Os Concepts

Operating System Concepts

System Structure „ System structured as a series of levels - each level performs a

related subset of functions „ Each level relies on the next lower level for more primitive functions „ Layers Concepts z

Hardware abstraction layer

z

Microkernel

z

Device drivers

Operating System Concepts

Computer System Structure „ Computer system can be divided into four components z

Hardware – provides basic computing resources  CPU,

z

memory, I/O devices

Operating system  Controls

and coordinates use of hardware among various applications and users

z

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

z

Users  People,

machines, other computers

Operating System Concepts

Four Components of a Computer System

Operating System Concepts

Operating System ?? „ Computer applications today require a single machine to perform

many operations and the applications may compete for the resources of the machine. „ This demands a high degree of coordination „ This coordination is handled by system software known as the

operating system

Operating System Concepts

Operating System ?? „ What is an operating system? z

Hard to define precisely, because operating systems arose historically as people needed to solve problems associated with using computers.

z

How about… “Software that makes computing power available to users by controlling the hardware.” “A collection of software modules including device drivers, libraries, and access routines.”

Operating System Concepts

Operating System Objectives „ Operating systems are among the most critical pieces of software

ever developed z

Convenience  Makes

z

the computer more convenient to use

Efficiency  Allows

computer system resources to be used in an efficient manner

z

Ability to evolve  Permit

effective development, testing, and introduction of new system functions without interfering with service

Operating System Concepts

Operating System Services „ Program development z

Editors, debuggers, frameworks

„ Program execution z

Initialization, scheduling

„ Access to I/O devices z

Uniform interface, hides details

„ Controlled access to files z

Authorization, sharing, caching

„ System access z

Protection, authorization, resolve conflicts

„ Error detection and response z

Hardware errors: memory error or device failure

z

Software errors: arithmetic errors, access forbidden memory locations, allocation errors

Operating System Concepts

OS as a Resource Manager „ OS executes same way as ordinary computer software - it is set of

computer programs „ The key difference is in the intent z

Directs use of resources

z

Relinquishes control of the processor to execute other programs

„ Kernel or nucleus z

Portion of operating system that is in main memory

z

Contains most-frequently used functions

Operating System Concepts

Evolution of Operating Systems „ Operating systems have evolved because z

New types of hardware and hardware upgrades

z

Development of new services and needs

z

Fixes to OS faults

Distributed Processing Systems Time-Sharing Systems Multi-programmed Batch Systems Simple Batch Processing Serial Processing

Operating System Concepts

OS for batch jobs z

Program execution required significant preparation of equipment

z

Program execution (job)

z

OS was a system to simplify program setup and simplify transition between jobs

z

Physical separation of users and equipment led to computer operators

z

Users left jobs with the operator and came back the next day (batch jobs)

z

Users had no interaction with computer during program execution. Maybe okay for some applications, but not for all.

Operating System Concepts

OS for Interactive Processing z

Allowed programs to carry on dialogue with user via remote terminals (workstations)

z

Real-time processing

z

Users demand timely response

z

Machines too expensive to serve only one user

z

Common for several users to want interactive services at the same time

Operating System Concepts

OS for time-sharing z

To accommodate multiple real-time users, the OS rotates its various jobs in and out of execution via time-sharing

z

Each job gets a predetermined “time slice”

z

At end of time slice current job is set aside and a new one starts

z

By rapidly shuffling jobs, illusion of several jobs executing simultaneously is created

z

Without time slicing, a computer spends most of its time waiting for peripheral devices or users

z

A collection of tasks can be completed in less time with timesharing than when completed sequentially

Operating System Concepts

Different Operating Systems on the Same Machine ? „ It is possible to have more than one operating system available to

be used on a machine. „ Only one operating system is run at a time, though. „ Examples: z

PCs -- DOS, Windows, or Linux

Operating System Concepts

Distributed Processing Systems „ Distributed operating systems z

provides the illusion of a single main memory and single secondary memory space

z

used for distributed file system

Operating System Concepts

Modern OS Architecture

Operating System Concepts

Computer Startup „ bootstrap program is loaded at power-up or reboot z

Typically stored in ROM or EPROM, generally known as firmware

z

Initializates all aspects of system

z

Loads operating system kernel and starts execution

Operating System Concepts

Computer System Organization „ Computer-system operation z

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

z

Concurrent execution of CPUs and devices competing for memory cycles

Operating System Concepts

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.

