1OSAR Autumn 2001

Course Info

Course Web page http://homer.csm.port.ac.uk/teaching/osarc/

Text Book Andrew S. Tanenbaum, Modern Operating

Systems , 2nd Ed (January 2001) Prentice Hall ISBN: 0130313580

Contact Info Hong Ong - hong.ong@port.ac.uk Garry Smith - garry.smith@computer.org

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What is an Operating System (OS)?

An OS is the interface between the user and the hardware It implements a virtual machine that is easier to

program than bare hardware. An OS is the resource allocator

It manages and allocates resources such that the computer hardware can be used in an efficient manner.

Examples of resources: processes, CPU(s), memory, file system, networking, etc.

The goal in OS development is to make the machine convenient to use and efficient.

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Why study Operating Systems?

Most likely you won't write an OS, so why study them?

Primary intersection point. It draws on many CS areas - software engineering, computer

architecture, data structures, networking, algorithms. Can apply techniques (data structures, conflict resolution,

concurrency issues, resource management, etc.) used in OS to other areas.

"Grungy" things sometimes do require modifications to OS you can't do this if you don't understand them!

Curiosity – “look under the hood” “Operating systems are among the most complex

pieces of software yet developed”, William Stallings, 1994

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Introduction - OS Overview

History of Operating Systems. Operating System Components. Computer Hardware Review. Operating System Interface. Operating System Structure.

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History of Operating Systems.

Once upon a time … Human

Computer

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History of Operating Systems (cont)

First generation 1945 - 1955 vacuum

tubes, plug boards

ENIAC (mid-1940’s) Just part of

it!!

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History of Operating Systems (cont)

Second generation 1955 - 1965 transistors, batch systems

Early batch system (overlapped CPU & I/O operations) Buffer slow I/O onto fast tape drives connected to CPU,

replicate I/O devices Spool data to disk

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History of Operating Systems (cont)

Third generation 1965 – 1980 ICs and

multiprogramming Multiple jobs are on

disk, waiting to run Multi-programming –

run several programs at the same time

IBM 360 series (mid-1960s)

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History of Operating Systems (cont)

Interactive timesharing Lots of cheap terminals and one computer

All users interact with the system at once Debugging is much easier

Disks are cheap so put programs and data online 1 punch card = 100 bytes

New problems: Need pre-emptive scheduling to maintain adequate

response time Need to avoid thrashing (swapping programs in and out

of memory too often) Need to provide adequate security measures

UNIX developed at Bell Labs (Thompson, Ritchie)

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History of Operating Systems (cont)

Fourth generation 1980 – present Personal computing

CPUs are cheap enough for everyone, yet powerful enough to be useful.

SUPERBOWLIBM 8086

vs.

The only real competitor left…There can be only one…IBM PC

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More Recent Development in OS

Parallel Operating Systems. Distributed Operating Systems. Clustered Operating Systems. Real time Operating Systems. Embedded Operating Systems.

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Parallel Operating Systems

Symmetric multiprocessing (SMP) Each processor runs an identical copy of the

operating system. Many processes can run at once without

performance deterioration. Most modern operating systems support SMP

Asymmetric multiprocessing Each processor is assigned a specific task;

master processor schedules and allocated work to slave processors.

More common in extremely large systems

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Distributed Operating Systems

Distribute the computation among several physical processors.

Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed networks or telephone lines.

Advantages of distributed systems. Resources Sharing Computation speed up – load sharing Reliability Communications

Requires networking infrastructure. Local area networks (LAN) or Wide area networks (WAN) May be either client-server or peer-to-peer systems.

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Clustered Operating Systems

Clustering allows two or more systems to share storage. Also known as SAN (Systems Area Network).

Provides high reliability. Asymmetric clustering: one server runs the

application while other servers standby. Symmetric clustering: all N hosts are

running the application

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Real time Operating Systems

Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.

Real-Time systems may be either hard or soft real-time. Hard - Deadlines are urgent; “Everything”

must be bounded Soft - Deadlines should be met, but if occasionally

a deadline can't be met, likely no one dies

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Embedded Operating Systems

Personal Digital Assistants (PDAs) Some Issues:

Limited memory “Slow” processors Small display screens.

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OS’s: The Bad News...

Modern operating system are: Enormous

Small OS = 100K lines of code Big OS = 10M lines

Complex Poorly understood – too large for one person to

comprehend. (Always) full of bugs and (often)

unreliable Dependent on hardware in strange ways

(makes porting difficult)

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OS Components

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Hardware Components

Components of a simple personal computer

Monitor

Bus

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CPU

(a) A three-stage pipeline(b) A superscalar CPU

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CPU Protection

Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.

