An Introduction to Operating Systems
Your computer can have an expensive motherboard, the fastest
processor, lots of memory, and superior graphics, but it is completely useless
without an operating system (OS). Every computer must have one. What is the
operating system? You can look at it as being the “middle man”. Application
software such as a spreadsheet cannot communicate directly with hardware.
Whether pressing a key on your keyboard or clicking your mouse, it’s the
computer operating system that provides the interface between software and
hardware. It handles all of the computer’s resources and has the ability to
support devices from many different vendors via device drivers. All these tasks
are quietly working behind the scenes without the user’s knowledge. [Source:
Learning about Computers]
An Introduction to Operating Systems
Photo by ktenter
The 1960’s definition of an operating system is “the
software that controls the hardware”. However, today, due to microcode we need
a better definition. We see an operating system as the programs that make the
hardware useable. In brief, an operating system is the set of programs that
controls a computer. Some examples of operating systems are UNIX, Mac, MS-DOS,
MS-Windows, Windows/NT, Chicago, OS/2, MacOS, VMS, MVS, and VM. [Source: Kent
State University]
An operating system is a collection of system programs that
manages all the other programs (i.e,application programs) in a computer as well
as the allocation and use of hardware resources such as the CPU (central
processing unit), memory and the hard disk drive (HDD). It controls and
facilitates the overall operation of a computer. It acts as an interface
between the hardware and the user level program.
Any operating system can be conceptually divided into three
sets of components: a kernel, low level utilities and other system programs,
and a user interface(s). The kernel is the most important and basic part of any
operating system. It has complete control over everything that occurs in it and
communicates directly with the hardware. Kernel controls low-level processes
that the average user usually cannot see: it controls how memory is read and
written, the order in which processes are executed, how information is received
and sent by devices like the monitor, keyboard and mouse, and decides, how to
interpret information received from networks. Kernel of an operating system is
copied from storage (hard disk) into a computer’s memory (RAM) as the computer
boots by a boot program and remains there until the computer is shut down.
An important feature of Unix-like operating systems is that
these components are completely separate entities, thus enabling users to run
various combinations of kernels, user interfaces and utility programs. In other
operating systems the separation is only conceptual
Whether it’s Windows, Mac, or Linux, operating systems have
a Kernel. The Kernel is the heart of an operating system, the part that does
the invisible work such as allowing user applications access to hardware,
managing resources like the memory and CPU, scheduling tasks, and storing
device drivers.It is usually the first part of the OS that is loaded after you
turn on your computer and it completes the POST (Power On Self Test). If a
problem occurs at any point when the OS is loading, usually an error message appears
on the screen.
Utilities are small programs that are used to help manage
the operating system and hardware. System programs also include daemons, which
monitor the system for certain types of events and respond to them.
The user interface is a component that interacts with the
computer user directly, allowing them to control and use programs. The user
interface may be graphical with icons and a desktop, or textual, with a command
line. The most basic, but also the most powerful, type of user interface is a
shell, which, as its name implies, is an outer wrapper to the kernel that
enables users to interact with computers in a text-only mode. Most modern
operating systems also have a graphical user interface (GUI), which provides
windows, icons and menus in a desktop metaphor that are easily manipulated with
a mouse or similar pointing device. GUIs have the advantage of being much more
intuitive and enjoyable for most new users.
The operating system is usually stored on a HDD (hard disk
drive), but it can also be stored on other media, including a CDROM (or even a
single floppy disk in the case of some very small operating systems such as
muLinux).
Whereas users interact with the system through a shell or a
GUI, application programs make use of the operating system and communicate with
each other by making requests for services through applicationprogram
interfaces (APIs) that are defined by the operating system.
Application programming interfaces provide services and code
libraries that let applications developers write modular code reusing well
defined programming sequences in user space libraries or in the operating
system itself. Which features are considered part of the operating system is
defined differently in various operating systems. For example, Microsoft
Windows considers its user interface to be part of the operating system, while
many versions of Linux do not. [Source: Wikipedia]
Since Windows ME, the Windows family has been based on the
WinNT architecture. The first version was NT 3.1, and there were other versions
in the 3.x group which was followed by NT 4.0. Linux and Mac are based on Unix.
