Loader

What is Loader?

A loader refers to the specific component that loads the necessary programs and libraries into the main memory of the computer to facilitate their execution, especially during the startup phase.

Technically, the loader is the component that reads the contents of the executable object codes generated by the compiler and assembler and performs the other necessary preparatory tasks to run them.

Understanding Loader

A loader in a computer memory is an important component that helps in starting any application by loading the program into the memory and preparing it for execution.

It is for this reason that the loader is typically located permanently in the memory of most of the operating systems. This helps in the faster loading of the programs.

Typically, every operating system that supports loading programs will use loaders. However, loaders may be absent in:

Extremely specialized computer systems are designed for running a particular set of special programs.
Embedded systems in which the codes are typically executed directly from the Read Only Memory (ROM).

However, there are quite a few operating systems supporting virtual memory that may have the loader stored in a pageable area in the memory.

In such situations, the loader will not copy the object codes or the content of it to the memory.

Instead, it will simply inform the subsystem of the virtual memory that there is a mapping between the contents of the related executable file and the specific space of the memory assigned for containing the codes of the program running.

This will actually help the virtual memory subsystem to know the following:

Which specific part of the memory is to be filled when required
When exactly the program execution will hit those unfilled areas in the memory

This means that:

Portions of the code of a program will not be really copied into the memory till these are not actually used.
The unused codes of the program may not be copied into the memory at all.

Functions of Loader

The loader is a specific type of a system program located in the memory that accepts object codes from the language translators as input for a program and helps in preparing them for execution.

It typically coordinates the operating system to get the original loading address of a file and loads it into the memory.

The functions of the loader are typically performed in four steps such as allocation, linking, relocation, and loading, where specific operations are performed in each step, as mentioned below.

In the allocation stage, the loader performs the following functions:

Assigning a memory space to load an object program for execution
Calculating the size of the program before allocating the space

In the linking stage, the functions performed by the loader include:

Linking two or more object codes
Providing information necessary for referencing between the codes
Resolving symbolic references between the object modules
Assigning addresses to user and library subroutines

In the relocation stage, the loader performs these basic functions:

Modifying the object program
Altering specific instructions to load at different addresses than the originally specified location
Adjusting address constants depending on the allocated space

In the final loading stage, the functions performed by the loader are:

Bringing the object codes for execution into the memory
Placing the data and machine instructions of relevant subroutines and programs into the memory

In addition to the above major functions, here are some other functions performed by the loader, depending on the type of operating system:

Validation
Memory mapping
Copying command-line arguments to virtual memory
Initializing registers
Initializing structures in the DLL
Creating a heap
Assigning environment variable blocks
Assigning PATH blocks
Adding NTDLL and executable to the module list
Loading KERNEL32.DLL
Loading executable imports
Garbage collection
Calling ntcontinue on the context parameter

Types of Loaders

There are five major types of loaders that perform diverse sets of functions such as, Compile and Go Loader, Absolute Loader, Relocatable Loader Dynamic Run-time Loader, and Bootstrap Loader.

Compile and Go Loader – This loader reads every line of the compiled program and produces machine code that is placed directly in the assigned memory location.
Absolute Loader – This loader takes a relocated object file and puts it in a particular memory location without needing relocation information as it is in the program itself.
Relocatable Loader – This loader can assign space to the executable codes for proper execution at any place within the main memory.
Dynamic Run Time Loader – This loader detects and loads all shared libraries and executable codes while a program is being executed.
Boot Strap Loader – This loader is executed when the computer system is turned on for the first time or restarted and loads the operating system in the main system memory.

Advantages

There is no need to translate a program every time because an object program is created when a source program is executed.
There is no wastage of memory since the loader, which is located in the memory, is smaller than an assembly and therefore occupies a smaller space.
The source program can be written in multiple languages and with multiple programs.

Disadvantages

A modified program must be retranslated.
A specific portion of the memory needs to be reserved for it.

Loader Vs Linker

The loader is an important component of the operating system that loads programs and libraries. Linker is a utility program that takes other codes and joins them into one executable file.
Loader uses the input of executable files generated by the linker. On the other hand, the linker takes object codes from the compiler and assembler.
The job of the loader is to load the executable files into the memory, but the linker produces the executable files.
The loader allocates the addresses to the executable files. In comparison, the linker joins all different modules.
The loader is responsible for setting up references to be used in the program. On the other hand, the linker is responsible for object management in the program space.
There are usually four different types of loaders, but in comparison there are only two types of linkers such as Linkage Editor Linker and Dynamic Linker.
The loader is located after the linker, but the linker is located after the language translators, such as the assembler and compiler.

Conclusion

The loader is a useful component in the memory that accepts object codes generated by the compilers and assemblers and helps in executing them by bringing them to the main memory.

Situated after the linker, it is also responsible for setting up references that will further help the programs in their operation.