Provide hardware support to differentiate between at least two modes of operations. User mode – execution done on behalf of a user. Monitor mode (also kernel mode or system

mode) – execution done on behalf of operating system.

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CPU Protection (cont)

Mode bit added to computer hardware to indicate the current mode: kernel (0) or user (1).

When an interrupt or fault occurs hardware switches to kernel mode.

Privileged instructions can be issued only in monitor mode.

kernel user

Interrupt/fault

set user mode

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Memory Structure

Typical memory hierarchy numbers shown are rough approximations

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Cache

Use of high-speed memory to hold recently-accessed data.

Requires a cache management policy. Caching introduces another level in storage

hierarchy. This requires data that is simultaneously stored in more than one level to be consistent.

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Memory

Volatile Random Access Memory (RAM)

Ranges from tens to hundreds of Megabytes, Program that can not fit into cache go to Memory.

Complementary Metal Oxide Semiconductor (CMOS) For storing the current time and date, and system setup

parameters. Non-Volatile

Write once Read only Memory (ROM)

Write many EEPROM (Electrically Erasable Programmable ROM)

Flash RAM (special type of EEPROM); The difference is Flash RAM write in block.

Usually, use for storing BIOS (bootstrap loader)

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Memory Protection

One base-limit pair and two base-limit pairs

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I/O Devices

I/O devices and the CPU can execute either synchronously or asynchronously.

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.

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I/O Devices Protection

All I/O instructions are privileged instructions.

Must ensure that a user program could never gain control of the computer in monitor mode (I.e., a user program that, as part of its execution, stores a new address in the interrupt vector).

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OS Interfaces (Shell, API)

Command interpreter: Interface between OS and your fingers (or mouth…or toes!)

Commands to manipulate files, run and kill programs, access remote machines, change protections on files, etc.

Some OS’s have “built-in” command interfaces, some allow an interface to be “plugged in”

Unix: “shell” (csh, tcsh, bash, ash, zsh, …)

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System calls - Programmer Interface

System calls are the programmer’s interface to the operating system

In modern systems, generally available from a high-level language such as C

Examples: fopen(), ioctl() (see MOS Fig 1.18 for more systems calls)

Previously: Most of OS was written in assembly and system calls were made from assembly language

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System Structure…Or Not?

Simplest system structure: little or none Example: MS-DOS Written to squeeze the most out of limited

hardware Small, some structure, but programmer is

free to go around whatever structure is present anyway

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UNIX (Traditional)

Two “layers” Systems programs, user programs monolithic (one big “blob” of code) kernel

provides: file system scheduling device management etc.

Components are separated by “discipline” and hacking requires care

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Complications & Solutions, Cont.

Use microkernel approach instead: Make kernel very small (provides only a limited

amount of the total OS functionality, like inter-process communication & basic H/W control)

“Plug in” virtual memory system, file system, etc.

Gives encapsulation w/o strict layering

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Virtual Machines

Virtual machine: fancy software which exports an interface that looks like a particular type of “machine”

Can create illusions of multiple computer systems, which can be like the physical hardware or completely different!

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Popular Virtual Machine OS: VM (IBM)

Each user on the machine can run an entirely different OS

One application: Can debug OS while others are working

Another application: Can work on custom OS modifications without taking the system down

What’s more, VM can run VM! Why?

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Virtual Machines, Cont.

Sometimes a VM environment actually produces a virtual machine that's faster than the real thing…on the same hardware! Example: VDM's under OS/2 (Virtual Dos

Machines) How can this be?

VM’s can be very difficult for OS’s like MS-DOS. Why?

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The Java Virtual Machine

Java achieves (relative) platform-independence through the JVM (Java Virtual Machine)

JVM (java) is implemented for each target platform

Java compiler (javac) emits bytecode (as *.class files) rather than architecture-specific machine code

JVM interprets the bytecode

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The Java Virtual Machine, Cont.

Problem: Speed! Java code typically executes more slowly than

“equivalent” C or C++ code Solutions … (more like workaround!)

JIT (Just In Time) compiler, which compiles the bytecode into native machine code immediately before execution

AOT (Ahead of Time) compiler, which compiles the bytecode into native machine code.

Interesting issue: caching

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Summary

A pseudo definition of OS: A software program that acts as an intermediary

between a user or an application program of a computer and the computer hardware

A brief history of OS. A high-level of overview of OS

From top-down perspective, CPU, Memory, Disk, I/O and interrupt handling

From bottom-up perspective, Command interpreter and system calls.

From within, Operating Systems structure.

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For the rest of this semester …

Process Management Threads, scheduling, mutual exclusion, deadlock

, IPC, etc. Memory Management

Paging, caching, swapping I/O System Management

File, Disk, Clock, device driver. Protection and Security. Intro to Distributed Systems.