Loader

• A loader is a system program, which takes the object code of a program as input and prepares it for execution.

• Programmers usually define the Program to be loaded at some predefined location in the memory.

• But this loading address given by the programmer is not be coordinated with the OS.

• The loader does the job of coordinating with the OS to get initial loading address for the .EXE file and load it into the memory.

Diagram

Loader Function: The loader performs the following functions:

1) Allocation

2) Linking

3) Relocation

4) Loading

Allocation:

• Allocates the space in the memory where the object program would be loaded for Execution.

• It allocates the space for program in the memory, by calculating the size of the program. This activity is called allocation.

• In absolute loader allocation is done by the programmer and hence it is the duty of the programmer to ensure that the programs do not get overlap.

• In reloadable loader allocation is done by the loader hence the assembler must supply the loader the size of the program.

Linking:

• It links two or more object codes and provides the information needed to allow references between them.

• It resolves the symbolic references (code/data) between the object modules by assigning all the user subroutine and library subroutine addresses. This activity is called linking.

• In absolute loader linking is done by the programmer as the programmer is aware about the runtime address of the symbols.

• In relocatable loader, linking is done by the loader and hence the assembler must supply to the loader, the locations at which the loading is to be done.

Relocation:

• It modifies the object program by changing the certain instructions so that it can be loaded at different address from location originally specified.

• There are some address dependent locations in the program, such address constants must be adjusted according to allocated space, such activity done by loader is called relocation.

• In absolute Loader relocation is done by the assembler as the assembler is aware of the starting address of the program.

• In relocatable loader, relocation is done by the loader and hence assembler must supply to the loader the location at which relocation is to be done.

Loading:

• It brings the object program into the memory for execution.

• Finally it places all the machine instructions and data of corresponding programs and subroutines into the memory. Thus program now becomes ready for execution, this activity is called loading.

• In both the loaders (absolute, relocatable) Loading is done by the loader and hence the assembler must supply to the loader the object program.