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important questions from system programming and compiler construction
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important questions from system programming and compiler construction
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1. What is loader? Explain different functions of loader?Ans:
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 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.
, 2. Explain the working of a direct linking loader with a proper
example. Clearly show the entries in the different databases built by
the direct linking loader.
Ans:
• Loader is the system program which is responsible for preparing the object program for
execution and initiate the execution.
• 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.
Dynamic Linking Loader:
• Dynamic Linking Loader is a general re-locatable loader
• Allowing the programmer multiple procedure segments and multiple data segments and
giving programmer complete freedom in referencing data or instruction contained in other
segments.
• The assembler must give the loader the following information with each procedure or data
segment.
• Dynamic linking defers much of the linking process until a program starts running. It
provides a variety of benefits that are hard to get otherwise.
• Dynamically linked shared libraries are easier to create than static linked shared libraries.
• Dynamically linked shared libraries are easier to update than static linked shared libraries.
• The semantics of dynamically linked shared libraries can be much closer to those of
unshared libraries.
• Dynamic linking permits a program to load and unload routines at runtime, a facility that
can otherwise be very difficult to provide.
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