2. What is an Operating System (1)?
• A modern computer consists of:
• One or more processors
• Main memory
• Secondary Storage
• Various input/output devices
• Managing all these varied components requires a layer of
software – the
Operating System (OS).
3. What is an Operating System (2)?
An Operating System is a System program that acts as an -
• intermediary/interface between a user of a computer and the
computer hardware.
• Resource Allocator
• Manages all resources
• Decides between conflicting requests for efficient and fair resource use
• Platform – creates an environment in which application programs run.
• Control program - Controls execution of programs to prevent errors
and improper use of the computer
OS goals:
• Control/execute user/application programs.
• Make the computer system convenient to use.
• Ease the solving of user problems.
• Use the computer hardware in an efficient manner.
4. OS goals:
• Control/execute user/application programs.
• Make the computer system convenient to use.
• Ease the solving of user problems.
• Use the computer hardware in an efficient manner.
A. Frank - P. Weisberg
6. 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
7. Services provided by an OS
• Facilities for program creation
• editors, compilers, linkers, debuggers, etc.
• Program execution
• loading in memory, I/O and file initialization.
• Access to I/O and files
• deals with the specifics of I/O and file formats.
• System access
• resolves conflicts for resource contention.
• protection in access to resources and data.
8. Why are Operating Systems Important?
Important to understand and know how to correctly
use when writing user applications.
Large and complex systems that have a high
economic impact and result in interesting problems
of management.
Few actually involved in OS design and
implementation but nevertheless many general
techniques to be learned and applied.
Combines concepts from many other areas of
Computer Science: Architecture, Languages,
Data Structures, Algorithms, etc.
9. COURSE SYLLABUS
Introduction to OS
Process Scheduling
Process Synchronization
Memory Management
Unit Title: File System
Unit Title: I/O System
10. Evolution of Operating System
• The evolution of operating systems
went through seven major phases
• Six of them significantly changed the
ways in which users accessed
computers through the open shop,
batch processing, multiprogramming,
timesharing, personal computing, and
distributed systems.
• In the seventh phase the foundations
of concurrent programming were
developed and demonstrated in
model operating systems .
Open
Shop
The 1940's - First Generations
The 1950's - Second Generation
The 1960's - Third Generation
Fourth Generation
11. Evolution of Operating System
Open Shop (Idea of OS)
• The story of operating systems in 1954 when computers had no operating systems
but were operated manually by their users.
• Each user was allocated a minimum 15-minute slot , of which time he usually
spent 10 minutes in setting up the equipment to do his computation . . . By the
time he got his calculation going, he may have had only 5 minutes or less of actual
computation completed-wasting two thirds of his time slot.
• Example: IBM 701 Open shop
12. Evolution of Operating System
Serial Processing OS
• Programming in 1's and 0's (machine language) was quite
common for early computer systems.
• They were now asked to prepare their programs and data on
punched cards and submit them to a computing center for
execution.
• Instruction and data used to be fed into the computer through
card reader using Punch Cards.
• Job execution was done in FIFO fashion.
• Job was given to Computer using Punch Cards then later on
Magnetic Tapes were also used.
13. Evolution of Operating System
Batch OS (BOS)
• Operators collected decks of punched
cards from users and used a satellite
computer to input a batch of jobs from
punched cards to a magnetic tape.
• The jobs were now input and run one at
a time in their order of appearance on
the tape. (FCFS).
• The running jobs output data on another
tape which used to get printed on a line
printer.
• Example: The SHARE operating system
for the IBM 709
14. Multiprogramming OS (needed for efficiency)
• Single user cannot keep CPU and I/O devices busy at
all times
• Multiprogramming organizes jobs (code and data) so
CPU always has one to execute
• A subset of total jobs in system is kept in memory
• One job selected and run via job scheduling
• When it has to wait (for I/O for example), OS
switches to another job
Evolution of Operating System
15.
16. Spooling:
• Spooling is an acronym for simultaneous
peripheral operations online.
• Data is sent to and stored in memory until
the program for the execution. When data
is temporarily held to be used and
executed by a program, device or the
system, then this process is called
Spooling.
• It overlaps I/O of one job with the
computation of other job and uses disk as
very large device. It’s a combination of
buffering and queueing.
Evolution of Operating System
17. Time Sharing OS (Multitasking)
• Logical extension of Multiprogramming.
• CPU executes many jobs while switching between them frequently.
• The time that each task gets to execute is called quantum(< 1 second).
