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Chapter 1: Introduction• ICS 240: Operating Systems

– William Albritton• Information and Computer Sciences

Department at Leeward Community College– Original slides by Silberschatz, Galvin, and Gagne

©2007 from Operating System Concepts with Java, 7th Edition with some modifications

– Also includes material by Dr. Susan Vrbsky from the Computer Science Department at the University of Alabama

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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.– Hardware includes

• Processors, main memory, clocks, terminals, disks, network interfaces, I/O (Input & Output) devices

– OS (operating system) is a layer of software on top of the hardware• OS can give the illusion of a machine different

from the actual hardware

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OS Goals• Three general goals of an OS

1. Execute user programs

2. Make computer system convenient to use

3. Use computer hardware in an efficient manner

• Usually must find a tradeoff between efficiency & convenience, as they can conflict with one another

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OS Functions• Three specific functions of OS

1. OS controls and coordinates the use of the hardware

• OS used to be called the control program

2. OS prevents errors and improper use of computer.

3. OS is a resource allocation manager.• Resources are: CPU time, memory space, file

storage space, etc.

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Computer System Structure• Computer systems can be divided into

four components1. Hardware

• provides basic computing resources– CPU, memory, I/O devices

2. Operating system• Controls and coordinates use of hardware

among various applications and users

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Computer System Structure• Computer systems can be divided into

four components (continued)3. 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

4. Users• People, machines, other computers

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Computer System’s Parts

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Operating System Definition• No universally accepted definition

– “Everything a vendor ships when you order is an operating system” is good approximation

• But varies wildly

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Operating System Definition• Common definition

– An OS is “the one program running at all times on the computer”

• Called the kernel

• Everything else is either a system program (enables hardware & software to work together) or an application program (user program)

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The Kernel• The kernel is the fundamental portion of

the OS needed to use the hardware– OS traditionally seen as the kernel, however,

vendors now packaging kernel plus system programs (e.g. compilers, editors) as OS

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The Busy Kernel• Typical tasks performed by the kernel

– manages memory– manages devices (e.g.. disks, printers)– manages files (tree structured directory)– implements processes (schedules, synchronizes,

communication between)– provides protection– provides error detection– implements network access

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Computer Startup• The bootstrap program is the initial

program that is run at power-up or reboot– Initializes all aspects of system– Loads operating system kernel and starts

execution

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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

• CPU (central processing unit) performs calculations on data

• Memory (memory storage unit) stores instructions & data while a computer program is running

– Note that the CPU & memory work closely together in order to run a program

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Buses• Set of wires or connections for data

transfer1. Address bus

– address of memory location (or I/O port)

2. Control bus– read (fetch) or write (store) commands

3. Data bus – data to be stored, or instructions to be used

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Computer System Organization• Computer-system operation

– CPU & devices compete for memory

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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.

• A buffer is a intermediate place where data is temporarily stored while being moved from one location to another

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Computer-System Operation• CPU moves data from/to main memory

to/from local buffers

• I/O moves data from the device to local buffer of controller.– Device controller informs CPU that it has

finished its operation by causing an interrupt.• An interrupt is a signal indicating that an event

has occurred

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Common Functions of Interrupts• When the CPU receives an interrupt,

– it must save the current state of the running program & execute the interrupt routine

• An interrupt routine is a method

• Once the CPU is finished running the interrupt routine, – the CPU must transfer back to the original

program from the place where it left off

• Note that the CPU can only execute one instruction from a program at a time

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Common Functions of Interrupts• An operating system is interrupt driven.

– OS invoked when (different events may cause an interrupt)

• System Calls - user issues I/O request

• Hardware devices - I/O device

• Errors - division by 0

• Console (operator interrupt) - operator can signal interrupt

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Interrupt Timeline

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Structure of CPU• CPU – central processing unit

• EU: execution unit• Data, pointer, & index registers (storage

locations)– A register is a high-speed storage location in the

CPU (think of it as a hardware variable)

• ALU – arithmetic and logic unit• CU – control unit (executes instructions)

• BIU: bus interface unit• Segment & IP registers (memory addressing) • BCU: bus control unit (data transfer)

• Clock – synchronizes operations

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Instruction Execution Cycle• CPU keeps looping through these steps

1. Fetch (read) instruction from memory & increment instruction pointer (IP) register• Has address of next instruction to be executed• Also called the “Program Counter” (PC)

