific variables or functions are placed in designated sections, influencing the final memory map.

5. Default Linker Behavior

If no explicit linker script is provided, the linker uses default settings or built-in scripts. These defaults typically follow conventions for common architectures and memory layouts but may be less optimized for specific applications.

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How It All Comes Together

During the linking process:

1. Linker Reads the Linker Script: Determines the layout of memory regions and section placements.
2. Parses Object Files: Gathers section sizes and symbols.
3. Combines Sections: Places sections from object files into memory regions as specified by the linker script.
4. Calculates Addresses: Assigns memory addresses based on section placements and alignment.
5. Generates the Map File: Provides a comprehensive view of the final memory layout, including section addresses and sizes.

The linker’s ability to generate a map file relies on a combination of inputs, including the linker script, object files, compiler specifications, linker options, and default behaviors.