Linker Scripts

Notice: This article, generated in part or in whole by an Artificial Intelligence, is intended as a brief introduction to the topic. Despite our best efforts to ensure accuracy, there may be some errors or omissions. Corrections and/or suggestions are welcomed!

For a comprehensive understanding, please refer to:

• OSDev Wiki Linker Scripts page.

Understanding Linker Scripts

Linker scripts are powerful tools that control how a program is laid out in memory. They are written in the Linker Command Language and are primarily used in systems programming, where control over memory layout is crucial.

What is a Linker Script?

A linker script is a description of the memory layout for the final executable file. It tells the linker where to place code and data in memory. It's used to specify the memory regions available, the start address of the program, and how sections in the input files should be mapped into the output file, among other things.

Key Concepts in a Linker Script

Symbols

Symbols are names used to represent memory locations. They can be defined in a linker script and used to control the memory layout. For example, a symbol can be used to set the start address of the program.

__rpi_phys_binary_load_addr = 0x80000;

Entry Point

The entry point of a program is the first instruction that gets executed when the program starts. It can be set in a linker script using the ENTRY keyword.

ENTRY(__rpi_phys_binary_load_addr)

Program Headers (PHDRS)

When we talk about creating an operating system, one of the key aspects we need to understand is how a program gets loaded into memory. This is where Program Headers, or PHDRS, come into play.

Think of a Program Header as a guide or a map for the system. It's part of a binary executable file - the kind of file your computer can run directly. This guide tells the system how to load different parts of that file into memory when you want to run the program.

Program Headers are defined in a special block in the linker script called PHDRS. Each Program Header within this block has two main components: a type and flags.

• The type tells the system what kind of segment (or part) of the binary file this header is describing. For example, PT_LOAD is a type that means this part of the binary file should be loaded into memory when the program runs.

• The flags are like switches that can be turned on or off to set permissions for this segment. These permissions can be things like whether the segment is readable (the system can read the data), writable (the system can change the data), or executable (the system can run the data as code).

Here's an example of what this might look like in a linker script:

PHDRS
{
    segment_code PT_LOAD FLAGS(5);
}


### Program Headers (PHDRS)

Program headers describe how sections of the binary should be mapped into memory. They are defined in the `PHDRS` block. Each program header has a type (like `PT_LOAD`) and flags to set permissions (like readable, writable, and executable).

```ld
PHDRS
{
    segment_code PT_LOAD FLAGS(5);
}

Sections

Sections are portions of the program, such as the .text section, which typically contains the executable code of the program. The SECTIONS command is used to describe the memory layout of the output file.

SECTIONS
{
    . =  __rpi_phys_binary_load_addr;

    .text :
    {
        KEEP(*(.text._start))
    } :segment_code
}

Location Counter (.)

The location counter (.) is used in linker scripts to represent the current output location in memory. It can be set to a symbol or a numeric value.

KEEP Directive

The KEEP directive tells the linker to keep certain sections even if they are not referenced. This is useful for sections that must be present in the final binary, like the start of the program.

Linker Scripts and Memory Management

Linker scripts play a pivotal role in memory management, particularly in systems programming where precise control over memory layout is essential. They allow developers to dictate how different parts of a program are mapped into memory, which can have significant implications for the program's performance and functionality.

One of the key ways linker scripts contribute to memory management is through the definition of memory regions. By specifying the start address and size of each region, a linker script can control where in memory different parts of the program (like code, data, and stack) are placed. This is especially important in resource-constrained environments, such as embedded systems, where efficient use of memory can make a big difference.

Linker scripts also manage memory by controlling the placement of 'sections' within the memory. Sections, like .text for code or .data for initialized data, are chunks of the program that the linker can place at specific locations in memory. By controlling the order and location of these sections, a linker script can optimize memory usage and potentially improve performance.

In addition, linker scripts can influence memory management through the use of symbols and the location counter. Symbols can represent specific memory locations, and the location counter (.) can be used to control the current output location in memory. These tools give developers fine-grained control over memory layout, allowing them to optimize memory usage and address specific system requirements.

Conclusion

Linker scripts are a powerful tool for controlling the memory layout of a program. Understanding them can be crucial in systems programming, especially when working with specific hardware or architectures. While they can seem complex at first, with a bit of practice, they become an invaluable tool in a programmer's toolkit.

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