Module 1 – Introduction/OS OverviewModule 1 – Introduction/OS Overview

Reading: Chapter 1 and 2 (Silberchatz)Reading: Chapter 1 and 2 (Silberchatz)

Objective:Objective: Quick overview of computer system Quick overview of computer system

organization – the processor (CPU), memory, organization – the processor (CPU), memory, and input/output, architecture and general and input/output, architecture and general operations.operations.

Introduce operating systems to understand its Introduce operating systems to understand its role and principal functions.role and principal functions.

Computer System

Organization

Operating System

Computer System

Organization

Operating System

Hardware

CPUBus

Device Controllers

Memory

Computer System OrganizationComputer System Organization Computer-system operation

One or more CPUs, device controllers connected through common bus providing access to shared memory

Concurrent execution of CPUs and devices competing for memory cycles

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

MainMemory

SecondaryStorage

Storage StructureStorage Structure Primary storage

Main memory, directly accessible by the CPU Program must be in main memory in order to be

executed Main memory usually not large enough to hold all

programs and data Main memory is volatile – loses contents on power

loss/reboot Secondary storage

holds large quantities of data, permanently

In general, we have a hierarchy of storage devices varying by speed, cost, size and volatility

Storage-Device HierarchyStorage-Device Hierarchy

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

I/O StructureI/O Structure Controller has registers for accepting commands

and transferring data (i.e. data-in, data-out, command, status)

Device driver for each device controller talks to the controller The driver is the one who knows details of

controller Provides uniform interface to kernel

I/O operation Device driver loads controller registers

appropriately Controller examines registers, executes I/O

I/O StructureI/O Structure

How does the driver know when the I/O completes? Periodically read the status register

• Called direct I/O• Low overhead if I/O is fast• If I/O is slow, lots of busy waiting

Any idea how to deal with slow I/O? Do something else and let the controller

signal device driver (raising an interrupt) that I/O has completed

• Called interrupt-driven I/O• More overhead, but gets rid of busy waiting

Interrupt TimelineInterrupt Timeline

I/O StructureI/O Structure

Interrupt – driven I/O still has lots of overhead if used for bulk data transfer An interrupt for each byte is too much

Ideas? Have a smart controller that can talk

directly with the memory Tell the controller to transfer block of data

to/from memory The controller raises interrupt when the

transfer has completed Called Direct Memory Access (DMA)

How a Modern Computer WorksHow a Modern Computer Works

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

Architecture

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

Single *processorMulti

processor

ClustersDistributed

Computer-System ArchitectureComputer-System Architecture Single-processor system

From PDAs to mainframes Almost all have special-purpose processors for

graphics, I/O• Not considered multiprocessor

Multi-processor systems Increase throughput Economy of scale Increased reliability Asymmetric multiprocessing

• Each processor assigned a specific task Symmetric multiprocessing (SMP) most common

• All processors perform tasks within the OS

Clusters, distributed systems

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

Architecture

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

Single *processorMulti

processor

ClustersDistributed

UserView

Whatis it?

Services User InterfaceGUI or CLI

The Role of the Operating SystemThe Role of the Operating System

User View of a ComputerUser View of a Computer This is my box, only I am using it

i.e. desktop system with one user monopolizing its resources OS maximizes the work (or play) user is performing OS designed mostly for ease of use, not for resource utilization Handheld systems – usability + low hardware demands

This is The Big Holy Computer, I am blessed to get some CPU time i.e. mainframe or minicomputer OS is designed to maximize resource use (CPU, memory, I/O)

Communism in practice - Let’s share our computers i.e. workstations connected to networks of servers Dedicated and shared resources OS compromises between individual usability and resource

utilization What? There is a computer inside?!

Embedded systems designed to run without user intervention

Why Operating Systems?Why Operating Systems?If there are several users/applications sharing the

computer i.e. Big Holy Computer who should get which resources? when? what is each user/application allowed to do? how should we bill the users?

Ok, ok, if there are many users/programs, something has to manage them, but what if there is single user? (i.e. for my desktop/PDA)?

hardware abstraction I might still want to run several applications

concurrently

What Operating Systems Do?What Operating Systems Do? Give me a 1-2 sentence summary of what

OS does OS is program most involved with the

hardware• hardware abstraction

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

Defining Operating SystemsDefining Operating Systems So, what is an operating system? No universally accepted definition “Everything a vendor ships when you order an

operating system” is good approximation But varies wildly (see system programs)

“The one program running at all times on the computer” is the one generally used in this course This is the kernel Everything else is either a system program (ships

with the operating system) or an application program

Ha! What does it mean “running at all times”? When I am playing Tetris, Tetris is running on the CPU, no?