After this time interval is over OS switches over to the next task.
• Each user has at least one program executing in memory - process
• If several jobs ready to run at the same time - CPU scheduling
• If processes don’t fit in memory, swapping moves them in and out to run
• Virtual memory allows execution of processes not completely in
memory
• Uses CPU scheduling and Multiprogramming simultaneously.
Evolution of Operating System
18. Parallel OS
• A parallel computing system consists of multiple processors that
communicate with each other using a shared memory.
• Single computer is required. Many operations are performed
simultaneously.
• It may have shared or distributed memory.
• Processors communicate with each other through bus
Evolution of Operating System
19. Distributed OS
• System components are located at different locations.
• Uses multiple computers. Multiple computers perform multiple
operations.
• It have only distributed memory
• Computer communicate with each other through message passing.
• Improves system scalability, fault tolerance and resource sharing
capabilities.
Evolution of Operating System
20. OS Evolution
OS for Interactive Processing
Allowed programs to carry on dialogue with user via remote
terminals (workstations)
Real-time processing
Users demand timely response
Machines too expensive to serve only one user
Common for several users to want interactive services at the
same time
21. Different Operating Systems on the Same
Machine ?
It is possible to have more than one operating system available to be
used on a machine.
Only one operating system is run at a time, though.
Examples:
VAX - VMS or Ultrix
IBM PCs - DOS or Linux
22. Operating-System Operations
Interrupt driven (hardware and software)
Hardware interrupt by one of the devices
Software interrupt (exception or trap):
Software error (e.g., division by zero)
Request for operating system service
Other process problems include infinite loop,
processes modifying each other or the
operating system
23. Operating-System Operations (cont.)
Dual-mode operation allows OS to protect itself and other system
components
User mode and kernel mode
Mode bit provided by hardware
Provides ability to distinguish when system is running
user code or kernel code
Some instructions designated as privileged, only
executable in kernel mode
System call changes mode to kernel, return from call
resets it to user
24. Transition from User to Kernel Mode
Timer to prevent infinite loop / process hogging resources
Timer is set to interrupt the computer after some time period
Keep a counter that is decremented by the physical clock.
Operating system set the counter (privileged instruction)
When counter zero generate an interrupt
Set up before scheduling process to regain control or terminate
program that exceeds allotted time
26. The OS Shell
Defines interface between OS and users
Windows GUI
UNIX command line
UNIX users can choose among a variety of shells
csh is the “C shell”
tcsh is an enhanced “C shell”
Shell programming
28. The OS Kernel
The internal part of the OS is often called the Kernel
Kernel Components
Process Manager
Memory Manager
Storage Manager
I/O Manager
Protection and Security
30. Process Management
A process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.
Process needs resources to accomplish its task
CPU, memory, I/O, files
Initialization data
Process termination requires reclaim of any reusable resources
Single-threaded process has one program counter specifying
location of next instruction to execute
Process executes instructions sequentially, one at a time, until completion
Multi-threaded process has one program counter per thread
Typically system has many processes, some user, some operating
system running concurrently on one or more CPUs
Concurrency by multiplexing the CPUs among the processes / threads
31. Process Management Activities
Creating and deleting both user and system processes
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling
The operating system is responsible for the following activities in
connection with process management:
32. Memory Management
To execute a program all (or part) of the instructions must be in
memory
All (or part) of the data that is needed by the program must be
in memory.
