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Introduction to Operating System

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It includes an introduction to Operating system (OS): Computer system structure and organization. OS definition, function, history. Categories, OS services, and operations. Note: If this data is useful for you , may you recommended it.
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[Dr. Qasim Mohammed Hussein] Page 1
Assistant Prof. Dr.
Qasim Mohammed Hussein
Lecture Notes on Operating System
[Dr. Qasim Mohammed Hussein] Page 2
An operating system (OS) is a set of programs that control the
execution of application programs and act as an intermediary between
a user of a computer and the computer hardware. OS is software that
manages the computer hardware as well as providing an environment
for application programs to run.
Examples of OS are: Windows, Windows/NT, OS/2 and MacOS.
The objectives of OS are:
(1) To make the computer system convenient and easy to use for the user.
(2) To use the computer hardware in an efficient way.
(3) To execute user programs and make solving user problems easier.
A computer system can be divided into four components: the
hardware, the operating system, the application programs and the
users. The abstract view of system components is shown in figure 1.
1. Hardware: such as CPU, memory and I/O devices.
2. Operating system: provides the means of proper use of the hardware in the
operations of the computer system, it is similar to government.
3. Application programs: solve the computing problems of the user, such as :
compilers, database systems and web browsers.
4. Users: peoples, machine, or other computer.
Introduction to operating
Computer System
Operating system Objectives
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Figure 1: computer system
1. Computer-System Operation
A modern general-purpose computer system consists of one or more CPUs and a
number of device controllers connected through a common bus that provides
access to shared memory (Figure 1.2). Each device controller is in charge of a
specific type of device (for example, disk drives, audio devices, and video
displays). The CPU and the device controllers can execute concurrently,
competing for memory cycles. To ensure orderly access to the shared memory, a
memory controller is synchronizing access to the memory.
For a computer to start running-for instance, when it is powered up or rebooted-
it needs to have an initial program to run. This initial program, or bootstrap
program, tends to be simple. Typically, it is stored in read-only memory (ROM)
or electrically erasable programmable read-only memory (EEPROM),known by
the general term firmware, within the computer hardware. It initializes all
aspects of the system, from CPU registers to device controllers to memory
contents. The bootstrap program must know how to load the operating system
Computer System Organization
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and to start executing that system. To accomplish this goal, the bootstrap
program must locate and load into memory the operating system kernel. The
operating system then starts executing the first process, such as "init," and waits
for some event to occur.
Figure 2
2. Storage Structure
Computer programs must be in main memory (also called RAM) to be executed.
Main memory is the only large storage area that the processor can access
directly. It forms an array of memory words. Each word has its own address.
Interaction is achieved through a sequence of load or store instructions to
specific memory addresses. The load instruction moves a word from main
memory to an internal register within the CPU, whereas the store instruction
moves the content of a register to main memory.
The instruction-execution cycle includes:
1) Fetches an instruction from memory and stores that instruction in the
instruction register. And increment the PC register.
2) Decode the instruction and may cause operands to be fetched from memory
and stored in some internal register.
3) Execute the instruction and store the result in memory.
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The programs and data are not resided in main memory permanently for the
following two reasons:
1) Main memory is usually too small to store all needed programs and. Data
2) Main memory is a volatile storage device that loses its contents when power
is turned off or otherwise lost.
Thus, most computer systems provide secondary storage as an extension of main
memory to hold large quantities of data permanently.
The wide variety of storage systems in a computer system can be organized in a
hierarchy (figure 2). The main differences among the various storage systems
lie in speed, cost, size, and volatility. The higher levels are expensive, but they
are fast.
Figure 3: Storage device hierarchy
3. I/O Structure
A computer system consists of CPUs and multiple device controllers that are
connected through a common bus. The device controller is responsible for
moving the data between the peripheral devices that it controls and its local
3. I/O Structure
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buffer storage. Typically, operating systems have a device driver for each device
To start an I/O operation, the device driver loads the appropriate registers within
the device controller. The device controller examines the contents of these
registers to determine what action to take. The controller starts the transfer of
data from the device to its local buffer. Once the transfer of data is complete, the
device controller informs the device driver via an interrupt that it has finished
its operation. The device driver then returns control to the operating system . For
other operations, the device driver returns status information.
For moving bulk data, direct memory access (DMA) is used. After setting up
buffers, pointers, and counters for the I/O device, the device controller transfers
an entire block of data directly to or from its own buffer storage to memory, with
no intervention by the CPU. Only one interrupt is generated per block, to tell the
device driver that the operation has completed, rather than the one interrupt per
byte generated for low-speed devices.
There are different categories for designing a computer system according to the
number processors used.
1. Single-processor system: there is one CPU for executing instructions.
2. Multiprocessor system: It contains two or more processors that share bus,
clock, physical memory and peripheral devices. The advantages of
multiprocessors are:
a) Increase throughput.
b) Economy scale (less cost).
c) Increase reliability.
