Operating Systems

Part I: Introduction

“I think that there is a world market for five computers”

- Thomas J. Watson (1945)

Why Study Operating Systems?

We want to have an efficient O/S because it– consumes more resources than any other program. – is the most complex program. – is necessary for any use of the computer. – is used by many users.

Efficiency is measured through– Functionality– Performance: Time and Utilization– Convenience and Cost

Goals of This Course

Understand what an operating system is Understand the key components of an

operating system Have a deeper understanding of common

operating systems in the market (e.g. Windows, Unix, MS-DOS) and the issues associated with them

To be able to use performance measures

What is an O/S?

•A layer of abstraction between the HW and SW•A resource coordinator•Virtual machine•Reactive system

Operating Systems: A Definition

A collection of programs that integrate the hardware resources of the computer and make those resources available to the user in a productive, timely, and efficient manner

Operating Systems Ease the Pain

Performs the interface task with the hardware (file operations, memory paging, etc.) which should have been done by the user if the OS did not exist

High-level interface (GUI, command line a.k.a. CUI) The O/S’s capability for multiuser and multitasking

utilize the hardware efficiently Makes visible the “virtual” component of the system Allows program interaction

Why are Operating Systems Difficult to Create and Maintain?

Size– Too big for one person; current systems have

millions of lines of code and involve 10-100 man years to build

Lifetime – Operating systems remain longer than the

programmers who originally wrote them. Code is written and rewritten and original intent is forgotten (Unix designed to be cute, small system - now several volumes thick!)

Why are Operating Systems Difficult to Create and Maintain?

Complexity – The system must do difficult things -- deal with ugly

I/O devices, multiplexing/juggling act, handle errors

Multitasking– Must do several things at once.

General purpose

A Brief History: Early 1950’s, Mainframes Rule!

Early systems– No O/S! Programmer is also operator– Large machines run from a console; programs loaded through

switches and card readers

Simple batch systems were the first real OS– Setup time was a problem -> hire an operator– Operator ran related jobs together– O/S was a simple program stored in one part of memory

Loads a single job from card reader into memory Transfers control from one job to the next

Offline Processing

Allowed jobs to be read ahead of time onto tape

CardReader CPU Line

printer

CardReader CPU Line

printer

TapeDrive

TapeDrive

TapeDrive

TapeDrive

On-line processing

Off-line processing

History: Spooling

Allowed jobs to be read ahead onto disk Spool (Simultaneous Peripheral Operation On-

Line)

CardReader CPU

Lineprinter

disk

Multiprogrammed Systems

Multiprogrammed batch systems provided increased utilization– Keeps several jobs in memory simultaneously– I/O processing of one job overlaps with computation

of another– Analogy: Lawyer working on several cases; while

waiting to go to trial on one, can work on another– Needs CPU scheduling

Timesharing/Multitasking Systems

Timesharing supported interactive use– Each user feels as if

he/she has the entire machine

– Tries to optimize response time

– Based on time-slicing; divide CPU equally among others

Desktop Systems

First appeared in the 1970s More popularly known as personal computers

(PCs) Breakthroughs in hardware allowed downsizing

from expensive mainframes

Multiprocessor Systems

Also known as parallel systems or tightly-coupled systems

Three main advantages– Increased throughput (more CPUs = more work in less time)– Economy of scale (saves money, CPUs share peripherals)– Increased reliability (provides redundancy and fault tolerance)

Symmetric multiprocessing (SMP): All CPUs do the same thing

Asymmetric multiprocessing: each CPU has specific role (usually master-slave)

Distributed (Loosely-Coupled) Systems

Facilitates use of geographically distributed computing resources Supports communications between parts of a job or different jobs Supports sharing of distributed resources, both hardware and

software Client-server systems vs. Peer-to-peer systems

Clustered Systems

Makes several CPUs work together to accomplish computational task

Most likely share storage and linked through a local area network (LAN)

Possible clustering schemes:– Symmetric mode (two or more hosts running applications and

monitoring each other)– Asymmetric clustering (one is in hot standby mode while

another is running applications; switches to backup if active fails)

Real Time Systems Used for specialized applications: subway systems,

flight-control, factories, power plants Basic idea: O/S guarantees response to physical

events will occur within a fixed amount of time Problem faced : Schedule activities so as to meet all

time constraints Hard real time system

– Deadline is critical– Typically used to control a device

Soft real time system– Deadline is important but not critical– Example : Video applications (Use in PC environment)

Handheld Systems

Used in PDAs and cellular phones Common concerns:

– Limited main memory– Processor speed– Small display screens

General Structure of an O/S

Resident Programs

Programs which are critical to the operation of the system

KERNEL

Non-resident Programs

Loaded into memory only when needed

An Example: MS-DOS Structure

Memory resident components– Command interface shell (eg. ver, dir, date, time) :

COMMAND.COM– Set of I/O routines which control each I/O devices

(drivers) -- e.g. BIOS : IO.SYS– File Management System : MSDOS.SYS

Non-resident components– FORMAT.COM, XCOPY.EXE, LABEL.EXE, etc.

How MS-DOS Programs are Loaded in Main Memory

At System Start-up Running a Program

Operating System Components

Process Management– Process: a program in execution– Keeps track of each process and it’s state– Create, delete, suspend, resume processes;

synchronize process communications, handle deadlocks

– Possibly support threads (executable parts of a process)

Operating System Components

Main Memory Management– Keep track of which parts of memory are in

use– Allocates and deallocates memory as

needed– Decides which processes must be loaded in

main memory when space becomes available

Operating System Components

File Management– Keeps tracks of available space on the system– Maintains directory structure and hierarchy– Supports file manipulation commands– Keeps track of file information (inode, name,

timestamp) I/O System Management

– Allows for a standard methodology for access of each device

– Use of device drivers for modularity

Operating System Components

Secondary Storage Management– Free space management– Storage allocation– Disk scheduling

Networking– Transfer protocols– Routing and connection strategies

Operating System Components

Protection System– Provide mechanism for controlling access to

programs, processes, or users– Essential in multitasking and multiuser systems

Command Interpreter System– GUI vs. Command Line Interface– Redirection, Piping, and Parameter Passing

Operating Systems Architectures

Monolithic Layered Virtual Machine Microkernel

Monolithic

Easy to implement “The Big Mess” –

virtually no structure! Kernel is one large

structure Each procedure is visible

to every other procedure Not used anymore

Layered

Not easy to implement because some functionalities are mutually dependent.

Inefficient because it requires a high number of traversals of interfaces

Virtual Machine

Each user has a “virtual machine” and he can choose which OS to run on that machine

Elegant, but does not deal with questions of resource management and responsiveness

Non-virtual Machine Virtual Machine

Microkernel

Used in Mac/OSF/NT Takes out as much

functionality as possible from kernel -- allows modularity and portability across platforms

Interactions between processes involve the kernel, thereby requiring high efficiency in transfer