Session1 intro to_os

Session 1
Introduction to Operating System
By
Kalyani Patil

1. What operating systems do
2. Definition of Operating System
3. Computer System Organization
4. Computer System Architecture
5. Operating System Structure
6. Operating System Operation
7. Process Management
8. Memory Management

What operating systems do
Your computer's operating system (OS) manages all of
the software and hardware on the computer. Most of the time, there are
several different computer programs running at the same time, and they all
need to access your computer's central processing unit (CPU), memory,
and storage. The operating system coordinates all of this to make sure each
program gets what it needs.
OS is System Software
It acts like intermediate between H/W and user
Resource Manager
Provide platform on which other application program are installed.

The operating system manages a computer's
hardware resources, including:
Input devices such as a keyboard and mouse
Output devices such as display monitors,
printers and scanners
Network devices such as modems, routers and
network connections
Storage devices such as internal and external
drives

User View
The user's view of the computer varies according to
the interface being used. Most computer users sit in
front of a PC, consisting of a monitor/ keyboard/
mouse, and system unit. Such a system is designed
for one user to monopolize its resources.
The goal is to maximize the work (or play) that the
user is performing. In this case/ the operating system
is designed mostly for with some attention paid to
performance and none paid to various hardware and
software resources are shared. Performance is, of
course, important to the user.

 In other cases, a user sits at a terminal connected to a
mainframe or a minicomputer. Other users are accessing the
same computer through other terminals. These users share
resources and may exchange information. The operating
system in Such cases is designed to maximize resource
utilization to assure that all available CPU time, memory, and
I/0 are used efficiently and that no individual user takes more
than her fair share.
Workstation users sit at connected to networks of other
workstations and These users have dedicated resources at their
disposal, but they also share resources such as networking and
servers-file, compute, and print servers. Therefore, their
operating system is designed to compromise between

System view
 Resource allocator : From system’s point of view OS is resource allocator.
 A computer system has many resources that may be required to solve a
problem:
 CPU time, memory space,
 file-storage space,
 I/0 devices, and so on.
 The operating system acts as the manager of these resources. Facing
numerous and possibly conflicting requests for resources, the operating
system must decide how to allocate them to specific programs and users
so that it can operate the computer system efficiently and fairly.

 As we have seen, resource allocation is especially important
where many users access the same mainframe or
minicomputer. A slightly different view of an operating
system emphasizes the need to control the various I/0
devices and user programs.
 An operating system is a control program. A manages the
execution of user programs to prevent errors and improper
use of the computer. It is especially concerned with the
operation and control of I/O devices.

Operating System Definition
We can also view a computer system as consisting of
hardware/ software and data. The operating system
provides the means for proper use of these resources
in the operation of the computer system.
An operating system is similar to a government. Like a
government, it performs no useful function by itself. It
simply provides an environment within which other
programs can do useful work.

An operating system is the most important software that runs
on a computer. It manages the
computer's memory and processes, as well as all of
its software and hardware. It also allows you
to communicate with the computer without knowing how to
speak the computer's language.
Without an operating system, a computer is useless.

Computer System Organization
Computer System

For a computer to start running-for instance, when it is
powered up or rebooted
Bootstrap program (Initial program)
firmware( stored on rom or EEPROM)
Operating system then start executing the first process
such as init.

I/O structure
 There are many types of IO devices used in computer system .
Managing these devices is major task for the operating
system.
 I/O operation affect the reliability and performance of a
system.

 Main memory is usually too small to store all needed programs
and data permanently. Main memory is a volatile storage device
that loses its contents when power is turned off or otherwise lost.
 Thus, most computer systems provide as an extension of main
memory. The main requirement for secondary storage is that it
be able to hold large quantities of data permanently. The most
common secondary-storage device is a which provides storage
for both programs and data. Most programs (system and
application) are stored on a disk until they are loaded into
memory. Many programs then use the disk as both the source
and the destination of their processing. Hence, the proper
management of disk storage is of central importance to a
computer system

Computer System Architecture
According to no of general purpose resister computer
system is organized.
Single Processor
Multiprocessor
 Symmetric Multiprocessor
 Asymmetric Multiprocessor
Clustered System

Single processor
 Most of the system uses a single processor . From PDA to
mainframe system are single processor.
 One main CPU used in single processor.
 This CPU execute a general purpose instruction set, also it can
execute instruction from user processes.
 Other device specific processor or I/O processor can be used
in single processor system.