3. Clustered system: it consists of multiple computer systems connected by a
local area network.
Computer system structure
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Operating systems have been evolving through the years. Following table shows
the history of OS.
OS performs many functions such as:
1. Implementing user interface.
2. Sharing HW among users.
3. Allowing users to share data among themselves.
4. Preventing users from interfering with one another.
5. Scheduling resource among users.
6. Facilitating I/O operations.
7. Recovering from errors.
8. Accounting for resource storage.
9. Facilitating parallel operations.
10. Organizing data for secure and rapid access.
11. Handling network communications.
Operating system Functions
Operating system History
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The main categories of modern OS may be classified into three groups which
are distinguished by the nature of interaction that takes place between the
computer and the user:
1. Batch system
In this type of OS, users submit jobs on regular schedule (e.g. daily, weekly,
monthly) to a central place where the user of such system did not interact
directly with computer system. To speed up the processing, jobs with similar
needs were batched together and were run through the computer as a group.
Thus, the programmer would leave the programs with the operator. The output
from each job would send to the appropriate programmer. The major task of this
type was to transfer control automatically from one job to the next.
Disadvantages of Batch System
1. Turnaround time can be large from user standpoint.
2. Difficult to debug program.
2. Time-Sharing System
This type of OS provides on-line communication between the user and the
system, the user gives his instructions directly and receives intermediate
response, and therefore it called interactive system.
The time sharing system allows many user simultaneously share the
computer system. The CPU is multiplexed rapidly among several programs,
which are kept in memory and on disk. A program swapped in and out of
memory to the disk.
Time sharing system reduces the CPU ideal time. The disadvantage is more
Operating system Categories
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3. Real time operating system
Real Time System is characterized by supplying immediate response. It
guarantees that critical tasks complete on time. This type must have a pre-
known maximum time limit for each of the functions to be performed on the
computer. Real-time systems are used when there are rigid time requirements
on the operation of a processor or the flow of data and real-time systems can
be used as a control device in a dedicated application.
The airline reservation system is an example of this type.
1. On-line and off-line operation
A special subroutine was written for each I/O device called a device
controller. Some I/O devices has been equipped for either on-line
operation (they are connected to the processor), or off-line operations
(they are run by control unit).
2. Buffering
A buffer is an area of primary storage for holding data during I/O transfer.
On input, the data are placed in the buffer by an I/O channel, when the
transfer is complete the data may be accessed the processor. The buffing
may be single or double.
3. Spooling (Simultaneously Peripheral Operation On-Line)
Spooling uses the disk as a very large buffer. Spooling is useful because
device access data that different rates. The buffer provides a waiting
station where data can rest while the slower device catches up.
Spooling allows overlapping between the computation of one job and I/O
of another job.
Performance development of OS
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4. Multiprogramming
In multiprogramming several programs are kept in main memory at the
same time, and the CPU is switching between them , thus the CPU always
has a program to be execute. The OS begins to execute one program from
memory, if this program need wait such as an I/O operation, the OS
switches to another program. Multiprogramming increases CPU
utilization. Multiprogramming system provide an environment in which
the various system resources are utilized effectively, but they do not
provide for user interaction with the computer system.
a) High CPU utilization.
b) It appears that many programs are allotted CPU almost
a) CPU scheduling is requires.
b) To accommodate many jobs in memory, memory management is
5. Parallel system
There are more than on processor in the system. These processors share
the computer bus, clock, memory and I/O devices.
The advantage is to increase throughput (the number of programs
completed in time unit).
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6. Distributed system
Distribute the computation among several physical processors. It involves
connecting 2 or more independent computer systems via communication
link. So, each processor has its own O.S. and local memory; processors
communicate with one another through various communications lines,
such as high-speed buses or telephone lines.
Advantages of distributed systems:
a) Resources Sharing You can share files and printers.
b) Computation speed up A job can be partitioned so that each
processor can do a portion concurrently (load sharing).
c) Reliability If one processor failed the rest still can function with no
d) Communications Such as electronic mail, ftp
7. Personal computer
Personal computers computer system dedicated to a single user. PC
operating systems were neither multi-user nor multi-tasking. The goal of
PC operating systems were to maximize user convenience and
responsiveness instead of maximizing CPU and I/O utilization.
•Examples: Microsoft Windows and Apple Macintosh
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An operating system provides services to programs and to the users of
those programs. The common services provided by the operating
system are:
1. Program execution: Operating system loads a program into memory
and executes the program. The program must be able to end its
execution, either normally or abnormally.
2. I/O Operation: I/O means any file or any specific I/O device.
Program may require any I/O device while running. So operating
system must provide the required I/O.