2. Decode the instruction

3. Fetch (read) data from memory

4. Execute the instruction

5. Store (write) result in memory

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Storage Structure• Main memory

– Within the memory storage unit– Only large storage media that the CPU can

access directly.– Another name for main memory is RAM

• RAM (Random Access Memory) means that the data stored in memory can be accessed at random (in any order)

– In other words, where the data is stored in memory does not influence the time required to return the data

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Storage Structure• To run a program, the program must

have be loaded into main memory– Ideally programs & data should be stored

in main memory permanently– Not practical because

1. Main memory too small & too expensive

2. Main memory is volatile (needs power, or data is lost)

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Storage Structure• Secondary storage

– Extension of main memory that provides large nonvolatile storage capacity.

• A typical example is 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.

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Storage Hierarchy

• Storage systems organized in hierarchy.– Speed

• Faster access to data at top of hierarchy

– Cost• More expensive storage at top of hierarchy

– Volatility• Power is required to store data at top of hierarchy, with

cutoff at electronic disk

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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 being used is copied from slower

storage to faster storage temporarily• This temporary storage place is called a “cache”

– Pronounced like “cash”

• For example, because data in memory is much slower to access than data in the CPU, the CPU has a cache to store recently used instructions and data

– See Figure 2-2 on the Processor Architecture link

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Caching

• Faster storage (cache) checked first to determine if data is there– If it is, data read directly from the cache– If not, read data from the slower storage & put a

copy in the faster cache• For example, web browsers have a cache of the most

recently viewed web pages

• Caches have a limited size– Cache management important design problem

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Various Levels of Storage

• Details on levels of storage hierarchy

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Acronyms

• CMOS– Complementary metal–oxide–semiconductor

• Type of integrated circuit

• SRAM– Static random access memory

• Does not have to be refreshed, but is volatile

• DRAM– Dynamic random access memory

• Charge does need refreshing (within 1 millisecond) & is volatile

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Data Migration

• Migration of Integer A from Disk to Register– Data is stored in multiple places when it is being

moved from storage to the CPU & back• Must be careful to use most recent value, no matter

where it is stored in the storage hierarchy

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Data Migration

• Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent data value in their cache

• Distributed environment situation even more complex– Several copies of the data can exist on

different computers

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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, so CPU always has one to execute

• A job is a program in execution (a running program)

– A subset of total jobs in system is kept in memory– Job scheduling is an algorithm (a plan) to select

which job to run– When it has to wait (for I/O for example) in one

job, OS switches to another job

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Operating System Structure• Timesharing (multitasking)

– Logical extension of multiprogramming– 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– A program loaded into memory & running is called a process– In other words, a process is a program in execution

• For example, a server running a portal such as MyUH

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Memory for Multiprogramming

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Operating-System Operations• Similar to the interrupts created by

hardware, software errors or requests create exceptions or traps– Division by zero, request for operating

system service

• Other process problems include infinite loop, processes modifying each other or the operating system

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Operating-System Operations• 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 mode, return from call resets it to user mode

– This prevents user programs from writing over the operating system with data

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User & Kernel Mode

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Process Management• A process is a program in execution.

– It is a unit of work within the system.

• A program is a passive entity, process is an active entity.

• Process needs resources to accomplish its task– CPU, memory, I/O, files

• Process termination requires reclaim of any reusable resources

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Process Management Activities• The operating system is responsible for

the following activities in connection with process management

1. Creating and deleting both user and system processes

2. Suspending and resuming processes

3. Providing mechanisms for process synchronization, process communication, & deadlock handling

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Memory Management• Memory management determines what is in

memory when– All data for a process must be placed in

memory at some time in order for the CPU to access the data

• Data stored on a disk must be placed in memory for a CPU to directly access it

– All instructions for a process must also be placed in memory in order for a program to execute

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Memory Management

• Memory management activities that the OS is responsible for

1. Keeping track of which parts of memory are currently being used and by whom

2. Deciding which processes (or parts thereof) and data to move into and out of memory

3. Allocating & deallocating memory space • For example, a local variable in a method is allocated

to memory when that method is called, then the local variable is deallocated when the method returns

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Storage Management• OS provides uniform, logical view of

information storage– A file is a logical storage unit which is an

abstraction of physical properties• For example, the user does not have to think about

exactly how a file is stored on a magnetic disk, as these details are abstracted away by the OS

– 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.