Operating System Concepts

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 z

Disk surface is logically divided into tracks, which are subdivided into sectors.

z

The disk controller determines the logical interaction between the device and the computer.

Operating System Concepts

Storage Hierarchy „ Storage systems organized in hierarchy. z

Speed

z

Cost

z

Volatility

„ Caching – copying information into faster storage system; main

memory can be viewed as a last cache for secondary storage.

Operating System Concepts

Storage-Device Hierarchy

Operating System Concepts

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 z

If it is, information used directly from the cache (fast)

z

If not, data copied to cache and used there

„ Cache smaller than storage being cached z

Cache management important design problem

z

Cache size and replacement policy

Operating System Concepts

Migration of Integer A from Disk to Register „ Multitasking environments must be careful to use most recent

value, no matter where it is stored in the storage hierarchy

„ Multiprocessor environment must provide cache coherency in

hardware such that all CPUs have the most recent value in their cache „ Distributed environment situation even more complex z

Several copies of a datum can exist

z

Various solutions covered in Chapter 17

Operating System Concepts

Memory Layout for Multiprogrammed System

Operating-System Structures

Operating System Concepts

The OS Kernel „ The internal part of the OS is often called the kernel „ Kernel Components z

File Manager

z

Device Drivers

z

Memory Manager

z

Scheduler

z

Dispatcher

Operating System Concepts

User Operating System Interface - CLI CLI allows direct command entry  Sometimes

implemented in kernel, sometimes by systems

program  Sometimes  Primarily –

multiple flavors implemented – shells

fetches a command from user and executes it

Sometimes commands built-in, sometimes just names of programs »

If the latter, adding new features doesn’t require shell modification

Operating System Concepts

User Operating System Interface - GUI „ User-friendly desktop metaphor interface z

Usually mouse, keyboard, and monitor

z

Icons represent files, programs, actions, etc

z

Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder)

z

Invented at Xerox PARC

„ Many systems now include both CLI and GUI interfaces z

Microsoft Windows is GUI with CLI “command” shell

z

Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and shells available

z

Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)

Operating System Concepts

System Calls „ Programming interface to the services provided by the OS „ Typically written in a high-level language (C or C++) „ Mostly accessed by programs via a high-level Application

Program Interface (API) rather than direct system call use „ Three most common APIs are Win32 API for Windows, POSIX API

for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM) „ Why use APIs rather than system calls?

(Note that the system-call names used throughout this text are generic)

Operating System Concepts

API – System Call – OS Relationship

Write()

Write() Implementation of Write()

Operating System Concepts

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

Operating System Concepts

Simple Structure „ MS-DOS – written to provide the most functionality in the least

space z

Not divided into modules

z

Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

Operating System Concepts

MS-DOS Layer Structure

Operating System Concepts

Layered Operating System

Operating System Concepts

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

passing „ Benefits: z

Easier to extend a microkernel

z

Easier to port the operating system to new architectures

z

More reliable (less code is running in kernel mode)

z

More secure

„ Detriments: z

Performance overhead of user space to kernel space communication

Operating System Concepts

Solaris Modular Approach

Operating System Concepts

Virtual 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 operating system creates the illusion of multiple

processes, each executing on its own processor with its own (virtual) memory

Operating System Concepts

Virtual Machines (Cont.) „ The resources of the physical computer are shared to create the

virtual machines z

CPU scheduling can create the appearance that users have their own processor

z

Spooling and a file system can provide virtual card readers and virtual line printers

z

A normal user time-sharing terminal serves as the virtual machine operator’s console

