ch1-introduction-to-os.ppt

1.1 Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition
Ch1: Introduction to OS
(12M)

What is an Operating System?
It is a program/software that that’s controls the execution of applications
programs such as compilers, database systems, games etc
It acts as an intermediary between a user of a computer and the
computer hardware.
In other words it is an interface between applications and hardware.
OS manages the computer hardware resources
The operating system provides the means for the proper use of these
resources in the operation of the computer system.
Operating system goals:
Execute user programs and make solving user problems easier
Make the computer system convenient to use
Use the computer hardware in an efficient manner

Operating systems as a User interface
Operating systems interact directly with the hardware to provide an interface
to other system software.
System software and hard ware exist to support the creation and effective
use application software.

 A computer system has a set of resources. These resource provides
various functions to the user like data movement, storing of data and
program, operation on data which are control by an operating system.
 The OS is responsible for controlling the use of a computer’s resources,
such as I/O, main and secondary memory, and processor execution time.
 Below Fig. shows OS as a resource manager

Operating System as Resource Manager

 The operating system is responsible for managing the all resources. A
portion of the OS is in main memory. This portion of the OS is called kernel.
 User program and data are also stored in remaining parts of the memory.
 I/O device is controlled by OS and it decides when an I/O device can be
used by program in execution. Processor is one type of resource and OS
control the execution of user program on the processor.
 Allocation of main memory is controlled by operating system with the help of
memory management hardware.
 Resource management includes sharing resources in different ways. Time
and space are the two concept for resource sharing.

The services provided by OS
The OS typically provides services in the following areas:
 Program development: The OS provides a variety of facilities and
services, such as editors and debuggers, to assist the programmer in
creating programs.
 Program execution: A number of steps need to be performed to execute a
program. Instructions and data must be loaded into main memory, I/O
devices and files must be initialized, and other resources must be prepared.
 Access to I/O devices: Each I/O device requires its own peculiar set of
instructions The OS provides a uniform interface that hides these details so
programmers can access such devices.
 Controlled access to files: For file access, the OS must reproduce a
detailed understanding of the nature of the I/O device (disk drive, tape drive)
and the structure of the data contained in the files on the storage medium.
 Error detection and response, In each case, the OS must provide a
response that clears the errors such as : internal and external hardware
errors and various software errors condition with the least impact on running
applications.

Mainframe systems
Batch
Multi Programmed
Multitasking
Time sharing
Desktop

Batch
To speed up processing, operators batched together jobs with similar needs
and ran them through the computer as a group. Thus, the programmers
would leave their programs with the operator. The operator would sort
programs into batches with similar requirements and, as the computer
became available ,would run each batch

Multi Programmed
When two or more programs are in the memory at the same time ,sharing
the processor is referred to the multiprogramming operating system.
Multiprogramming increases CPU utilization by organizing jobs so that the
CPU always has one to execute.
Resource management is the main aim of multiprogramming OS
The operating system keeps several jobs in memory simultaneously
As long as at least one job needs to execute, the CPU is never idle
Multiprogramming OS do not provide user interaction with the computer
system.

 In multiprogramming operating system, programs are competing for
resources.
 Suppose there are four programs for execution. All four programs are loaded
into the memory. CPU select first program for execution. Normally programs
contain instruction for CPU and I/O operation
 CPU bound instructions : c = a + b
 I/O bound instructions : printf, scanf etc
 When any I/O instruction is run into the program, CPU select next program for
processing. CPU select second program for execution and I/O system select
first program for performing I/O operation.
 In this way multiprogramming operating system monitors the state of all active
programs and system resources.

Multitasking (time sharing)
 Multi-tasking means that the computer can work with more than one program
at a time.
 Time sharing (or multitasking) is a logical extension of multiprogramming OS
The CPU executes multiple jobs by switching among them, but the switches
occur so frequently that the users can interact with each program while it is
running.
The response time should be short typically within 1 second or so.
A time-shared operating system allows many users to share the computer
simultaneously
 User interaction with program is possible in time sharing OS. During execution
of the program, user interacts directly with the program, supplying information
to the program.
 Many users share the computer system simultaneously in time sharing
operating system. Time sharing system uses multiprogramming and CPU
scheduling. Each user has at least one separate program in memory.

 In In time sharing system, each user is given a time slice for executing
his/her job in round robin fashion, job continues until the time slice ends.
 Time sharing system is more complex than multiprogramming operating
system.
 Major problem with time sharing system is protection and security of data.

Multiprocessor System
 Multiprocessor system means more than one processor in close
communication. All the processor share common bus, clock, memory and
peripheral devices.
 By increasing the number of processors, we expect to get more work done in
less time
 Multiprocessor system is also called parallel system or tightly coupled
systems. They handle system calls, do memory management, provide a file
system, and manage I/O devices.
 The most common multiprocessor systems use symmetric multiprocessing
(SMP), in which each peer CPU processor performs all tasks, including
operating-system functions and user processes.
 Features of Multiprocessor Systems
 1. The processor should support efficient context switching operation
 2. It supports large physical address space and larger virtual address space.
 3. If one processor fails, then other processors should save the interrupted
process state so that execution of the process can continue

 Multiprocessor systems are of two types: symmetric multiprocessing and
asymmetric multiprocessing.

End of Chapter 1

ch1-introduction-to-os.ppt

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