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Chapter 1: Introduction
Chapter 1: Introduction
 What Operating Systems Do
 Computer-System Organization
 Computer-System Architecture
 Operating-System Structure
 Operating-System Operations
 Process Management
 Memory Management
 Storage Management
 Protection and Security
 Distributed Systems
 Special-Purpose Systems
 Computing Environments
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Objectives
 To provide a grand tour of the major operating systems
components
 To provide coverage of basic computer system organization
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What is an Operating System?
 A program that acts as an intermediary 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.
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Operating System
 Manages the computer hardware
 Varies from machine type to machine type.

Efficiency versus convenience
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Computer System Structure
 Computer system can be divided into four components

Hardware – provides basic computing resources


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


CPU, memory, I/O devices
Word processors, compilers, web browsers, database
systems, video games
Users

People, machines, other computers
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Four Components of a Computer System
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OS from two viewpoints
 User View (Ease of use, performance, resource utilization)

PC

Mainframe/Minicomputer terminals

Networks of Workstations and Servers

Handheld computers
 System View

OS as a resource allocator

OS as a control program
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Operating System Definition
 OS is a resource allocator

Manages all resources (CPU time, memory space, file space...)

Decides between conflicting requests for efficient and fair resource
use
 OS is a control program

Controls execution of programs to prevent errors and improper
use of the computer. It is especially concerned with the operation
and control of I/O devices.
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Operating System Definition (Cont.)
 No universally accepted definition
 The fundamental goal of computer systems is to execute user
programs and make solving user problems easier. For this,
hardware is constructed for which application programs are
developed. The common functions of controlling and allocating
resources are then brought together into “Operating System”.
 “Everything a vendor ships when you order an operating system”
is good approximation

But varies wildly
 “The one program running at all times on the computer” is the
kernel. Everything else is either a system program (ships with
the operating system) or an application program
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Computer Startup
 bootstrap program is loaded at power-up or reboot

Typically stored in ROM or EEPROM, generally known as
firmware

Initializates all aspects of system, from CPU registers to device
controllers to memory contents

Loads operating system kernel and starts execution

The OS then starts executing the first process, such as “init”,
and waits for some event to occur.

The occurrence of an event is usually signaled by an interrupt
from either hardware or the software. Hardware may trigger an
interrupt at any time by sending a signal to the CPU, usually by
way of the system bus. Software may trigger an interrupt by
executing a special operation called a system call.
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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
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Computer-System Operation
 I/O devices and the CPU can execute concurrently.
 Each device controller is in charge of a particular device type.
 Each device controller has a local buffer.
 CPU moves data from/to main memory to/from local buffers
 I/O is from the device to local buffer of controller.
 Device controller informs CPU that it has finished its operation by
causing an interrupt.
 To ensure orderly access to the shared memory, a memory
controller is provided whose function is to synchronize access to
memory.
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Common Functions of Interrupts
 Interrupt transfers control to the interrupt service routine generally,
through the interrupt vector, which contains the addresses of all the
service routines.
 Interrupt architecture must save the address of the interrupted
instruction (on the stack)
 Incoming interrupts are disabled while another interrupt is being
processed to prevent a lost interrupt.
 A trap is a software-generated interrupt caused either by an error or
a user request.
 An operating system is interrupt driven.
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Interrupt Handling
 The operating system preserves the state of the CPU by storing
registers and the program counter.
 Determines which type of interrupt has occurred:

polling

vectored interrupt system
 Separate segments of code determine what action should be taken
for each type of interrupt
 Interrupts must be handled quickly

A table of pointers (vectors) to interrupts routines can be used

Interrupt vector addresses is then indexed by a unique device
number, given with the interrupt request, to provide the address
of the interrupt service routine for the interrupting device.
 After the interrupt is serviced, the saved return address is loaded
into the program counter and the interrupted computation resumes.
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Interrupt Timeline
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I/O Structure
 Each device controlller is in charge of a specific type of device. Depending on the
controller, there may be more than one attached device (eg. SCSI)
 Device controller maintains some local buffer storage and a set of special purpose
registers. Controller moves the data between the peripheral devices and its local
buffer storage.
 OSs have a device driver for each device controller. Device driver presents a
uniform interface to the device to the rest of OS.
 To do I/O

Device driver loads the appropriate registers within the device controller.

Device controller examines the contents of these registers to determine action.

Controller starts the transfer from the device to its local buffer

Once the transfer is complete, the controller informs the device driver via an
interrupt.

