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Unit 3: Operating System Structure

best relegated to the operating systems to manage it than giving individual users the control of Notes

it. It is not difficult for the user-level programs to provide these services but for above mentioned
reasons it is best if this services left with operating system.
Communications

There are instances where processes need to communicate with each other to exchange
information. It may be between processes running on the same computer or running on the
different computers. By providing this service the operating system relieves the user of the
worry of passing messages between processes. In case where the messages need to be passed to
processes on the other computers through a network it can be done by the user programs. The

user program may be customized to the specifics of the hardware through which the message
transits and provides the service interface to the operating system.
Error Detection

An error is one part of the system may cause malfunctioning of the complete system. To avoid
such a situation the operating system constantly monitors the system for detecting the errors.
This relieves the user of the worry of errors propagating to various part of the system and causing
malfunctioning.

This service cannot allowed to be handled by user programs because it involves monitoring
and in cases altering area of memory or deallocation of memory for a faulty process. Or may be
relinquishing the CPU of a process that goes into an infinite loop. These tasks are too critical to

be handed over to the user programs. A user program if given these privileges can interfere with
the correct (normal) operation of the operating systems.

3.2 System Calls

System calls provide an interface between a running program and operating system. System calls
are generally available as assembly language instructions. Several higher level languages such as
C also allow to make system calls directly.

In UNIX operating system the system call interface layer contains entry point in kernel code. All
system resources are managed by the kernel. Any request from user or application that involves
access to any system resource must be handled by kernel code. The user process must not be given
open access to kernel code for security reason. Many opening into kernel code called system calls
are provided to user so that the user processes can invoke the execution of kernel code. System
calls allow processes and users to manipulate system resources.
There are three general methods that are used to pass information (parameters) between a
running program and the operating system.
1. One method is to store parameters in registers.
2. Another is to store parameters in a table in memory and pass the address of table.
3. The third method is to push parameters on stack and allow operating system to pop the
parameters off the stack.

System Calls for Process Management

These types of system calls are used to control the processes. Some examples are end, abort, load,
execute, create process, terminate process etc.

Example: The exit() system call ends a process and returns a value to it parent.

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