Both the Windows and Linux/Mac architecures consist of a layered structure with
User mode sitting on top of the Kernel. In order to make user applications work
it takes an API (ApplicationProgram Interface) & libraries. An API consists
of pre-made tools that aid in writing software. A library is a collection of
often used functions or other code that can be implemented by various programs.
That way, a programmer doesn’t have to rewrite the same code every time it’s
needed. Windows uses Dynamic Link Libraries, which simply means the library is
called during run time, i.e. when a program is actually running. Another way of
looking at it is the library is used only when needed. Dynamic Link Libraries
usually have a .dll extension. [Source: Learning about Computers]
Kernel (the core component), Shell (the user interface) and
File System are the major 3 components of operating system.
Numerous operating systems have been developed over the
years, but only a few are widely used. Microsoft Windows is the dominant family
of operating systems for personal computers, with a share generally estimated
at in excess of 90 percent worldwide. However, its shares are much smaller for
other types of applications, such as mainframes, servers and embedded devices.
An embedded device is a combination of computer hardware and software that is
built into some other product, such as an electric appliance, industrial
production equipment, cell phone or disk drive.
For a list of the most commonly used operating systems along
with a brief description of each, see The Most Popular Operating Systems, by
The Linux Information Project, May 2006
In the opinion of many computer experts, the best and most
successful (as measured in terms of such criteria such as stability,
efficiency, flexibility and longevity) operating systems that have been
developed to date are UNIX and its various descendants (such as Linux and Mac
OS X), commonly referred to as Unix-like operating systems. This is largely a
result of their emphasis (although not always fully achieved) on simplicity,
modularity and transparency.
The first computers did not have operating systems. Rather,
a user would typically load punched paper tape or cards containing a program
and data directly into a tape reader or card reader connected to the computer.
The computer would then commence work and continue until the program was
completed or until it crashed (which was even more common then than now).
The development of operating systems represented a major
improvement in the efficiency of computer utilization. However, the early
operating systems were not standardized, with each computer manufacturer
developing one or more specific to its particular computer models. This
situation continued into the 1960s, when IBM developed the System/360 series of
mainframe computers, all of whose models ran a single basic operating system,
referred to as OS/360.
UNIX, the first version of which was developed in 1969 at
Bell Labs by Ken Thompson, represented a major advance over earlier operating
systems because of its simplicity and portability (i.e., ability to be used on
a wide variety of computers). This, together with the facts that it was
initially essentially free as well as easily obtainable and easily modified,
led to its widespread acceptance by universities and businesses. It also
resulted in its serving as the starting point for the development of other
operating systems.
UNIX was so successful, in fact, that it was difficult for
any subsequently developed systems, including MS-DOS and the other Microsoft
systems, to not be influenced by it.
Although many people have attempted to develop new operating
systems, very few have met with any success. The reason is that it is an
extremely complicated project and it can take years to complete even a most
basic version. Moreover, it would become an even more difficult and time
consuming project if the system were to incorporate some truly novel features.
For example, it took Linus Torvalds more that a year of to make Linux even
marginally useful despite the fact that it was basically just a clone of
existing operating systems, and it took many more years for Linux to take off
and become truly useful.
Types of Operating Systems
There are four major types of computer operating systems: A
Real Time OS is for industry and machines. There’s basically no user
involvement. A single-user single task operating system is for one person to do
one thing at a time. A single-user multitask OS allows one person to run more
than one application on a computer simultaneously. A multi-user operating
system permits multiple users to access a single computer. [Source:Learning
about Computers]
Batch Operating Systems
A batch operating system is a single stream system that
processes the data and program submitted in a group or batch.
Batch processing requires the program , data and appropriate
system commands to be submitted together in the form of a job to the computer
operator. The job is usually in the form of punch cards . Computer executes the
submitted commands and data to process and afer some time the computer produces
output. The output consists of the result of program as well as a dump of the
final memory and register contents for debugging.
Batch Operating system allow little or no interaction
between users and executing programs. Programs that do not require interaction
and programs with long execution times may be served well by Batch Operating
System .
Real-time
A real-time operating system is a multitasking operating
system that aims at executing real-time applications. Real-time operating
systems often use specialized scheduling algorithms so that they can achieve a
deterministic nature of behavior. The main objective of real-time operating
systems is their quick and predictable response to events. They have an
event-driven or time-sharing design and often aspects of both. An event-driven
system switches between tasks based on their priorities or external events while
time-sharing operating systems switch tasks based on clock interrupts.
