Structure Of Operating System

Structure Of Operating System.

There are two types of structure
 1. Monolithic structure
 2. Layered structure

1. Monolithic Operating System Structure

A monolithic kernel is an operating system structure where the entire operating system is
working in the kernel space and alone as supervisor mode.
All the functionality resides in a single large module.

1. It is first structured, but is very problematic from the standpoint of code correctness, since a code
error anywhere can result in a system crash.

2. It was also hard to port or extend, because code that needs to be changed tends to be spread
throughout the system, rather than being localized in one place.
structuring model for a monolithic system

This organization suggests a basic structure for the operating system:
1. The main program that invokes the requested service procedure.
2. A set of service procedures that carry out the system calls.

3. A set of utility procedures that help service procedures.

2. Layered Approach

This approach breaks up the operating system into different layers. The system had 6 layers :

Layer 0 split with an allocation of the processor, switching between processes when interrupts
occurred or timers expired. Layer 0 provided the basic multiprogramming of the CPU.

Layer 1 did memory management. It allocated space for processes in the main memory. Layer 1
software took care of making sure pages were brought into memory whenever they were needed.

Layer 2 handled communication between each process and the operator console. Above this
layer each process effectively had its own operator console.

Layer 3 took care of managing the I/O devices and buffering the information streams to and
from them. Above layer 3 each process could deal with abstract I/O devices with nice properties,
instead of real devices with many peculiarities.

Layer 4 was where the user programs were found. They did not have to worry about the process,
memory, console, or I/O management.

The system operator process was located in layer 5.

So, With a layered approach, the bottom layer is the hardware, while the highest layer is the user
interface.

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