In these notes, Covered all the topics of operating systems briefly.
COs -
Analyze the structure of OS and basic architectural components involved in OS design.
Compare and contrast various CPU scheduling algorithms.
Evaluate the requirements for the process synchronization and co-ordinatio...
(1) What is operating System? Explain the abstract view of the components of a
computer system.
• An operating system (OS) is a collection of software that manages computer
hardware resources and provides various services for computer programs. It acts as an
intermediary between the user of a computer and the computer hardware.
Figure 1-1. A computer system consists of hardware, system programs, and application
programs.
• The placement of the operating system is shown in Fig. 1-1. At the bottom is the
hardware, which, consists of integrated circuit chips, wires, disks, a key board, a
monitor and similar physical devices.
• On the top of the hardware is the software.
• Operating system runs on the bare hardware and it provides base for the rest of the
software.
• Most computers have two modes of operation: kernel mode and user mode.
• The operating system is the most fundamental piece of software and runs in kernel
mode.
• In this mode it has complete access to all the hardware and can execute any instruction
that the machine is capable of executing.
• The rest of the software runs in user mode, in which only a subset of the machine
instructions is available. Here we find the command interpreter (shell), compilers,
editors, and other system programs.
• Finally, above the system programs are the application programs. These programs are
purchased or written by the users to solve their particular problems, such as word
processing, spreadsheets, web browser or music player.
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, 1 - Introduction
• To hide complexity of hardware, an operating system is provided. It consists of a layer
of software that (partially) hides the hardware and gives the programmer a more
convenient set of instructions to work with.
(2) Give the view of OS as an extended machine.
Operating systems perform two basically unrelated functions: providing a clean abstract set
of resources instead of the messy hardware to application programmers and managing
these hardware resources.
Operating System as an Extended Machine
• The architecture (instruction set, memory, I/O, and bus structure) of most computers at
the machine level language is primitive and awkward to program, especially for input /
output operations.
• Users do not want to be involved in programming of storage devices.
• Operating System provides a simple, high level abstraction such that these devices
contain a collection of named files.
• Such files consist of useful piece of information like a digital photo, e mail messages, or
web page.
• Operating System provides a set of basic commands or instructions to perform various
operations such as read, write, modify, save or close.
• Dealing with them is easier than directly dealing with hardware.
• Thus, Operating System hides the complexity of hardware and presents a beautiful
interface to the users.
Figure 1-2. Operating Systems turn ugly hardware into beautiful abstractions.
• Just as the operating system shields (protect from an unpleasant experience) the
programmer from the disk hardware and presents a simple file-oriented interface, it
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, 1 - Introduction
also conceals a lot of unpleasant business concerning interrupts, timers, memory
management, and other low level features.
• In each case, the abstraction offered by the operating system is simpler and easier to
use than that offered by the underlying hardware.
• In this view, the function of the operating system is to present the user with the
equivalent of an extended machine or virtual machine that is easier to work with than
the underlying hardware.
• The operating system provides a variety of services that programs can obtain using
special instructions called system calls.
(3) Give the view of OS as a Resource Manager.
• The concept of an operating system as providing abstractions to application programs is
a top down view.
• Alternatively, bottom up view holds that the OS is there to manage all pieces of a
complex system.
• A computer consists of a set of resources such as processors, memories, timers, disks,
printers and many others.
• The Operating System manages these resources and allocates them to specific
programs.
• As a resource manager, Operating system provides controlled allocation of the
processors, memories, I/O devices among various programs.
• Multiple user programs are running at the same time.
• The processor itself is a resource and the Operating System decides how much
processor time should be given for the execution of a particular user program.
• Operating system also manages memory and I/O devices when multiple users are
working.
• The primary task of OS is to keep the track of which programs are using which
resources, to grant resource requests, to account for usage, and to resolve conflicting
requests from different programs and users.
• An Operating System is a control program. A control program controls the execution of
user programs to prevent errors and improper use of computer.
• Resource management includes multiplexing (sharing) resources in two ways: in time
and in space.
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