This document discusses different methods for structuring operating systems, including monolithic, layered, and microkernel approaches. It provides examples of each type, such as MS-DOS as a monolithic OS and Windows NT 4.0 and XP as layered OSes. The document also outlines the key characteristics of microkernel systems, including moving most functionality out of the kernel into user space and using inter-process communication. Benefits of the microkernel approach include extensibility, reliability, portability, and support for distributed and object-oriented systems.

1. K.S.SENDHIL KUMAR
SCOPE,VIT University
Operating Systems – Structuring
Methods

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Operating System Layers
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Operating Systems Structures
 Structure/Organization/Layout of
OSs:
1. Monolithic (one unstructured program)
2. Layered
3. Microkernel

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Monolithic Operating System
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Monolithic OS – basic structure
 Application programs that invokes the requested
system services.
 A set of system services that carry out the
operating system procedures/calls.
 A set of utility procedures that help the system
services.
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MS-DOS System Structure
 MS-DOS – written to provide the most functionality in
the least space:
 not divided into modules (monolithic).
 Although MS-DOS has some structure, its interfaces
and levels of functionality are not well separated.

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MS-DOS Layer Structure
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Layered Approach
 The operating system is divided into a number of
layers (levels), each built on top of lower layers.
 The bottom layer (layer 0) is the hardware; the
highest (layer N) is the user interface.
 With modularity, layers are selected such that each
uses functions (operations) and services of only
lower-level layers.

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Layered Operating System
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An Operating System Layer
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General OS Layers
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Layered Architecture

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Microkernel System Structure (1)
 Move as much functionality as possible from the
kernel into “user” space.
 Only a few essential functions in the kernel:
 primitive memory management (address space)
 I/O and interrupt management
 Inter-Process Communication (IPC)
 basic scheduling
 Other OS services are provided by processes
running in user mode (vertical servers):
 device drivers, file system, virtual memory…
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Layered vs. Microkernel Architecture
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Microkernel System Structure (2)
 Communication takes place between user modules
using message passing.
 More flexibility, extensibility, portability and reliability.
 But performance overhead caused by replacing
service calls with message exchanges between
processes.

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Microkernel Architecture

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Benefits of a Microkernel Organization (1)
 Extensibility/Reliability
 easier to extend a microkernel
 easier to port the operating system to new architectures
 more reliable (less code is running in kernel mode)
 more secure
 small microkernel can be rigorously tested.
 Portability
 changes needed to port the system to a new processor
is done in the microkernel, not in the other services.
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Benefits of Microkernel Organization (2)
Distributed system support
message are sent without knowing
what the target machine is.
Object-oriented operating system
components are objects with clearly
defined interfaces that can be
interconnected to form software.

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Mac OS X Structure
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Windows NT 4.0 Architecture
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Windows XP Architecture
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22. Windows 7.0 Architecture
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