operating system

CMET 331 Operating Systems

1. Introduction

Chapter 1: Introduction • What is Operating System • 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 • Computing Environments

Objectives • To provide a grand tour of the major

components of operating systems • To provide coverage of basic computer system

organization

Operating System - Definitions • No universally accepted definition • A set of programs that provide convenience for

the user – no. • A program that acts as an intermediary between

a user of a computer and the computer hardware – yes.

• A source of overhead that runs when you are trying to get useful work done – yes.

• The fundamental enforcer of protection and resource arbitration in a multi-use computer system – yes.

What is an Operating System? • A operating system is a program that

manages the computer hardware. —A program that acts as an intermediary between a

computer user 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 – A System View • 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

History: How Did We Get Here • Early: no operating system at all (real

programmers use toggle switches). —“Computers may someday weigh less than

150 tons.” – Popular Electronics • Batch systems

—Single programming —Multiprogramming

• Time sharing systems (modern programmers eat quiche). —Networked and Distributed Operating

Systems • Multimedia and real-time systems • Future: secure, reliable operating systems

The “Core” of an Early Processor • Three types of instructions:

—Arithmetic (add, subtract, and, or, etc.) —Load/Store (transfer between memory, registers) —Control transfer (Jump, Call)

• No protection mechanism • One level memory hierarchy

—Memory used to be faster than CPU!

The “Core” of a Modern Processor • Four or five types of

instructions: — Arithmetic — Load/Store — Control transfer — Floating Point — Vector

• Protection — Address translation (virtual

memory) — Privilege violation

exceptions — Supervisor mode

• Five level memory hierarchy — Registers — L1 cache — L2 cache — Main memory — Disk

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 operation • 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 system call is a software-generated interrupt caused either by an error or a user request.

• An operating system is interrupt driven.

Interrupt Handling • Suspends the normal sequence of execution

• Disable further interrupts. • The operating system preserves the state of the CPU by

storing registers and the program counter. • Determines which type of interrupt has occurred:

— vectored interrupt system • Separate segments of code determine what action

should be taken for each type of interrupt • When interrupt processing is done, restore the saved

registers and the program counter. • Enable interrupts. • Resume program execution.

Interrupt Handling

Direct Memory Access (DMA) 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.

Sectors

Tracks

Cylinders

A disk is accessed as an array: sector 0 is the first sector of the top track of the outmost cylinder. The next sectors of the same track are then ordered. Then, sectors from the next track are ordered. After all the sectors of all the tracks are ordered, we move to the next cylinder. The innermost is the last cylinder.

Disk Arm

• Sectors • Tracks • Cylinders

Disk Structure

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

Cache Memory

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

Comparison

Batch Multiprogramming

Time Sharing

Main Objective Maximize Processor Utilization

Minimize response time to user commands

Outcome Throughput Interactive Work

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 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

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)

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 Skype and Bitcoin

Web-Based Computing • Web has become ubiquitous • PCs are 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

End of Chapter 1