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Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Chapter 1: Introduction

1.2 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
Open-Source Operating Systems

Objectives
To provide a grand tour of the major operating systems components
To provide coverage of basic computer system organization

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

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

Operating System Definition
OS is a resource allocator
Manages all resources
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

Operating System Definition (Cont.)
No universally accepted definition
“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.

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

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

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

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
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

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

Interrupt Timeline

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

Direct Memory Access Structure
Used for high-speed I/O devices able to transmit information at close
to memory speeds
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

Storage Structure
Main memory – only large storage media that the CPU can access
directly
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

Storage Hierarchy
Storage systems organized in hierarchy
Speed
Cost
Volatility
Caching – copying information into faster storage system; main
memory can be viewed as a last cache for secondary storage

Storage-Device Hierarchy

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

Computer-System Architecture
Most systems use a single general-purpose processor (PDAs through
mainframes).
Most systems have special-purpose processors as well.
Multiprocessors systems growing in use and importance
Also known as parallel systems, tightly-coupled systems
Advantages include:
1. Increased throughput
2. Economy of scale
3. Increased reliability – graceful degradation or fault
tolerance
Two types:
1. Asymmetric Multiprocessing
2. Symmetric Multiprocessing

How a Modern Computer Works

Symmetric Multiprocessing Architecture

A Dual-Core Design

Clustered Systems
Like multiprocessor systems, but multiple systems working together
Usually sharing storage via a storage-area network (SAN)
Provides a high-availability service which survives failures
Asymmetric clustering has one machine in hot-standby mode
Symmetric clustering has multiple nodes running applications,
monitoring each other
Some clusters are for high-performance computing (HPC)
Applications must be written to use parallelization

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 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
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

Memory Layout for Multiprogrammed System

Operating-System Operations
Interrupt driven by hardware
Software error or request creates exception or trap
Division by zero, request for operating system service
Other process problems include infinite loop, processes modifying each
other or the operating system
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

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

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

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:

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
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

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 dirs
Mapping files onto secondary storage
Backup files onto stable (non-volatile) storage media

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)

Performance of Various Levels of Storage
Movement between levels of storage hierarchy can be explicit or
implicit.

Migration of Integer 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

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

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

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 to be single system, then modems
Now firewalled, networked

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

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

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

Open-Source Operating Systems
Operating systems made available in source-code format rather than
just binary closed-source
Counter to the copy protection and Digital Rights Management
(DRM) movement
Started by Free Software Foundation (FSF), which has “copyleft”
GNU Public License (GPL)
Examples include GNU/Linux, BSD UNIX (including core of Mac OS
X), and Sun Solaris

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