ch1-1.pptx

Silberschatz, Galvin and Gagne ©2011
Operating System Concepts Essentials – 8th Edition
Chapter 1

9.2 Silberschatz, Galvin and Gagne ©2011
Contents
 Definition of an Operating System (OS)
 Role of an Operating System
 Classification of Operating Systems
 Layered Model of an OS
 Computer system organization
 Specific terms and concepts

The Definition of an OS
 A program that acts as an interface between a user of a computer and the computer
hardware
 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 System Definition (Cont.)
 No universally accepted definition
 “Everything a vendor ships when you order an operating system” is a good
approximation
 “The one program running at all times on the computer” is the kernel.

Computer System Structure
 Computer system can be divided into four components:
 Hardware – provides basic computing resources
 CPU, memory, I/O devices
 Operating system
 Controls and coordinates use of hardware among various applications and
users
 Application programs – define the ways in which the system resources are used
to solve the computing problems of the users
 Word processors, compilers, web browsers, database systems, video games
 Users
 People, machines, other computers

Four Components of a Computer System

The Role of an OS
 Extended or virtual machine
 The external or the user point of view
 A top-down perspective
 Hides the complexity of using the hardware devices
 Provides the user a more convenient view of the system resources
 Purpose: Convenience
 Resource manager
 The inside or the designer point of view
 A bottom-up perspective
 Brings the hardware resources in a functional state
 Provides each program with time and space for using resources
 Purpose: Efficiency

Computer System Organization
 Computer-system operation
 One or more CPUs, device controllers connect through common bus providing
access to shared memory
 Concurrent execution of CPUs and devices competing for memory cycles

Types Of Operating System
 Single User Operating System
 Batch Operating System
 Multi programmed System

Single User Operating System
 Personal computer----computer system dedicated to a single user
 Interactive
 User convenience and Responsiveness

Batch Operating System
 Historical perspective
 Current Perspective
 Example Application

Multi Programmed System
Several jobs are kept in main memory at the same time
And CPU is multiplexed among them
• CPU scheduling

Computer-System Operation
 I/O devices and the CPU can execute concurrently
 CPU moves data from/to main memory to/from local buffers
 Each device controller is in charge of a particular device type
 Each device controller has a local buffer
 Device controller informs CPU that it has finished its operation by causing an interrupt

Computer Startup
 bootstrap program is loaded at power-up or reboot
 Typically stored in ROM or EPROM, generally known as firmware
 Initializes all aspects of system
 Loads operating system kernel and starts execution

Chapter 1
 Interrupts
 an interrupt is an event generated by an I/O devices and peripheral devices
 To get the attention of CPU
 Example:
 Ringing Bell
 What to do?(in case of ringing)
 Stop work
 Open the Door
 Solve the problem
 Start work
 Same case in system environment
 CPU stop the work
 Check the interrupt
 Handle the interrupt
 Start the work

Chapter 1
 The occurrence of an event is usually signaled by an interrupt from either the
hardware or software
Classes of Interrupts
 Interrupts
(generated by I/O devices and peripheral devices)
 System calls
(generated by s/w)
 Trap
(generated by CPU itself)
 Signals
(generated by a process)

Chapter 1
How interrupts are Handle?
Different routines handle different interrupts called(ISR)
 When CPU is interrupted?
• It stop what it was going on and context is saved in register(pc)Which tells the state of CPU.
 Control moves to OS
• Examine the nature of interrupt
• Calls the corresponding (ISR)[stored in kernel and mange in vector table]
After serving the interrupt the save address is loaded again to pc to resume the process again.
Separate segments of code determine what action should be taken for each type of interrupt

Chapter 1
 Trap
IS an event generated by CPU itself and control moves to the OS
When CPU generate Trap?
Such instructions where CPU does not make decision what to do
e.g.(division by zero )so it generate a trap

Chapter 1
HOW TRAP ARE HANDLE
In trap, control moves to OS and code run that is called(TSR) but in this case we don’t need to save the state of
CPU
 Why state is not save?
Because program is trying to execute a instruction which the CPU does not want.
 What happened after TSR?
• The program will be terminate
• And the reason of termination (in this case division by zero)

Chapter 1
 SIGNAL
A signal is an event generated to get attention of a process. The signal generated in this case is
called SIGINT (Interrupt signal).The control is given to the process not to operating system. As the
process is a soft entity so its called soft interrupt
How signal are Handle
When signals are occurred how process mange it? There are 3 methods for it
• Completely ignore it
• To run the predefined action set by the O.S
• Define a program

Chapter 1
 System calls
is an event generated by s/w to get services of O.S.
Actually the system call interface is a layered of s/w on the top of the kernel which allows u to get into the kernel for
performing various tasks through kernel code.
HOW TO HANDLE SYSTEM CALLS
The control moves to O.S
• Run the code (code is called service code for system calls)

Chapter1
Polling
Polling is a systematic checking of devices by operating system whether thy want to generate interrupt or not.
ISSUE:
Operating System time will b waste

Chapter 1
Operating System Services
2 types of services are available
I. Services for the user
II. Services for the operating system itself
 User process’s services
Program Execution
I/O operations
File system Manipulation
Communication b/w processes
Error Detection and Handling
 Operating System Services
Resources Management
Accounting
Protection