The device driver then returns control to the operating system, possibly
returning the data or a pointer to data if the operation was a read.
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I/O Structure
 After I/O starts, control returns to user program only upon I/O
completion.
 Wait instruction idles the CPU until the next interrupt
 Wait loop (contention for memory access).
 At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
 After I/O starts, control returns to user program without waiting
for I/O completion.
 System call – request to the operating system to allow user
to wait for I/O completion.
 Device-status table contains entry for each I/O device
indicating its type, address, and state.
 Operating system indexes into I/O device table to determine
device status and to modify table entry to include interrupt.
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Two I/O Methods
Synchronous
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Asynchronous
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Device-Status Table
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Direct Memory Access Structure
 Interrupt driven I/O can produce high overhead when used for bulk
data movement. To solve this problem, direct memory access
(DMA) is used.

After setting up buffers, pointers, and counters for the I/O
device, controller transfers an entire block of data directly
to/from its own buffer to memory with no intervention by the
CPU.

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 one
interrupt per byte.

While the device controller is performing these operations, the
CPU is available to accomplish other work.
 Some high-end systems use switch rather than bus architecture
where the DMA is more effective.
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Storage Structure
 Main memory – only large storage media that the CPU can access
directly. Random Access Memory – RAM.

Implemented in semiconductor dynamic random-access
memory - DRAM technology.

Interaction is achieved through a sequence of “load” or “store”
instructions.

Also, CPU automatically loads instructions from memory

Instruction – execution cycle – von Neumann architecture

An instruction is fetched from memory, stored in Instruction
Register.

Instruction is then decoded and may cause operands to be
fetched from memory and stored in registers.

Instruction is executed and the result may be stored back in
memory.
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Storage Structure
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity.
 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.

Most programs are stored on a disk until they are loaded into
memory.
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Storage Hierarchy
 Storage systems organized in hierarchy.

Speed

Cost

Size

Volatility
 Caching – copying information into faster storage system; main
memory can be viewed as a last cache for secondary storage.
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Storage-Device Hierarchy
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Caching
 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
 Cache smaller than storage being cached

Cache management important design problem

Cache size and replacement policy
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Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit or
implicit. Data transfer from cache to CPU and registers is a
hardware function, whereas transfer from disk to memory is
controlled by OS.
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Migration of Integer A from Disk to Register
 Multitasking environments must be careful to use most recent
value, not 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 on different computers

Various solutions covered in Chapter 17
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Computer-System Architecture
 Single-Processor Systems
 Multiprocessor Systems
 Clustered Systems
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Computer System Architecture
 Single Processor Systems

One CPU

May have other special-purpose processors (such as disk
controllers)

Run a limited instruction set

Do not run user processes

Sometimes managed by OS

For example a disk controller processor receives a
sequence of requests from the main CPU and implements
its own disk queue and scheduling algorithm to relieve the
main CPU of the overhead of disk scheduling.
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Multiprocessor systems
 Two or more processors in close communication, sharing the
computer bus and sometimes the clock, memory, and peripherals.
 Advantages:

Increased throughput


Economy of scale


The speed-up ratio with N processors is not N however.
Sharing of peripherals, mass storage, power supplies
Increased reliability

Graceful degredation

Fault tolerance
–
Failure detection, diagnose and correction
–
Hardware duplication
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Multiprocessor Systems
 Two types:

Asymmetric multiprocessing in which each processor is
assigned a specific task. A master processor controls the
system, scheduling and allocating work to slave processors.

Symmetric multiprocessing (SMP) in which each processor
performs all tasks within the operating system. All processors
are peers.
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Clustered Systems
 Gather together multiple CPUs to accomplish computation
 Composed of two or more individual systems coupled together.
 High availability service.

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.
 Structure

Asymmetric clustering: One machine is in hot-standby mode
while the other is running applications. Hot-standby machine only
monitors the active server.

Symmetric clustering:Two or more hosts are running applications
and are monitoring each other
 Parallel Clusters allow multiple hosts to access the same data on the
shared storage. May need a distributed lock manager (DLM).
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Operating System Structure


Multiprogramming 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 (job pool) 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
Timesharing (multitasking) is logical extension in which CPU switches jobs
so frequently that users can interact with each job while it is running,
creating interactive computing

Response time should be < 1 second

Each user has at least one program executing in memory process

Job scheduling: Which jobs to bring to memory from job pool on disk.