Multi-user
A multi-user operating system allows multiple users to
access a computer system concurrently. Time-sharing system can be classified as
multi-user systems as they enable a multiple user access to a computer through
the sharing of time. Single-user operating systems, as opposed to a multi-user
operating system, are usable by a single user at a time. Being able to use
multiple accounts on a Windows operating system does not make it a multi-user
system. Rather, only the network administrator is the real user. But for a
Unix-like operating system, it is possible for two users to login at a time and
this capability of the OS makes it a multi-user operating system.
Multi-tasking vs. Single-tasking
When only a single program is allowed to run at a time, the
system is grouped under a single-tasking system. However, when the operating
system allows the execution of multiple tasks at one time, it is classified as
a multi-tasking operating system. Multi-tasking can be of two types:
pre-emptive or co-operative. In pre-emptive multitasking, the operating system
slices the CPU time and dedicates one slot to each of the programs. Unix-like
operating systems such as Solaris and Linux support pre-emptive multitasking,
as does AmigaOS. Cooperative multitasking is achieved by relying on each
process to give time to the other processes in a defined manner. 16-bit
versions of Microsoft Windows used cooperative multi-tasking. 32-bit versions,
both Windows NT and Win9x, used pre-emptive multi-tasking. Mac OS prior to OS X
used to support cooperative multitasking.
Distributed
A distributed operating system manages a group of
independent computers and makes them appear to be a single computer. The development
of networked computers that could be linked and communicate with each other
gave rise to distributed computing. Distributed computations are carried out on
more than one machine. When computers in a group work in cooperation, they make
a distributed system.
Embedded
Embedded operating systems are designed to be used in
embedded computer systems. They are designed to operate on small machines like
PDAs with less autonomy. They are able to operate with a limited number of
resources. They are very compact and extremely efficient by design. Windows CE
and Minix 3 are some examples of embedded operating systems.
Functions of Operating Systems
Operating system performs various tasks such as Process
Management, Memory Management, File Management, IO Management, Secondary
Storage Management, Networking and Protection System.
Process Management
The operating system manages many kinds of activities
ranging from user programs to system programs like printer spooler, name
servers, file server etc. Each of these activities is encapsulated in a
process. A process includes the complete execution context (code, data, PC,
registers, OS resources in use etc.).
It is important to note that a process is not a program. A
process is only ONE instant of a program in execution. There are many processes
can be running the same program. The five major activities of an operating
system in regard to process management are
• Creation
and deletion of user and system processes.
• Suspension
and resumption of processes.
• A
mechanism for process synchronization.
• A
mechanism for process communication.
• A
mechanism for deadlock handling.
Main-Memory Management
Primary-Memory or Main-Memory is a large array of words or
bytes. Each word or byte has its own address. Main-memory provides storage that
can be access directly by the CPU. That is to say for a program to be executed,
it must in the main memory.
The major activities of an operating in regard to
memory-management are:
• Keep
track of which part of memory are currently being used and by whom.
• Decide
which process are loaded into memory when memory space becomes available.
• Allocate
and deallocate memory space as needed.
File Management
A file is a collected of related information defined by its
creator. Computer can store files on the disk (secondary storage), which
provide long term storage. Some examples of storage media are magnetic tape,
magnetic disk and optical disk. Each of these media has its own properties like
speed, capacity, data transfer rate and access methods.
A file systems normally organized into directories to ease
their use. These directories may contain files and other directions.
The five main major activities of an operating system in
regard to file management are
1. The
creation and deletion of files.
2. The
creation and deletion of directions.
3. The
support of primitives for manipulating files and directions.
4. The
mapping of files onto secondary storage.
5. The back
up of files on stable storage media.
I/O System Management
I/O subsystem hides the peculiarities of specific hardware
devices from the user. Only the device driver knows the peculiarities of the
specific device to whom it is assigned.