Chapter 1: User process’s services
 Program execution: The system must be able to load a program into memory and to
run that programs. The program must be able to end its execution.
 I/O Operations: A running program may require I/O, which may involve a file or an
I/O device. For efficiency and protection user usually cannot control I/O devices
directly. The OS provides a means to do I/O.
For user User interface
For process API’S
 File System Manipulation: Programs need to read, write files. Also they should be
able to create and delete files by name.
 Communications: There are cases in which one program needs to exchange
information with another process. This can occur between processes that are
executing on the same computer or between processes that are executing on different
computer systems tied together by a computer network. They have to communicate with each other and have
need some services for communication to occur. In UNIX and LINUX theses services are
Pipes
Named pipes
Sockets
Message Queues

Chapter 1: User process’s services
 Error detection: – OS needs to be constantly aware of possible errors
 May occur in the CPU and memory , in I/O devices, in user program
 For each type of error, OS should take the appropriate action to ensure
correct and consistent computing
 Debugging facilities can greatly enhance the user’s and programmer’s
abilities to efficiently use the system

Chapter 1: Operating System Services
 Resource allocation: When multiple users are logged on the system or multiple jobs
are running at the same time, resources must be allocated to each of them. There are
various routines to schedule jobs, allocate plotters, modems and other peripheral
devices.
 Accounting: We want to keep track of which users use how many and which kinds of
Computer resources. This record keeping may be used for accounting .
 Protection: The owners of information stored in a multiuser or
networked computer system may want to control use of that information.
When several separate processes execute concurrently, it should not be
possible for one process to interfere with the others

Chapter 1
 Hardware Protection
Multi-programming put several programs in memory at the same time; while this
increased system utilization it also increased problems. With sharing, many processes could be
adversely affected by a bug in one program. One program could also
modify the program or data of another program. A file may overwrite another file or folder on disk. A
process may get the CPU
and never give up it. So the issues of hardware protection are:
 Dual Mode operation
 I/O protection
 Memory protection
 CPU protection.

Chapter 1: Hardware Protection
Provide hardware support to differentiate b/w at least two modes of operation
User Mode: Execution done on behalf of a user
System Mode: Execution done on behalf of O.S
How TO SWITCH USER MODE TO SYSTEM MODE
Through system call
How To Implement Dual Mode
Mode bit is added to computer hardware to indicate the current Mode
user=1
System=0

Chapter 1:Dual Mode Operation
When an interrupt or fault occur hardware switches to system mode
Interrupt/faults
Set user mode
All privileged instructions can be work in system mode
user System

Chapter 1

Chapter 1
I/O PROTECTION
To prevent users from performing illegal I/O, we define all I/O instructions to be
privileged instructions. Thus users cannot issue I/O instructions directly; they must do it
through the operating system. For I/O protection to be complete, we must be sure that a
user program can never gain control of the computer.

Chapter 1
Memory Protection
 Must Provide memory protection outside the address space of a process
 In order to have memory protection add two register that determine the range of
legal addresses a program may access
Base register
Hold the smallest legal physical memory address.
Limit Register
Contain the size of range

Chapter 1
Hardware address protection with base and limit registers

Chapter1
 CPU Protection
Timer:
• Commonly used to implement the time sharing
• interrupt CPU after specified period to ensure O.S maintain control
• Timer is decrement every clock tick
• When timer reaches the value “0” an interrupt occur
• Load timer is a privilege Instruction

Chapter 1:Direct Memory Access
 CPU is responsible to transfers blocks of data from buffer storage directly to main
memory
Direct Memory Access
 Device controller transfers blocks of data from buffer storage directly to main memory
without CPU intervention
 Only one interrupt is generated per block, rather than the one interrupt per byte.

Chapter 1:Storage Structure
 Main memory – only large storage media that the CPU can access directly
 Random access
 Typically volatile
 Secondary storage – extension of main memory that provides large nonvolatile
storage capacity
 Virtual memory
 Magnetic disks – rigid metal or glass platters covered with magnetic recording
material
 Disk surface is logically divided into tracks, which are subdivided into sectors
 The disk controller determines the logical interaction between the device and
the computer

Chapter 1:Storage Structure
 How to calculate the total capacity of hard disk
No of sectors: 400
No of tracks:100
Data stored in one Sector=512byte
Total number of tracks and sectors=400*100=40000
512 byte stored in 1 sector so total capacity of hard
disk=40000*512=20480000byte

Chapter 1: Storage Hierarchy
 Storage systems organized in hierarchy
 Speed
 Cost
 Volatility

Chapter 1: Storage Hierarchy

Chapter1:Caching
 Caching – copying information into faster storage system; main memory can be
viewed as a cache for secondary storage
 Important principle, performed at many levels in a computer (in hardware, operating
system, software)
 Information in use copied from slower to faster storage temporarily
 Faster storage (cache) checked first to determine if information is there
 If it is, information used directly from the cache (fast)
 If not, data copied to cache and used there

Chapter 1

Chapter 1: Cache
 Cache hit: Data found in cache. Results in data
transfer at maximum speed.
 Cache miss: Data not found in cache. Processor
loads data from M and copies into cache. This results in extra delay, called miss penalty.
 Cache Line
• Cache is partitioned into lines (also called blocks). Each line has 4-64 bytes in it.
During data transfer, a whole line is read
• Each line has a tag that indicates the address in M from which the line has been
copied.
• “Does this key match with any of the tags?” Data is read only if a match is found.

Chapter 1
 What are the level of Cache?
There are Three levels of Cache
Level1:Built with processor
Level2:Between L1 and RAM
Level 3:On motherboard
 So if your system has L1,L2 and L3 cache data fetching will be L1->L2->L3->RAM
i.e.. If data is not there in L1 it will check L2 then L3 then RAM.
What is Cache replacement policy?
I. LRU
II. LFU
What is cache coherence?

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