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 (physical memory versus logical memory)
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Memory Layout for Multiprogrammed System
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Operating-System Operations
 Interrupt driven by hardware
 Software error or request creates exception or trap

Division by zero, invalid memory access, request for operating system service
 Other process problems include infinite loop, processes modifying each other or
the operating system. An error should cause problems only for the one program
that was running.
 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. If tried to be executed in user mode, trapped to OS.
 System call changes mode to kernel, return from call resets it to user

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Operating System Operations
 Whenever a trap or interrupt occurs, the hardware switches from
user to kernel mode. The system always switches to user mode
before passing control to a user program.
 When a system call is executed, it is treated by the hardware as a
software interrupt. Control passes through the interrupt vector to a
service routine in the OS, and the mode bit is set to kernel mode.
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Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources

Set interrupt after specific period

Operating system decrements counter

When counter zero generate an interrupt

Set up before scheduling process to regain control or terminate
program that exceeds allotted time
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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
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Process Management Activities
The operating system is responsible for the following activities in
connection with process management:
 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
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Memory Management
 All data in memory before and after processing
 All instructions in memory in order to execute
 Memory management determines what is in memory when

Optimizing CPU utilization and computer response to users
 To improve utilization, computers must keep several programs in
memory, creating a need for memory management
 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
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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, datatransfer rate, access method (sequential or random)
 File-System management

OS implements the abstract concept of a file by managing mass
storage media and the devices that control them.
 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 dirs
Mapping files onto secondary storage
 Backup files onto stable (non-volatile) storage media

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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

Varies between WORM (write-once, read-many-times) and RW
(read-write)
 OS mount and unmount media in devices, allocating and
freeing the devices for exclusive use by processes, and
migrating data from secondary to tertiary storage.
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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
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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
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Distributed Systems
 Collection of physically separate, possibly heterogenous computer
systems that are networked to provide the users with access to the
various resources
 Networks vary by the protocols used (TCP/IP, ATM,...), the
distances and the transport media.
 Most OSs support TCP/IP. To an OS, a network protocol simply
needs a network adapter with a device driver to manage it, as well
as software to handle data.
 Networks (LAN, WAN, MAN)
 Media (copper wire, fiber strands, microwave dishes,...)
 Network Operating System is an OS providing features such as
file sharing accross the network and that includes a communication
scheme that allows different processes on different computers to
exchange messages.
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Special Purpose Systems
 Real-Time Embedded Systems

Limited OS features, little or no user interface, preferring to
spend their time monitoring and managing hardware devices.

Run real-time operating systems (RTOS). Processing must
be done within the defined constraints
 Multimedia Systems

Multimedia data consists of audio and video files as well as
conventional files. Must be delivered (streamed) according to
certain time restrictions (e.g. 30 frames per second).
 Handheld Systems

Special purpose embedded OSs.

Small size and memory

Small and slow processor to consume less power

Limited I/O (small size, web clipping)

Wireless technology, such as BlueTooth or 802.11, allowing
remote access to e-mail and web browsing.
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Computing Environments
 Traditional computer

Blurring over time

Office environment
 PCs
connected to a network, terminals attached to
mainframe or minicomputers providing batch and
timesharing
 Now
portals allowing networked and remote systems
access to same resources

Home networks
 Used
 Now

to be single system, then modems
firewalled, networked
Traditional time-sharing systems are uncommon. The same
scheduling technique is still in use on workstations and
servers, but frequently the processes are all owned by the
same user.
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Computing Environments (Cont.)

Client-Server Computing
 Dumb terminals supplanted by smart PCs
 Many systems now servers, responding to requests generated by
clients
 Compute-server provides an interface to client to request
services (i.e. database)
 File-server provides interface for clients to store and retrieve
files
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Peer-to-Peer Computing
 Another model of distributed system
 P2P does not distinguish clients and servers

Instead all nodes are considered peers

May each act as client, server or both

Node must join P2P network


Registers its service with central lookup service on network,
or

Broadcast request for service and respond to requests for
service via discovery protocol
Examples include Napster and Gnutella
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Web-Based Computing
 Web has become ubiquitous
 PCs most prevalent devices
 More devices becoming networked to allow web access
 New category of devices to manage web traffic among similar
servers: load balancers
 Use of operating systems like Windows 95, client-side, have
evolved into Linux and Windows XP, which can be clients and
servers
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End of Chapter 1