Memory management determines what is in memory and when
Optimizing CPU utilization and computer response to users
Memory management activities
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 deallocating memory space as needed
33. Storage Management
OS provides uniform, logical view of information storage
Abstracts physical properties to logical storage unit - file
Each medium is controlled by device (i.e., disk drive, tape drive)
Varying properties include access speed, capacity, data-transfer rate, access
method (sequential or random)
File-System management
Files usually organized into directories
Access control on most systems to determine who can access what
OS activities include
Creating and deleting files and directories
Primitives to manipulate files and directories
Mapping files onto secondary storage
Backup files onto stable (non-volatile) storage media
34. Mass-Storage Management
Usually disks used to store data that does not fit in main memory or
data that must be kept for a “long” period of time
Proper management is of central importance
Entire speed of computer operation hinges on disk subsystem and its
algorithms
OS activities
Free-space management
Storage allocation
Disk scheduling
Some storage need not be fast
Tertiary storage includes optical storage, magnetic tape
Still must be managed – by OS or applications
Varies between WORM (write-once, read-many-times) and RW (read-
write)
35. Migration of data “A” from Disk to Register
Multitasking environments must be careful to use most recent
value, no matter where it is stored in the storage hierarchy
Multiprocessor environment must provide cache coherency in
hardware such that all CPUs have the most recent value in their
cache
Distributed environment situation even more complex
Several copies of a datum can exist
Various solutions covered in Chapter 17
36. I/O Subsystem
One purpose of OS is to hide peculiarities of hardware devices
from the user
I/O subsystem responsible for
Memory management of I/O including buffering (storing data
temporarily while it is being transferred), caching (storing parts of data
in faster storage for performance), spooling (the overlapping of output
of one job with input of other jobs)
General device-driver interface
Drivers for specific hardware devices
37. OS Device Drivers
Software to communicate with peripheral devices or controllers
Each driver is unique
Translates general requests into specific steps for that device
38. Protection and Security
Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
Security – defense of the system against internal and external attacks
Huge range, including denial-of-service, worms, viruses, identity theft, theft
of service
Systems generally first distinguish among users, to determine who can
do what
User identities (user IDs, security IDs) include name and associated
number, one per user
User ID then associated with all files, processes of that user to determine
access control
Group identifier (group ID) allows set of users to be defined and controls
managed, then also associated with each process, file
Privilege escalation allows user to change to effective ID with more rights
39. OS Summary
Shell -- interface to user
File Manager -- manages mass memory
Device Drivers -- communicate with peripherals
Memory Manager -- manages main memory
Scheduler & Dispatcher -- manage processes
40. System Calls
Programming interface to the services provided
by the OS
Typically written in a high-level language (C or
C++)
Mostly accessed by programs via a high-level
Application Programming Interface (API)
rather than direct system call use
Three most common APIs are Win32 API for
Windows, POSIX API for POSIX-based
systems (including virtually all versions of
UNIX, Linux, and Mac OS X), and Java API
for the Java virtual machine (JVM)
41.
42. Types of System Calls
Process control
create process, terminate process
end, abort
load, execute
get process attributes, set process attributes
wait for time
wait event, signal event
allocate and free memory
Dump memory if error
Debugger for determining bugs, single step execution
Locks for managing access to shared data between processes
43. Types of System Calls
File management
create file, delete file
open, close file
read, write, reposition
get and set file attributes
Device management
request device, release device
read, write, reposition
get device attributes, set device attributes
logically attach or detach devices
44. Types of System Calls (Cont.)
Information maintenance
get time or date, set time or date
get system data, set system data
get and set process, file, or device attributes
Communications
create, delete communication connection
send, receive messages if message passing model to host
name or process name
From client to server
Shared-memory model create and gain access to memory
regions
transfer status information
attach and detach remote devices
45. Types of System Calls (Cont.)
Protection
Control access to resources
Get and set permissions
Allow and deny user access
46. Operating System Services
Operating systems provide an environment for execution of
programs and services to programs and users
One set of operating-system services provides functions that
are helpful to the user:
User interface - Almost all operating systems have a user
interface (UI).
Varies between Command-Line (CLI), Graphics
User Interface (GUI), batch
Program execution - The system must be able to load a
program into memory and to run that program, end
execution, either normally or abnormally (indicating
error)
I/O operations - A running program may require I/O,
which may involve a file or an I/O device
47. Operating System Services (Cont.)
One set of operating-system services provides functions that are helpful to the user
(Cont.):
File-system manipulation - The file system is of particular interest. Programs
need to read and write files and directories, create and delete them, search them, list
file Information, permission management.
Communications – Processes may exchange information, on the same computer or
between computers over a network
Communications may be via shared memory or through message passing
(packets moved by the OS)
Error detection – OS needs to be constantly aware of possible errors
May occur in the CPU and memory hardware, 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
48. Operating System Services (Cont.)
Another set of OS functions exists for ensuring the efficient
operation of the system itself via resource sharing
Resource allocation - When multiple users or multiple jobs
running concurrently, resources must be allocated to each of
them
Many types of resources - CPU cycles, main memory, file
storage, I/O devices.
Accounting - To keep track of which users use how much and
what kinds of computer resources
Protection and security - The owners of information stored in
a multiuser or networked computer system may want to control
use of that information, concurrent processes should not
interfere with each other
Protection involves ensuring that all access to system
resources is controlled
Security of the system from outsiders requires user
authentication, extends to defending external I/O devices
from invalid access attempts