Multiprocessor operating systems
 Multiprocessor operating systems are the server operating
systems with special features for the communication and the
connectivity.
 Multiprocessor operating systems are used where multiple
CPUs connected into a single system.
 Multiprocessor operating system (OS) is almost a regular OS
as they also handle system calls, do memory management,
provide file system, and also manage input/output devices.
 But, there are some extra features available in multiprocessor
operating systems, those extra features are listed below:
 Process synchronization
 Resource management
 Scheduling

 Here are the list of some various organizations of multiprocessor operating
systems:
 Each CPU has its own OS
 Master-Slave multiprocessors
 Symmetric multiprocessors
 Master-Slave Multiprocessors(Asymmetric)
• The master-slave models basically solves almost all the problems of the first model.
• In this model, there is a single data structure that keeps track of ready processes.
• Now, when a central processing unit goes idle in this model, then it asks the OS for a
process to run and it is assigned one.
 Symmetric Multiprocessors
• Symmetric multiprocessors (SMP) is the third model. In this model, there is one copy of
the OS in memory, but any central processing unit can run it.
• Now, when a system call is made, then the central processing unit on which the system
call was made traps to the kernel and then processes that system call.

Clustered System
 Another type of multiple-CPU system is the Like
multiprocessor systems, clustered systems gather together
multiple CPUs to accomplish computational work.
 They composed of two or more individual systems-or nodes-
joined together.
 The generally accepted definition is that clustered computers
share storage and are closely linked via a LAN’s or a faster
interconnect, such as InfiniBand.
 Clustering is usually used to provide service; that is, service
will continue even if one or more systems in the cluster faiL
 High availability is generally obtained by adding a level of
redundancy in the system.

 A layer of cluster software runs on the cluster nodes. Each node can
monitor one or more of the others (over the LAN). If the monitored
machine fails, the monitoring machine can take ownership of its
storage and restart the applications that were running on the failed
machine. The users and clients of the applications see only a brief
interruption of service.
 As a cluster consists of several clusters may also be used to provide
environments. Such systems can supply significantly greater
computational power than single-processor or even SMP systems
because they are capable of running an application concurrently on
all computers in the cluster. However, applications must be written to
take advantage of the cluster by using a technique known as which
consists of dividing a program into separate components that run in
parallel on individual computers in the cluster. Typically, these
applications are designed so that once each computing node in the
cluster has solved its portion of the problem, the results from all the
nodes are combined into a final solution.

Symmetric and Asymmetric Clustering
 Clustering can be structured or symmetrically.
 In Symmetric clustering one machine is in hot
standby mode, while the other is running the
applications
 The hot-standby host machine does nothing but
monitor the active server. If that server fails, the not-
standby host becomes the active server.
 In two or more hosts are running applications and
are monitoring each other. This mode is obviously
more efficient, as it uses all of the available hardware.

Operating System Operation
 Operating System is Interrupt driven.
 If there are no processes to execute, no I/0 devices to service,
and no users to whom to respond, an operating system will sit
quietly waiting for something to happen.
 Trap(Exception) is a software generated interrupt caused either
by error (divide by zero)or by a specific request from a user
program that an operating-system service be performed.
 interrupt-driven nature of an operating system defines that
system's general structure. 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.

1. Dual Mode Operation
User mode
System Mode or Kernel Mode or Supervised or Privileged
mode
 In order to ensure the proper execution of the operating system,
we must be able to distinguish between the execution of
operating-system code and user defined code .

 At the very least we need two separate modes of operation user
mode and kernel mode.
 A bit, called mode bit is added to the hardware of the computer
to indicate the current mode: kernel (0) or user (1).
 With the mode bit we are able to distinguish between a task that
is executed on behalf of the operating system and one that is
executed on behalf of the user.
 When the computer system is executing on behalf of a user
application, the system is in user mode. However, when a user
application requests a service from the operating system (via a ..
system call), it must transition from user to kernel mode to fulfill
the request.

 At system boot time, the hardware starts in kernel mode.
 The operating system is then loaded and starts user
applications in user mode.
 Whenever a trap or interrupt occurs, the hardware switches
from user mode to kernel mode (that is, changes the state of
the mode bit to 0).
 Thus, whenever the operating system gains control of the
computer, it is in kernel mode. The system always switches to
user mode (by setting the mode bit to 1) before passing
control to a user program.