Operating System Concepts

Virtual Machines

Non-virtual Machine

(a) Non-virtual machine

Virtual Machine

(b) virtual machine

Operating System Concepts

Virtual Machines (Cont.) „ The virtual-machine concept provides complete protection of system

resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources. „ A virtual-machine system is a perfect vehicle for operating-systems

research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation. „ The virtual machine concept is difficult to implement due to the effort

required to provide an exact duplicate to the underlying machine

Operating System Concepts

VMware Architecture

Operating System Concepts

The Java Virtual Machine

Operating System Concepts

Operating-System Operations „ Process Management „ Memory Management „ Storage Management „ Protection and Security „ Distributed Systems

Processes …

Operating System Concepts

Process Concept „ An operating system executes a variety of programs: z

Batch system – jobs

z

Time-shared systems – user programs or tasks

„ Textbook uses the terms job and process almost

interchangeably

„ Process – a program in execution; process execution must

progress in sequential fashion

„ A process includes: z

program counter

z

stack

z

data section

Operating System Concepts

Process in Memory

Operating System Concepts

Process State „

As a process executes, it changes state z

new: The process is being created

z

running: Instructions are being executed

z

waiting: The process is waiting for some event to occur

z

ready: The process is waiting to be assigned to a processor

z

terminated: The process has finished execution

Operating System Concepts

Diagram of Process State

Operating System Concepts

Process Control Block (PCB) Information associated with each process „ Process state „ Program counter „ CPU registers „ CPU scheduling information „ Memory-management information „ Accounting information „ I/O status information

Operating System Concepts

Process Control Block (PCB)

Operating System Concepts

CPU Switch From Process to Process

Operating System Concepts

Process Creation „ Parent process create children processes, which, in turn create

other processes, forming a tree of processes „ Resource sharing z

Parent and children share all resources

z

Children share subset of parent’s resources

z

Parent and child share no resources

„ Execution z

Parent and children execute concurrently

z

Parent waits until children terminate

Operating System Concepts

Process Creation (Cont.) „ Address space z

Child duplicate of parent

z

Child has a program loaded into it

„ UNIX examples z

fork system call creates new process

z

exec system call used after a fork to replace the process’ memory space with a new program

Operating System Concepts

Process Creation

Operating System Concepts

Process Termination „ Process executes last statement and asks the operating system to

delete it (exit) z

Output data from child to parent (via wait)

z

Process’ resources are deallocated by operating system

„ Parent may terminate execution of children processes (abort) z

Child has exceeded allocated resources

z

Task assigned to child is no longer required

z

If parent is exiting  Some

operating system do not allow child to continue if its parent terminates –

All children terminated - cascading termination

Operating System Concepts

Inter-process Communication (IPC) „ Mechanism for processes to communicate and to synchronize their

actions

„ Message system – processes communicate with each other

without resorting to shared variables

„ IPC facility provides two operations: z

send(message) – message size fixed or variable

z

receive(message)

„ If P and Q wish to communicate, they need to: z

establish a communication link between them

z

exchange messages via send/receive

„ Implementation of communication link z

physical (e.g., shared memory, hardware bus)

z

logical (e.g., logical properties)

Operating System Concepts

Communications Models

Operating System Concepts

Direct Communication „ Processes must name each other explicitly: z

send (P, message) – send a message to process P

z

receive(Q, message) – receive a message from process Q

„ Properties of communication link z

Links are established automatically

z

A link is associated with exactly one pair of communicating processes

z

Between each pair there exists exactly one link

z

The link may be unidirectional, but is usually bi-directional

Operating System Concepts

Client-Server Communication „ Sockets „ Remote Procedure Calls „ Remote Method Invocation (Java)

Operating System Concepts

Sockets „ A socket is defined as an endpoint for communication „ Concatenation of IP address and port „ The socket 161.25.19.8:1625 refers to port 1625 on host

161.25.19.8 „ Communication consists between a pair of sockets

Operating System Concepts

Socket Communication

Operating System Concepts

Remote Procedure Calls „ Remote procedure call (RPC) abstracts procedure calls between

processes on networked systems. „ Stubs – client-side proxy for the actual procedure on the server. „ The client-side stub locates the server and marshalls the

parameters. „ The server-side stub receives this message, unpacks the

marshalled parameters, and peforms the procedure on the server.

Note : Marshalling is the process of packaging and sending interface method parameters across thread, process or machine boundaries.

Operating System Concepts

Remote Method Invocation „ Remote Method Invocation (RMI) is a Java mechanism similar to

RPCs. „ RMI allows a Java program on one machine to invoke a method on

a remote object.

Threads

Operating System Concepts

Single and Multithreaded Processes

Operating System Concepts

User Threads „ Thread management done by user-level threads library „ Three primary thread libraries: z

POSIX Pthreads

z

Win32 threads

z

Java threads

Operating System Concepts

Kernel Threads „ Supported by the Kernel „ Examples z

Windows XP/2000

z

Solaris

z

Linux

z

Tru64 UNIX

z

Mac OS X

Operating System Concepts

Multithreading Models „ Many-to-One „ One-to-One „ Many-to-Many

Operating System Concepts

Many-to-One „ Many user-level threads mapped to single kernel thread „ Examples: z

Solaris Green Threads

z

GNU Portable Threads

Operating System Concepts

Many-to-One Model

Operating System Concepts

One-to-One „ Each user-level thread maps to kernel thread „ Examples z

Windows NT/XP/2000

z

Linux

z

Solaris 9 and later

Operating System Concepts

One-to-one Model

Operating System Concepts

Many-to-Many Model „ Allows many user level threads to be mapped to many

kernel threads „ Allows the operating system to create a sufficient

number of kernel threads „ Solaris prior to version 9 „ Windows NT/2000 with the ThreadFiber package

Operating System Concepts

Many-to-Many Model

Operating System Concepts

Two-level Model „ Similar to M:M, except that it allows a user thread to be

bound to kernel thread „ Examples z

IRIX

z

HP-UX

z

Tru64 UNIX

z

Solaris 8 and earlier

Operating System Concepts

Two-level Model

Operating System Concepts

Threading Issues „ Semantics of fork() and exec() system calls „ Thread cancellation „ Signal handling „ Thread pools „ Thread specific data „ Scheduler activations

Operating System Concepts

Thread Cancellation „ Terminating a thread before it has finished „ Two general approaches: z

Asynchronous cancellation terminates the target thread immediately

z

Deferred cancellation allows the target thread to periodically check if it should be cancelled

Operating System Concepts

Signal Handling „

Signals are used in UNIX systems to notify a process that a particular event has occurred

„

A signal handler is used to process signals

„

1.

Signal is generated by particular event

2.

Signal is delivered to a process

3.

Signal is handled

Options: z

Deliver the signal to the thread to which the signal applies

z

Deliver the signal to every thread in the process

z

Deliver the signal to certain threads in the process

z

Assign a specific thread to receive all signals for the process

Operating System Concepts

Thread Pools „ Create a number of threads in a pool where they await work „ Advantages: z

Usually slightly faster to service a request with an existing thread than create a new thread

z

Allows the number of threads in the application(s) to be bound to the size of the pool

Operating System Concepts

Java Thread States

Deadlocks

Operating System Concepts

Deadlock ??? A condition that occurs when two processes are each waiting for the other to complete before proceeding. The result is that both processes hang. Deadlocks occur most commonly in multitasking and client/server environments. Ideally, the programs that are deadlocked, or the operating system, should resolve the deadlock, but this doesn't always happen. A deadlock is also called a deadly embrace.

Operating System Concepts

Bridge Crossing Example

„

Traffic only in one direction.

„

Each section of a bridge can be viewed as a resource.

„

If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback).

„

Several cars may have to be backed up if a deadlock occurs.

„

Starvation is possible. z

In computer science, starvation is a multitasking-related problem, where a process is perpetually denied necessary resources. Without those resources, the program can never finish its task.

Main Memory

Operating System Concepts

Objectives „ To provide a detailed description of various ways of

organizing memory hardware „ To discuss various memory-management techniques,

including paging and segmentation „ To provide a detailed description of the Intel Pentium, which

supports both pure segmentation and segmentation with paging

Operating System Concepts

Background „ Program must be brought (from disk) into memory and placed

within a process for it to be run „ Main memory and registers are only storage CPU can access

directly „ Register access in one CPU clock (or less) „ Main memory can take many cycles „ Cache sits between main memory and CPU registers „ Protection of memory required to ensure correct operation

Operating System Concepts

Base and Limit Registers „ A pair of base and limit registers define the logical address space

Operating System Concepts

Binding of Instructions and Data to Memory „ Address binding of instructions and data to memory addresses

can happen at three different stages z

Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes

z

Load time: Must generate relocatable code if memory location is not known at compile time

z

Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers)

Operating System Concepts

Multi-step Processing of a User Program

Operating System Concepts

Logical vs. Physical Address Space „ The concept of a logical address space that is bound to a

separate physical address space is central to proper memory management z

Logical address – generated by the CPU; also referred to as virtual address

z

Physical address – address seen by the memory unit

„ Logical and physical addresses are the same in compile-time

and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme

Operating System Concepts

Contiguous Allocation „ Main memory usually into two partitions: z

Resident operating system, usually held in low memory with interrupt vector

z

User processes then held in high memory

„ Relocation registers used to protect user processes from each

other, and from changing operating-system code and data z

Base register contains value of smallest physical address

z

Limit register contains range of logical addresses – each logical address must be less than the limit register

z

MMU maps logical address dynamically

Operating System Concepts

Contiguous Allocation (Cont.) „ Multiple-partition allocation z

Hole – block of available memory; holes of various size are scattered throughout memory

z

When a process arrives, it is allocated memory from a hole large enough to accommodate it

z

Operating system maintains information about: a) allocated partitions b) free partitions (hole) OS

OS

OS

OS

process 5

process 5

process 5

process 5

process 9

process 9

process 8 process 2

process 10 process 2

process 2

process 2

File-System Interface

Operating System Concepts

File-System Interface „ File Concept „ Access Methods „ Directory Structure „ File-System Mounting „ File Sharing „ Protection

Operating System Concepts

File Concept „ Contiguous logical address space „ Types: z

Data  numeric  character  binary

z

Program

Operating System Concepts

File Structure „ None - sequence of words, bytes „ Simple record structure z

Lines

z

Fixed length

z

Variable length

„ Complex Structures z

Formatted document

z

Relocatable load file

„ Can simulate last two with first method by inserting appropriate

control characters

„ Who decides: z

Operating system

z

Program

Operating System Concepts

File Attributes „ Name – only information kept in human-readable form „ Identifier – unique tag (number) identifies file within file system „ Type – needed for systems that support different types „ Location – pointer to file location on device „ Size – current file size „ Protection – controls who can do reading, writing, executing „ Time, date, and user identification – data for protection, security,

and usage monitoring „ Information about files are kept in the directory structure, which is

maintained on the disk

Operating System Concepts

File Types – Name, Extension

Operating System Concepts

Example of Index and Relative Files

Operating System Concepts

Directory Structure „ A collection of nodes containing information about all files

Directory

Files

F1

F2

F3

F4 Fn

Both the directory structure and the files reside on disk, Backups of these two structures are kept on tapes

Operating System Concepts

A Typical File-system Organization

Operating System Concepts

Single-Level Directory „ A single directory for all users

Naming problem Grouping problem

Operating System Concepts

Two-Level Directory „ Separate directory for each user

„ Path name „ Can have the same file name for different user „ Efficient searching „ No grouping capability

Operating System Concepts

Tree-Structured Directories

Operating System Concepts

Tree-Structured Directories (Cont) „ Absolute or relative path name „ Creating a new file is done in current directory „ Delete a file

rm „ Creating a new subdirectory is done in current directory

mkdir Example: if in current directory /mail mkdir count

mail prog

copy prt exp count

Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”

Operating System Concepts

Acyclic-Graph Directories „ Have shared subdirectories and files

Operating System Concepts

Protection „ File owner/creator should be able to control: z

what can be done

z

by whom

„ Types of access z

Read

z

Write

z

Execute

z

Append

z

Delete

z

List

Mass-Storage Systems

Operating System Concepts

Overview of Mass Storage Structure „

Magnetic disks provide bulk of secondary storage of modern computers z

Drives rotate at 60 to 200 times per second

z

Transfer rate is rate at which data flow between drive and computer

z

Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)

z

Head crash results from disk head making contact with the disk surface 

That’s bad

„

Disks can be removable

„

Drive attached to computer via I/O bus z

Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI

z

Host controller in computer uses bus to talk to disk controller built into drive or storage array

Operating System Concepts

Moving-head Disk Machanism

Operating System Concepts

Disk Structure „ Disk drives are addressed as large 1-dimensional arrays of logical

blocks, where the logical block is the smallest unit of transfer. „ The 1-dimensional array of logical blocks is mapped into the

sectors of the disk sequentially. z

Sector 0 is the first sector of the first track on the outermost cylinder.

z

Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.

I/O Systems

Operating System Concepts

I/O Systems „ I/O Hardware „ Application I/O Interface „ Kernel I/O Subsystem „ Transforming I/O Requests to Hardware Operations „ Streams „ Performance

Operating System Concepts

Objectives „ Explore the structure of an operating system’s I/O subsystem „ Discuss the principles of I/O hardware and its complexity „ Provide details of the performance aspects of I/O hardware and

software

Operating System Concepts

I/O Hardware „ Incredible variety of I/O devices „ Common concepts z

Port

z

Bus (daisy chain or shared direct access)

z

Controller (host adapter)

„ I/O instructions control devices „ Devices have addresses, used by z

Direct I/O instructions

z

Memory-mapped I/O

Operating System Concepts

Polling „ Determines state of device z

command-ready

z

busy

z

Error

„ Busy-wait cycle to wait for I/O from device

Operating System Concepts

Interrupts „ CPU Interrupt-request line triggered by I/O device „ Interrupt handler receives interrupts „ Maskable to ignore or delay some interrupts „ Interrupt vector to dispatch interrupt to correct handler z

Based on priority

z

Some nonmaskable

„ Interrupt mechanism also used for exceptions

Operating System Concepts

Interrupt-Driven I/O Cycle

Operating System Concepts

A Kernel I/O Structure

SCSI : small computer system interface

Operating System Concepts

Block and Character Devices „ Block devices include disk drives z

Commands include read, write, seek

z

Raw I/O or file-system access

z

Memory-mapped file access possible

„ Character devices include keyboards, mice, serial ports z

Commands include get, put

z

Libraries layered on top allow line editing

Operating System Concepts

Kernel I/O Subsystem „ Scheduling z

Some I/O request ordering via per-device queue

z

Some OSs try fairness

„ Buffering - store data in memory while transferring between

devices z

To cope with device speed mismatch

z

To cope with device transfer size mismatch

z

To maintain “copy semantics”

Operating System Concepts

Kernel I/O Subsystem „ Caching - fast memory holding copy of data z

Always just a copy

z

Key to performance

„ Spooling - hold output for a device z

If device can serve only one request at a time

z

i.e., Printing

„ Device reservation - provides exclusive access to a device z

System calls for allocation and deallocation

z

Watch out for deadlock

Operating System Concepts

I/O Protection „ User process may accidentally or purposefully attempt to disrupt

normal operation via illegal I/O instructions z

All I/O instructions defined to be privileged

z

I/O must be performed via system calls  Memory-mapped

and I/O port memory locations must be

protected too z

Note : a system call, or software interrupt is the mechanism used by an application program to request service from the operating system. System calls often use a special machine code instruction which causes the processor to change mode (e.g. to "supervisor mode" or "protected mode"). This allows the OS to perform restricted actions such as accessing hardware devices or the memory management unit.

Operating System Concepts

Kernel Data Structures „ Kernel keeps state info for I/O components, including open file

tables, network connections, character device state „ Many, many complex data structures to track buffers, memory

allocation, “dirty” blocks „ Some use object-oriented methods and message passing to

implement I/O

Operating System Concepts

I/O Requests to Hardware Operations „ Consider reading a file from disk for a process: z

Determine device holding file

z

Translate name to device representation

z

Physically read data from disk into buffer

z

Make data available to requesting process

z

Return control to process

Operating System Concepts

Life Cycle of An I/O Request

Operating System Concepts

Performance „ I/O a major factor in system performance: z

Demands CPU to execute device driver, kernel I/O code

z

Context switches due to interrupts

z

Data copying

z

Network traffic especially stressful

Security

Operating System Concepts

The Security Problem „ Security must consider external environment of the system, and

protect the system resources „ Intruders (crackers) attempt to breach security „ Threat is potential security violation „ Attack is attempt to breach security „ Attack can be accidental or malicious „ Easier to protect against accidental than malicious misuse

Operating System Concepts

A Boot-sector Computer Virus

Operating System Concepts

Cryptography as a Security Tool „ Broadest security tool available z

Source and destination of messages cannot be trusted without cryptography

z

Means to constrain potential senders (sources) and / or receivers (destinations) of messages

„ Based on secrets (keys)

Operating System Concepts

Secure Communication over Insecure Medium

Operating System Concepts

Digital Certificates „ Proof of who or what owns a public key „ Public key digitally signed a trusted party „ Trusted party receives proof of identification from entity and

certifies that public key belongs to entity „ Certificate authority are trusted party – their public keys included

with web browser distributions z

They vouch for other authorities via digitally signing their keys, and so on

Operating System Concepts

Encryption Example - SSL „ Insertion of cryptography at one layer of the ISO network model

(the transport layer)

„ SSL – Secure Socket Layer (also called TLS) „ Cryptographic protocol that limits two computers to only exchange

messages with each other z

Very complicated, with many variations

„ Used between web servers and browsers for secure

communication (credit card numbers)

„ The server is verified with a certificate assuring client is talking to

correct server

„ Asymmetric cryptography used to establish a secure session key

(symmetric encryption) for bulk of communication during session

„ Communication between each computer theb uses symmetric key

cryptography

Distributed System Structures

Operating System Concepts

Motivation „ Distributed system is collection of loosely coupled processors

interconnected by a communications network

„ Processors variously called nodes, computers, machines, hosts z

Site is location of the processor

„ Reasons for distributed systems z

Resource sharing  sharing

and printing files at remote sites

 processing  using

information in a distributed database

remote specialized hardware devices

z

Computation speedup – load sharing

z

Reliability – detect and recover from site failure, function transfer, reintegrate failed site

z

Communication – message passing

Operating System Concepts

A Distributed System

Operating System Concepts

Types of Distributed Operating Systems „ Network Operating Systems „ Distributed Operating Systems

Operating System Concepts

Network-Operating Systems „ Users are aware of multiplicity of machines. Access to

resources of various machines is done explicitly by: z

Remote logging into the appropriate remote machine (telnet, ssh)

z

Remote Desktop (Microsoft Windows)

z

Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism

Operating System Concepts

Distributed-Operating Systems „ Users not aware of multiplicity of machines z

Access to remote resources similar to access to local resources

„ Data Migration – transfer data by transferring entire file, or

transferring only those portions of the file necessary for the immediate task „ Computation Migration – transfer the computation, rather than the

data, across the system

Operating System Concepts

Distributed-Operating Systems (Cont.) „ Process Migration – execute an entire process, or parts of it, at

different sites z

Load balancing – distribute processes across network to even the workload

z

Computation speedup – subprocesses can run concurrently on different sites

z

Hardware preference – process execution may require specialized processor

z

Software preference – required software may be available at only a particular site

z

Data access – run process remotely, rather than transfer all data locally

Operating System Concepts

The Future… „

In the future, computers will continue to become physically smaller and more portable.

„

Operating systems have to deal with issues like disconnected operation and mobility.

„

Media rich information within the grasp of common people - information with psuedo-real time components like voice and video.

„

Operating systems will have to adjust to deliver acceptable performance for these new forms of data.

Operating System Concepts

Finally „

Operating systems are so large no one person understands whole system. Outlives any of its original builders.

„

The major problem facing computer science today is how to build large, reliable software systems.

„

Operating systems are one of very few examples of existing large software systems, and by studying operating systems we may learn lessons applicable to the construction of larger systems.