Secondary-Storage Management
Generally speaking, systems have several levels of storage,
including primary storage, secondary storage and cache storage. Instructions
and data must be placed in primary storage or cache to be referenced by a
running program. Because main memory is too small to accommodate all data and
programs, and its data are lost when power is lost, the computer system must
provide secondary storage to back up main memory. Secondary storage consists of
tapes, disks, and other media designed to hold information that will eventually
be accessed in primary storage (primary, secondary, cache) is ordinarily
divided into bytes or words consisting of a fixed number of bytes. Each
location in storage has an address; the set of all addresses available to a
program is called an address space.
The three major activities of an operating system in regard
to secondary storage management are:
1. Managing
the free space available on the secondary-storage device.
2. Allocation
of storage space when new files have to be written.
3. Scheduling
the requests for memory access.
Networking
A distributed systems is a collection of processors that do
not share memory, peripheral devices, or a clock. The processors communicate
with one another through communication lines called network. The
communication-network design must consider routing and connection strategies,
and the problems of contention and security.
Protection System
If a computer systems has multiple users and allows the
concurrent execution of multiple processes, then the various processes must be
protected from one another’s activities. Protection refers to mechanism for
controlling the access of programs, processes, or users to the resources
defined by a computer systems.
Command Interpreter System
A command interpreter is an interface of the operating
system with the user. The user gives commands with are executed by operating
system (usually by turning them into system calls). The main function of a
command interpreter is to get and execute the next user specified command.
Command-Interpreter is usually not part of the kernel, since multiple command
interpreters (shell, in UNIX terminology) may be support by an operating
system, and they do not really need to run in kernel mode. There are two main
advantages to separating the command interpreter from the kernel.
1. If we
want to change the way the command interpreter looks, i.e., I want to change
the interface of command interpreter, I am able to do that if the command
interpreter is separate from the kernel. I cannot change the code of the kernel
so I cannot modify the interface.
2. If the
command interpreter is a part of the kernel it is possible for a malicious
process to gain access to certain part of the kernel that it showed not have to
avoid this ugly scenario it is advantageous to have the command interpreter
separate from kernel.
Points to Remember
• I found
many students gets confused with multiprogramming and multiprocessing. A trick
to remember would be to look on the word itself. multiprogramming means
multiple programs, so if an operating system can execute multiple programs in
one processor, it is multiprogramming. Similarly multiprocessing is multiple
processing, so if an operating system can use multiple processors to run one
program, it is multiprocessing operating system.
• Operating
system can have graphical user interface (GUI) or character user interface
(CUI). Windows is GUI operating system and DOS is CUI operating system. GUI
offers desktop, icons and menus to click, drag and drop to issue commands where
as CUI offers a command line where you will type the command in correct syntax.
• Batch
Operating System, Real Time Operating System, Single User Single Tasking
Operating System, Single User Multi Tasking Operating System, Multi User
Operating System, Distributed Operating System, Embedded Operating System are
the different types of operating systems.
• Operating
Systems are responsible to:
• Provide
the instructions to display the on-screen elements. These elements are
collectively known as user interface.
• Load
programs into computer memory so that users can use.
• Control
the flow of data in and out of computer as well as to and from peripheral
devices.
• Manage
the data stored in secondary storage
• Provide
security by preventing unauthorized access to the computer’s resources.
• Monitor
the status of critical computer components to detect failures that affect
processing.
• Assign
processors to different tasks being performed by the computer system.
Windows
operating system supports multitasking, multithreading and multiprogramming.
DOS
supports only single user and single tasking.
Operating
system works like an air traffic controller to coordinate the activities within
the computer.
CP/M is
the first PC operating system developed by Dr. Gary A. Kildall in 1974.
In 1981
Microsoft purchased an unauthorized clone of CP/M from Seattle Software Works
and licensed this clone to IBM. IBM marketed it as PC-DOS on the first IBM PC
in 1981. Microsoft marketed it to all other PC as MS-DOS.
In
Windows XP, the term XP stands for eXPerience.
Windows
XP was first released on 25th October 2001.
Windows
Vista released on 30th January 2007.
Windows
7 is released on 22nd July 2009.
Windows
CE is an operating system developed by Microsoft for embedded systems. Windows
CE are used in industrial controllers, communication hubs, POS terminals,
cameras, tablets, interactive televisions.
Pointer,
pointing device, icons, desktop, windows, menus, buttons, toolbars etc are the
basiccomponents of GUI (Graphical User Interface).