 The dual mode of operation provides us with the means for
protecting the operating system from errant users-and errant users
from one another. We accomplish this protection by designating
some of the machine in instructions that may cause harm as
privileged instruction.
 The hardware allow’s privileged instruction only, If an attempt is
made to execute a privileged instruction in user mode, the
hardware does not execute the instruction but rather treats it as
illegal and traps it to the operating system.

Timer
 We must ensure the operating system maintains control over
the CPU.A user program must be prevented from getting
struck in an infinite loop or not calling system services and
never returning control to the operating. To accomplish this
goal, we can use a timer.
 A timer can be set to interrupt the computer after a specified
period.
 The period may be fixed (for example, 1/60 second) or
variable (for example, from 1 millisecond to 1 second).

Process Management
 A process is a program in execution for exm. Batch job is a
process.
 A program dose nothing unless its instructions are executed
by a CPU .
 A time shared user program such as compiler is a process.
 A system task (Spooling O/P to printer) is a process.
 A process needs certain resources---including CPU time,
memory, files, and I/O devices to accomplish its task These
resources are either given to the process when it is created or
allocated to it while it is running.

 We emphasize that a program by itself is not a process :a
process is a passive entity , such as a content of file stored on
disk, whereas a process is an active entity.
 A single threaded process has one program counter
specifying the next instruction to execution.
 The execution of such a process must be sequential. The CPU
executes one instruction of the process after another, until the
process completes.
 A process is the unit of work in a system. Such a system
consists of a collection of processes, some of which are
operating-system processes (those that execute system code)
and the rest of which are user processes (those that execute
user code).

Activity
1. Scheduling processes and threads on the CPUs
2. Creating and deleting both user and system
processes
3. Suspending and resuming processes
4. Providing mechanisms for process
synchronization
5. Providing mechanisms for process
communication
6. Provide Mechanism for Deadlock Handling

Memory Management
 For the CPU to process data from disk, those data must first be transferred
to main memory by CPU-generated I/0 calls. In the same way, instructions
must be in memory for the CPU to execute them.
 For a program to be executed, it must be mapped to absolute addresses
and loaded into memory.
 As the program executes, it accesses program instructions and data from
memory by generating these absolute addresses.
 Eventually, the program terminates, its memory space is declared available,
and the next program can be loaded and executed.
 To improve both the utilization of the CPU and the speed of the
computer's response to its users, general-purpose computers must keep
several programs in memory, creating a need for memory management.

Activity
 Keeping track of which parts of memory are currently
being used and by whom
 Deciding which processes (or parts thereof) and data
to move into and out of memory
 Allocating and de allocating memory space as
needed

File-System Management
 A file is a collection of related information defined by its
creator.
 Commonly, files represent programs (both source and object
forms) and data.
 Data files may be numeric, alphabetic, alphanumeric, or
binary. Files may be free-form (for example, text files), or they
may be formatted rigidly (for example, fixed fields).
 The operating system implements the abstract concept of a
file by managing mass-storage media, such as tapes and
disks, and the devices that control them. Also, files are
normally organized into directories to make them easier to
use. Finally, when multiple users have access to files, it may be
desirable to control by whom and in what ways (for example,
read, write, append) files may be accessed.

Activities
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

Mass Storage Management
Most programs-including compilers, assemblers, word
processors, editors, and formatters-are stored on a disk
until loaded into memory and then use the disk as both the
source and destination of their processing. Hence, the
proper management of disk storage is of central
importance to a computer system.
1. Free-space management
2. Storage allocation
3. Disk scheduling

Caching
 Caching is an important principle of computer systems. Information is
normally kept in some storage system (such as main memory). As it is
used, it is copied into a faster storage system-the cache-on a temporary
basis. When we need a particular piece of information, we first check
whether it is in the cache. If it is, we use the information directly from the
cache; if it is not, we use the information from the source, putting a copy
in the cache under the assumption that we will need it again soon.
 Internal programmable registers, . such as index registers, provide a high-
speed cache for main memory. The programmer (or compiler) implements
the register-allocation and register-replacement algorithms to decide
which information to keep in registers and which to keep in main memory.
There are also caches that are implemented totally in hardware. For
instance, most systems have an instruction cache to hold the instructions
expected to be executed.

Session1 intro to_os

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