System ProgramsSystem Programs Are these part of the Operating System? Whatever is not in the kernel, but is shipped with the OS

Of course, depends on the OS and the vendor Provide a lot of system-type functionality, i.e. most

commands you type in Unix CLI are not shell commands, but standalone system programs that perform system tasks

Most users’ view of the operation system is defined by system programs, not the actual system calls

System ProgramsSystem Programs System programs provide a convenient

environment for program development and execution. They can be divided into: File management – i.e. copy, rm, ls, mkdir Status information – i.e. ps, who, regedit File modification - editors Programming language support – i.e. cc, javac Program loading and execution – loaders, debuggers Communications – ssh, ftp Application programs – browsers, spreadsheets,

games• Also called system utilities

Operating System ServicesOperating System ServicesServices provided to user programs:

I/O operations • User program cannot directly access I/O hardware, OS

does the low level part for them Communications

• Both inter-process on the same computer, and between computers over a network

• via shared memory or through message passing Error detection

• Errors do occur: in the CPU and memory hardware, in I/O devices, in user programs

• OS should handle them appropriately to ensure correct and consistent computing

• Low level debugging tools really help

Operating System Services (Cont.)Operating System Services (Cont.)Services for ensuring efficient operation of the system

itself Resource allocation and management

• Needed when there are multiple users/ multiple jobs running concurrently

• Some resources need specific management code• CPU cycles, main memory, file storage

• Others can be managed using general code - I/O devices Accounting

• Which users use how much and what kinds of computer resources

Protection and security • Between different processes/users in the computer• From the outsiders in a networked computer system• Protection: ensuring that all access to system resources is

controlled• Security: user authentication, defending against external I/O

devices from invalid access attempts

OS interface for users - CLIOS interface for users - CLI

Command Line Interface allows direct command entry from keyboard Command interpreter is itself a program that reads

command lines typed in by the user • Often called a shell (UNIX terminology)

Execution of commands accomplished in one of two ways

• The command interpreter executes the command itself• Programming instructions allow command interpreters to

execute “shell” programs• The command is used to invoke a separate program

(system program)

OS interface for users - GUIOS interface for users - GUI User-friendly desktop metaphor interface

Usually mouse, keyboard, and monitor Icons represent files, programs, actions, etc Various mouse buttons over objects in the interface

cause various actions (provide information, options, execute function, open directory

Invented at Xerox PARC Many systems now include both CLI and GUI interfaces

Microsoft Windows is GUI with CLI “command” shell Apple Mac OS X as “Aqua” GUI interface with UNIX

kernel underneath and shells available Solaris and Linux are CLI with optional GUI interfaces

(Java Desktop, KDE)

CLI vs GUICLI vs GUISo, which one is better? What would your grandma prefer? True hacker would never use GUI!

By definition, anybody using GUI is wimp, not a true hacker!

Depends on what you want to do…

GUI benefits Intuitive, easy to use

Ha! If well designed…not always.

CLI benefits Easier to execute complex commands

OS InterfacesOS Interfaces

CLI and GUI – interfaces for the user What other interfaces the Operating Systems have?

Interface to the programs running on the computer and requesting OS services

• System Call interface

Interface to the hardware• Interrupts, drivers device controllers

Shall talk about the System Call interface a little later We will talk about the interface to the hardware in

Ch13 - Kernel I/O subsystem

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

Architecture

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

Single *processorMulti

processor

ClustersDistributed

UserView

Whatis it?

Services User InterfaceGUI or CLI

OperationDualMode

ProcessManag.

MemoryManagement

Input/OutputManagement

A Short History of Operating SystemsA Short History of Operating Systems Problem in 60’s

Computers expensive, need to efficiently utilize them

Single job cannot efficiently use the computer• CPU idle when doing I/O, I/O devices idle when

computing Solution:

Multiprogramming Keeping several jobs in memory in order to

efficiently utilize resources One job selected and run via job scheduling When it has to wait (for I/O for example), OS

switches to another job In general, no guarantee of low response time

A Short History of Operating SystemsA Short History of Operating Systems Later on: computers becoming cheaper, but many

resources still expensive (fast laser printers, mass storage, …) Share the resources in the network Every user wants to have fast interactive environment Solution:

Timesharing (multitasking): Switch jobs frequently, even if they would like to

compute more (preemption) Timer interrupts are used for preemption If several jobs ready to run at the same time CPU

scheduling

A Short History of Operating SystemsA Short History of Operating Systems Lots of stuff invented to make this work well

If processes don’t fit in memory, swapping moves them in and out

Virtual memory allows execution of processes not completely in memory

Introduced in 60’s and 70’s for mainframes Early PCs started simple and primitive (MS-

DOS) Advanced stuff added later as computing

power and user requirements grew

Operating-System OperationsOperating-System Operations

OS is interrupt driven Interrupts raised by hardware and software

Mouse click, division by zero, request for operating system service

Timer interrupt (i.e. process in an infinite loop), memory access violation (processes trying to modify each other or the operating system)

Some operations should be performed only by a trusted party Accessing hardware, memory-management registers A rogue user program could damage other programs,

steal the system for itself, … Solution: dual-mode operation

Transition from User to Kernel ModeTransition from User to Kernel Mode 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

System CallsSystem Calls Programming interface to the services provided

by the OS Process control

• i.e. launch a program File management

• i.e. create/open/read a file, list a directory Device management

• i.e. request/release a device Information maintenance

• i.e. get/set time, process attributes Communications

• i.e. open/close connection, send/receive messages

System CallsSystem Calls Typically written in a high-level language (C or C++) Called from user program by invoking trap instruction

Software interrupt that sets the mode bit to kernel mode and jumps to the appropriate routine, chosen from the system call table based on the provided trap number

• Linux example:

http://docs.cs.up.ac.za/programming/asm/derick_tut/syscalls.html

Upon completion, the mode is switched back to user mode and status of the system call and any return values are returned

API – System Call – OS RelationshipAPI – System Call – OS Relationship

System CallsSystem Calls Mostly accessed by programs via a high-level Application

Program Interface (API) rather than direct system call use Three most common APIs:

Win32 API for Windows POSIX API for POSIX-based systems

• UNIX, Linux, and Mac OS X Java API for the Java virtual machine (JVM)

The caller need know nothing about how the system call is implemented Just needs to obey API and understand what OS will do as a

result call Most details of OS interface hidden from programmer by API

• Managed by run-time support library (set of functions built into

libraries included with compiler)

Standard C Library ExampleStandard C Library Example C program invoking printf() library call, which calls write()

system call

Process ManagementProcess Management Program is passive, process is active, unit of work within

system Single-threaded process has one program counter specifying

location of the 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

Process needs resources to accomplish its task CPU, memory, I/O, files Initialization data

When process terminates, its resources need to be reclaimed

OS needs to facilitate interaction between processes

Process Management ActivitiesProcess 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

Computer Startup and ExecutionComputer Startup and Execution How does a computer start?

bootstrap program is loaded at power-up or reboot Typically stored in ROM or EEPROM, generally known as

firmware Basic hardware initialization (CPU registers, memory) Loads operating system kernel and starts execution

How does execution look like? Typical situation: User program runs, kernel waits for an

event to occur Interrupt from either hardware or software

• Hardware can trigger an interrupt at any time• Software triggers interrupt by executing system call

Stops current execution, transfers execution to a fixed location

• Interrupt service routine is executed, then the execution is resumed where it was interrupted

• Usually a service routine for each device / function• Interrupt vector dispatches interrupt to appropriate routine

Example: MS-DOS executionExample: MS-DOS execution

(a) At system startup (b) running a program

Simple, single-user process control

Example: Unix ShellExample: Unix Shell When you log in, a shell program is launched for you Shell interprets the command line and launches the programs

you specify So, what happens when you type mkdir rrr and press enter?

The shell asks the system to launch a program called ‘mkdir’ with command line argument ‘rrr’

It actually has to figure out where is that program located …

… and then tell the kernel to launch it In Unix, the launching of a program is actually two-step

process: fork() system call to make a new process - copy of itself exec() system call to replace the shell body of this new

process by the body of the program We will talk about this in detail when discussing process

management

Example: Unix ShellExample: Unix Shell The ‘mkdir’ program creates a directory ‘rrr’

and returns exit status to the parent process (i.e. the shell) by making system call exit()

Now, what is the shell process doing meanwhile? By default: waiting until the launched process

finishes• Launching foreground process

But you can also launch a process in the background – in such case the shell continues immediately

Memory ManagementMemory Management Program being executed needs its data and instructions in

the main memory We want to execute several programs concurrently, possibly

with memory requirements larger than the available physical memory

Memory management determines what is in memory and when. Goal: 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 de-allocating memory space as needed