3. File system manipulation: Program needs to read a file or write a
file. The operating system gives the permission to the program for
operation on file.
4. Communication: Data transfer between two processes is required
for some time. The both processes are on the one computer or on
different computer but connected through computer network.
Communication may be implemented by two methods:
a. Shared memory
b. Message passing.
5. Error detection: error may occur in CPU, in I/O devices or in the
memory hardware. The operating system constantly needs to be
aware of possible errors. It should take the appropriate action to
ensure correct and consistent computing.
Operating system with multiple users provides efficient system
1. Resource allocation: For simultaneously executing job.
Operating system Service
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2. Accounting: For account billing and usage statistics.
3. Protection: Ensure access to system resource is controlled.
Modern operating systems are interrupt driven. If there are no processes to
execute, no I/O devices to service, and no users to whom to respond, an
operating system will sit quietly, waiting for something to happen. Events are
almost always signaled by the occurrence of an interrupt or a trap. A trap is a
software-generated interrupt caused either by an error (for example, division by
zero or invalid memory access) or by a specific request from a user program
that an operating-system service be performed. For each type of interrupt,
separate segments of code in the operating system determine what action should
be taken. An interrupt service routine is provided that is responsible for dealing
with the interrupt.
Since the operating system and the users share the hardware and software
resources of the computer system, we need to make sure that an error in a user
program could cause problems only for the one program that was running. With
sharing, many processes could be adversely affected by a bug in one program. A
properly designed operating system must ensure that an incorrect (or
malicious) program cannot cause other programs to execute incorrectly.
A) Dual-Mode Operation
We must be able to distinguish between the execution of operating-system code
and user defined code. The approach is to separate the two modes of operation:
user mode and kernel mode (also called supervisor mode, system mode, or
privileged mode). A bit, called the mode bit is added to the hardware of the
computer to indicate the current mode: kernel (0) or user (1). The dual mode of
operation provides us with the means for protecting the operating system from
errant users-and errant users from one another.
Operating system operations
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System calls provide the means for a user program to ask the operating system to
perform tasks reserved for the operating system on the user program's behalf.
B) Protection CPU
To ensure that the operating system maintains must control over the CPU. We
must prevent a user program from getting stuck in an infinite loop or not
calling system services and never returning control to the operating system. To
accomplish this goal, we can use a timer. A timer can be set to interrupt the
computer after a specified fixed or variable period.
The operating system components are :
In multiprogramming environment, OS decides which process gets the processor
when and how much time. The operating system is responsible for the following
activities in regard to process management:
1) Creating and deleting both user and system processes
2) Suspending and resuming processes
3) Providing mechanisms for process synchronization
4) Providing mechanisms for process communication
5) Providing mechanisms for deadlock handling
Main memory is a large array of words or bytes where each word or byte has its
own address. The operating system is responsible for the following activities in
regard to memory management:
1) Keeping track of which parts of memory are currently being used and by
2) Deciding which processes (or parts thereof) and data to move into and out of
1. Process Management
Memory Management
Operating System Components
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3) Allocating and deallocating memory space as needed.
The operating system is responsible for the following activities in regard to file
1) Creating and deleting files
2) Creating and deleting directories to organize files
3) Supporting primitives for manipulating files and directories
4) Mapping files onto secondary storage
5) Backing up files on stable (nonvolatile) storage media
OS provides the following activities in connection with disk management:
1. Free-space management.
2. Storage allocation
3. Disk scheduling.
System calls provide an interface between the running program and the
operating system. User cannot execute privileged instructions; the user must ask
OS to execute them- system calls. System calls are implemented using traps.
OS gains control through trap, switches to kernel mode, performs service,
switches back to user mode, and gives control back to user.
Example about how system calls are used from the OS to read data from one
files and copy them to another file, shown in figure . the programmer never see
this level of details
System Call and System Program
File system Management
Secondary storage Management
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Example system call sequence
System program provide basic function to users, so that they don’t need to write
their own environment for program development (editors, compilers) and
program execution (shells).
Protection refers to mechanism that control the access of programs or users to the
both system and resources. The protection mechanism must:
1. Distinguish between unauthorized and authorized user.
2. Specify the control to be imposed.
3. Provide a means of enforcement.
Security measures are responsible for defending a computer system from external
or internal attack.
There are many questionS the files:
1. Operating System: Questions and their answers: Processes and Deadlock
(Part 1)
2. Operating system questions with their answers (Memory management,
Virtual memory, Processes synchronization) Part two.
The link:
Acquire input filename
Write prompt to screen
Accept input
Acquire output filename
Write prompt to screen
Accept input
Open the input file
If file does not exist, abort
Create output file
If file exist, abort
Read from input file
Write to output file
Until read fail
Close output file
Write completion message on screen
Terminate normally
Protection and security
Module Questions
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