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Storage Management

• File-System management– Files usually organized into directories (folders)– Access control on most systems to determine who

can access what– OS activities include

1. Creating and deleting files and directories (folder)2. Mapping files onto secondary storage3. File & directory access control

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Disk 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 activities1. Free-space management

2. Storage allocation

3. Disk scheduling

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Protection and Security• Protection

– Any mechanism for controlling access of processes or users to resources defined by the OS

• Improves reliability by detecting errors• Protection against an unauthorized or incompetent user

• Security– Defense of the system against internal and external

attacks• Huge range, including denial-of-service, worms, viruses,

identity theft, theft of service– Because of the increase in security threats, security is becoming

more & more important to businesses as well as researchers

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Protection and Security• Systems generally first distinguish among

users, to determine who can do what– Most OS keep a list of user names & associated

user identifiers (user IDs) • Each user ID is unique, one per user

• User ID is associated with all files and processes that user has been given access control

• When the user logs into a system, the corresponding user ID is determined & the user is given access only to the user’s files & processes

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Computing Environments • Client-Server Computing

– Many systems are now servers, responding to requests generated by clients

• Clients ask for requests & servers provide services– For example, file-server, such as a web 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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Computing Environments • A current trend is to provide more access to

computers– Office environment

• Used to be terminals attached to mainframe, and then PCs connected to a network

• Now portals allowing networked and remote systems access to same resources

– Home environment• Used to be single system with no network connections,

then modems

• Now networked

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Web-Based Computing

• Web has become ubiquitous– Ubiquitous is a fancy word that computer folks like

to say which simply means being everywhere at once

• More devices becoming networked to allow web access– Use of operating systems like Windows 95, which

was only a web client, have evolved into Linux and Windows XP, which can be clients and servers

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History of OS

• 0th generation– 100 BCE

• Antikythera Mechanism by the Greeks– Complex system of gears for astronomical calculations

– http://antikythera-mechanism.gr/

– 1400s• Incas had digital computers using ropes & pulleys

– Knots in rope used as a binary code that could calculate such things as calendar dates & tax records

– http://www.osdata.com/book.pdf

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History of OS

• 0th generation– Charles Babbage (1792 - 1871)

• Designed the first digital computer

• Used wheels, cogs, gears

• Couldn't produce precision needed

• Ada Lovelace was world’s 1st programmer

– None of these 0th generation computer systems had an OS

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History of OS

• 1st generation of computer systems 1945 - 1955 – Hardware consisted of rooms with 20,000 or more

vacuum tubes• Had to use a plugboard to program the computer

– User acted as operator• Only could do basic calculations

• All programming in machine language

– Still no OS

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History of OS

• 2nd generation systems 1955 - 1965 – Hardware was transistors– Batch jobs were used for programming

• Many programs run one after another by reading punch cards into a magnetic tape which was carried to another computer for input

• Output produced another magnetic tape that was carried to another computer for printed output

– Hired operators instead of user to set up jobs• Therefore first OS needed and created (resident monitor)

– No interaction between user and program– Compilers added and loaded from tape

• FORTRAN (scientific computer language)• COBOL (business computer language)

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History of OS• 3rd generation systems 1965 - 1980

– Hardware was Integrated Circuits (IC) with a few transistors– Multiprogramming

• More than one job loaded into memory– illusion of more than one job running - pseudo parallelism

increases CPU utilization - not idle

– OS is more complex• Which job to schedule next?, etc.

– Time Sharing (multitasking)• Interactive - input from keyboard, as each user has a terminal

– OS expanded to include:• Memory management, disk management, file management

– Specific OS• OS/360 (business & science), Multics (UNIX forerunner)

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History of OS

• 4th generation systems (1980 - present)– Hardware is LSI (Large Scale Integration) chips

which now have billions of transistors– PCs (personal computer) dedicated to a single user

• OS becomes more simple with single users

– Examples of early PC operating systems• MS-DOS (Microsoft Disk Operating System), MAC OS,

UNIX

– Multiprocessors, distributed systems, network operating systems

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Future of OS

• 5th generation systems (the future)– 5th generation computers are thought to involve AI

(artificial intelligence)• Includes such abilities as as natural language recognition,

the ability to reason, and intelligent image processing– An example is the Fifth Generation Computer Systems project

(FGCS) which was funded by the Japanese Government (1982-1994)

– http://www.icot.or.jp/ARCHIVE/HomePage-E.html