Storage ManagementStorage 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 directories• Mapping files onto secondary storage• Backup files onto stable (non-volatile) storage media

Mass-Storage ManagementMass-Storage Management Usually disks used to store what won’t fit in

memory Proper management is of central importance Entire speed of computer operation hinges on the

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)

CachingCaching 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 is smaller than storage being cached Cache management important design problem Cache size and replacement policy

Why caching works? Principle of locality (temporal, spatial)

Performance of Various Levels of StoragePerformance of Various Levels of Storage

Movement between levels of storage hierarchy can be explicit or implicit

I/O SubsystemI/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

Migration of Integer A from Disk to Migration of Integer A from Disk to RegisterRegister

Multitasking environments must be careful to use most recent value, does not matter where it is stored in the storage hierarchy

Multiprocessor environments usually 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

A Kernel I/O StructureA Kernel I/O Structure

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

Architecture

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

Single *processorMulti

processor

ClustersDistributed

UserView

Whatis it?

Services User InterfaceGUI or CLI

OperationDualMode

ProcessManag.

MemoryManagement

Input/OutputManagement

DifferentFlavors

Design issues

Structure

LayerMicroKernelModules

Virtual Machine

Operating System Design and Operating System Design and ImplementationImplementation

The design is primarily affected by choice of hardware and the type of system Batch, time-shared, single-user, multi-user, distributed,

real-time, general purpose User goals and System goals

User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast

System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

Implementation Traditionally in assembler Nowadays mostly in C, with small assembler sections

(drivers, saving/restoring registers)

Operating System StructureOperating System Structure

Internal structure of different OSes can vary widely As the requirements vary widely

Low hardware resources, simple functionality Simple, monolithic structure

More functionality & resources Layered structure MSDOS and traditional Unix are OS-es with monolithic

structure that uses some layering Even more functionality & resources, focus on clean design

and flexibility Microkernel structure (MACH, QNX, early Windows NT)

Flexibility & efficiency Modular structure (Solaris, Windows NT)

MS-DOS Layer StructureMS-DOS Layer Structure

Layered ApproachLayered Approach The operating system is divided into a number of layers (levels),

each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers

Traditional UNIX System StructureTraditional UNIX System Structure UNIX was created as a simple, relatively low

functionality alternative to the complex mainframe OS-es of that time

Microkernel System Structure Microkernel System Structure Moves as much from the kernel into “user” space Communication takes place between user modules using

message passing Benefits:

Easier to extend a microkernel Easier to port the operating system to new architectures More reliable (less code is running in kernel mode) More secure

Detriments: Performance overhead of user space to kernel space

communication

ModulesModules Most modern operating systems implement kernel modules

Uses object-oriented approach Each core component is separate Each talks to the others over known interfaces Each is loadable as needed within the kernel

Virtual MachinesVirtual Machines

A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware

A virtual machine provides an interface identical to the underlying bare hardware

The VM creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

System ModelsSystem Models

(a) Nonvirtual machine (b) virtual machine

Non-virtual Machine Virtual Machine

Virtual Machines - ImplementationVirtual Machines - Implementation How to create several VM when we have only one

set of hardware resources? Sharing resources

CPU Time-shared scheduling

Printers/card readers Spooling

Memory Virtual memory

HD Ha! Its not possible to have full HD for each VM!

Other problems: real-time aspects

Advantages/Disadvantages of Virtual Advantages/Disadvantages of Virtual MachinesMachines

Advantage: Complete protection/isolation of system resources

between VMs Perfect vehicle for operating-systems research and

development• If the OS you are developing crashes, just restart the VM

Useful when you have applications for different OSs

Disadvantage No direct sharing of resources between different VMs Difficult to implement completely

Example: VMware ArchitectureExample: VMware Architecture

The Java Virtual MachineThe Java Virtual Machine

We are not going to talk aboutWe are not going to talk about

Distributed systems Real-time embedded systems Multimedia systems Handheld systems Different computing environments Peer to peer computing Web-based computing

Computer System

Organisation

Operating System

Hardware

CPUBus

Device Controllers

Memory

Storage Structure

I/O Structure

Architecture

MainMemory

SecondaryStorage

Direct I/O

InterruptDriven

DMA

Single *processorMulti

processor

ClustersDistributed

UserView

Whatis it?

Services User InterfaceGUI or CLI

OperationDualMode

ProcessManag.

MemoryManagement

Input/OutputManagement

DifferentFlavors

Design issues

Structure

LayerMicroKernelModules

Virtual Machine