What is Loader, How it is Different from Linker?

What is Loader?

A loader is a software component in an operating system that loads executable programs or software modules into memory for execution by the CPU. It performs tasks such as allocating memory space, relocating program code to the appropriate memory addresses, and resolving external references between different parts of the program. This process is essential for preparing a program to be executed effectively within the computer's memory space.

Functions of Loader

The loader is responsible for several crucial functions in the execution of programs:

1. Memory Allocation:
   The loader allocates memory space for the program in the computer's memory. It ensures that the program's instructions and data have the necessary memory addresses to operate properly.
2. Address Binding:
   It performs the process of address binding, which involves associating symbolic addresses used in the program with actual memory addresses. This can include absolute, relative, or dynamic addressing, ensuring that the program can access its required resources correctly.
3. Relocation:
   If the program is designed to be loaded at a different memory location than its original compilation address, the loader performs relocation. It adjusts the program's memory references to reflect the new starting address.
4. Linking and Symbol Resolution:
   The loader resolves references to external symbols, such as functions or variables, by matching them with their actual locations in other modules or libraries. This allows different parts of the program to communicate and interact seamlessly.
5. Loading and Execution:
   The loader physically loads the program's machine code and data into memory, making it ready for execution by the CPU. Once loaded, the loader transfers control to the program's starting point, initiating its execution.
6. Error Detection:
   The loader checks for potential errors during the loading process, such as insufficient memory, incompatible machine code formats, or unresolved symbols. It informs the user or programmer about any issues that could prevent successful execution.

Overall, the loader plays a critical role in preparing executable programs for smooth and efficient execution on an operating system by managing memory allocation, address binding, symbol resolution, and error detection.

What is an Absolute Loader?

An absolute loader is a type of loader used in the context of early computer systems and programming languages. It is responsible for loading a program into memory without performing any additional address relocation or modification. In other words, an absolute loader assumes that the program has been compiled to run at a specific memory location and loads it into that predetermined memory address.

Key characteristics of an absolute loader include:

1. No Address Relocation:
   Unlike more advanced loaders, absolute loaders do not perform any address relocation or modification of the program's code or data. The program is loaded directly into the memory addresses for which it was compiled.
2. Fixed Memory Locations:
   The program must be compiled to operate at specific memory locations. If the program needs to be loaded at a different memory address, it would require recompilation.
3. Simplicity:
   Absolute loaders are relatively simple in design and implementation since they don't need to handle complex address calculations or symbol resolution.
4. Limited Flexibility:
   Because the program's memory address is fixed, absolute loaders lack flexibility in terms of memory utilization. It can lead to issues if memory conflicts or fragmentation occur.
5. Historical Context:
   Absolute loaders were more common in early computing environments where memory management and dynamic loading were less sophisticated.

As computing technology evolved, absolute loaders gave way to more versatile loaders that could handle dynamic memory allocation, address relocation, and symbol resolution. These advancements led to more efficient memory usage and greater flexibility in program execution.

Algorithm

The algorithm of an absolute loader is relatively straightforward due to its simplicity. It involves reading machine code instructions and data from a file and loading them into memory locations specified in the program. Here's a basic outline of the algorithm for an absolute loader:

1. Read Header Information:

   • Open the executable file containing the program to be loaded.
   • Read any necessary header information that might include memory addresses and program size.

2. Load Program into Memory:

   • Read machine code instructions and data from the file one by one.
   • Load each instruction or data element into its predetermined memory address.

3. Repeat Until End of File:

   • Continue reading and loading instructions/data until the end of the file is reached.

4. Close File:

   • Close the executable file as it's no longer needed for loading.

5. Execution:

   • Transfer control to the starting address of the loaded program to begin its execution.

While this algorithm provides a basic framework for an absolute loader, it's important to note that real-world loaders, especially in modern operating systems, are much more complex due to the need for memory management, address relocation, symbol resolution, error handling, and support for various executable file formats. However, the core idea of loading a program into specific memory addresses without address relocation remains a characteristic of the absolute loader concept.

Flowchart

Now since we have learned the algorithm of absolute loader, let us summarize through a flowchart.

Difference between Linker and Loader

Let us now look into the following comparison table highlighting the main differences between loaders and linkers: