Operating Systems

國立清華大學資訊工程學系

CS1356 資訊工程導論

112/04/20

How does a computer start executing?

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Simple Answer

• The program counter is initiated with a particular address in a special memory when the computer is powered on– That address is start of a (special) program

to bring up other programs and the system– But DRAM is volatile!– So, we use read-only memory (ROM)

Suppose the computer runs only one program

…Full control of everything, e.g. CPU

Manage everything

Suppose the computer runs many programs …

•How do they get executed?

•How do they get the most important resource – the CPU?

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Grabbing the CPU

• In a single-processor computer, only one CPU to be shared by all programs

• Who can get the microphone?(analogy by 資工四年清班張明禾 )

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We Need a Chairperson!

• The chairperson decides who gets the microphone to speak next

• Two ways to schedule:– Let each speaker talk until he/she finishes– “Interrupts” the speaker to get back the

microphone and turn to another speaker time sharing

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Chairperson Can Do More

• Which portion of blackboarda speaker can write? memory management

• Who can use 幻燈機 (projector)?– Why not operated by chairperson?– Device driver and management

Chairperson is OS

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What Is a System?

• A set of interacting, interdependent entities forming an integrated whole. – From Wikipedia

• Five components– Hardware – Software– Data– Procedure– User

System

Hardware

Software

User

Data

Procedure

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Software Classification

• Operating system is one kind of software

(Fig. 3.3)

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Operating Systems

• One kind of software that controls the overall operation of a computer – Unix, Sun Solaris– Linux: Ubuntu, Redhat, ...– Microsoft Windows– Apple Mac OS X– Google Chrome OS

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Outline

• Components and functions

• Process management

• Handling competition for resources

• Security

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Components and Functions

Shell, kernel, file manager, device drivers, memory manager,

bootstrapping, scheduler, dispatcher

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Components of OS

• For user: shell, privilege control (security)

• For data: file manager

• For hardware: device manager, memory manager, and boot manager

• For software: – Where to store: file manger, registry– How to execute: scheduler, process manager

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How about Procedure?

• OS needs to define a set of rules or working flows for users and hardware/ software developers.– For example, you need to double click an icon

to open a program or a file.– Design a simple yet useful procedure for a

complicated system is not an easy job. – This is for books like “How to use

computers?” to talk about

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Shell

• Shell: an interface between users and the operating system– Text based: 命令提示字元– Graphical user interface

(GUI)• Windows, icons, menus,

pointers (WIMP)• Window manager

(Fig. 3.4)

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File Manager

• File manager: organizes and maintains the records of files in mass storages

• Hierarchical structure– Directory (or folder), directory path

• File descriptor– File name, extension, size, updated

date, permissions, attributes, …

• File operations– Copy, paste, creation, open…

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Device Manager

• Communicate with the controllers/devices– Drive the corresponding peripheral devices– Each device driver is uniquely designed for its

particular type of device

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Memory Manager

• Coordinates the use of memory

• Suppose computer runs only one program– The program can use any part of the main

memory (i.e., DRAM)– What if the program is larger than the memory?

store only the needed portion

– Who makes the move?

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Memory Manager

• Suppose computer runs many programs at the same time– What if programs are larger than the memory?– Which program uses which part of the

memory?– How to protect they from each other?

All done by OS!

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Virtual Memory

• Create the illusion of a memory space much larger than physical memory. How?– Store only needed portion in memory and the

remaining in disks– Shuffle portions between memory and disks– Program uses virtual address, while OS does

the mapping

• Paging: memory is grouped into pages to facilitate the mapping and shuffling

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Example of Paging

• There are 8 pages; each is of 4KB – Main memory is of size 16KB (4 pages)– Programs use virtual

address to access data and code

– OS does the mappingand paging

Disk

Page 0

Page 1

Page 2

Page 3

Page 4

Page 5

Page 6

Page 7

Virtual address

Main memory

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Get It Started: Bootstrapping

• Loader: a special program places machine programs to main memory for execution– Think about the problem 2 of homework 3– Usually part of the OS’s scheduler

• Who loads the OS to memory?– A “special memory” that contains a “program”

to load the OS after computer is powered on.

Read-only memory (ROM) Bootstrap

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The Booting Process

The program counter is initiated with a particular address in ROM where the bootstrap is stored

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BIOS and Firmware

• The bootstrap program and other basic input/output functions are contained in a special ROM, called BIOS (basic input/output system)

• A program stored in ROM is called firmware– Hardware or software?

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Process Management

History, today, and future

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A Program vs. a Process

• Program: a set of instructions, e.g., notepad.c, notepad.exe

• Process: activity of executing a program• A program can be run multiple times, each

instance/activity called a process• Interprocess communication

– The communication between processes (may from running one or more programs)

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Evolution of Computers

• Batch processing

• Interactive processing: requires real-time processing

• Time-sharing/multitasking: implemented by multiprogramming

• New challenges: multicore processors, and small devices

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Batch Processing

FIFO: first in first serve

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Interactive Processing

Text editing, music/movie playing, …

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Time-sharing/Multitasking

• Time-sharing between process A and process B

(Fig. 3.6)

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Context (Process State)

• Snapshot of current status of a process– A process identifier, or PID– Register values, Program Counter value– The memory space, I/O, files for the process– Can be saved and resumed as if the process is not

interrupted

• Another meaning: execution state of the process– Ready: ready for execution– Waiting: waiting for some I/O– Complete: finished process

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Scheduler

• Determines which processes should be considered for execution based on some priorities or concerns– Using process table for administration

• Process table– Ready or waiting– Priority– Non-scheduling information: memory pages,

etc.

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Dispatcher

• Gives time slices to a process that is ready

• Executes a context switch when the running process’s time slice is over– Time slice: a time segment for each execution– Interrupt: the signal generated by a hardware

timer to indicate the end of a time slice.– The interrupt handler (part of dispatcher)

starts after the interrupt to perform context switch

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Context Switch (process switch)

1. Get an interrupt from timer

2. Go to the interrupt handlera. Save the context of process A

b. Find a process ready to run(Assume that is process B)

c. Load the context of process B

3. Start (continue) process B

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Thread

• A task existing within a process that allows multiple independent instances to be executed concurrently– Multiple threads share resources such as

memory, program code, … – Each thread has its own program counter,

registers, and stack (local memory)

• The context switch of threads is much faster than that of processes

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Exercises

• Suppose an OS allocates time slices in 10 millisecond units and the time required for a context switch is negligible. How many processes can obtain a time slice is one second?  

• If it takes one microsecond to perform a context switch and processes use only half of their allotted 10 millisecond time slices, what percent of a CPU’s time is spent performing context switches rather than executing processes?

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New Challenges

• Multicore processor– A processing system composed of two or

more independent cores (or CPUs), which share resources, such as memory.

• Embedded systems, small devices– A computer system designed to perform one

or a few dedicated functions, often with real-time computing constraints.

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Multiprocessor Machines

• How to assign tasks to processors?– Load balance problem

• How to use processors to handle one task?– Parallelization, scaling problem

Processor 1

Processor 4

Processor 2

Processor 3

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Task 7

Task 8

Task 9

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OS for Small Devices

• Embedded systems, PDA, mp3 player, cell phone, GPS,…– Limited storage, limited power, – Usually has real time requirement

• Turkey system: store all programs and data in a persistent memory– No BIOS and program loader

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Handling Competition for Resources

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Competition for Resources

• What are resources?– CPU, memory, files, peripheral devices, …

• In a multitasking system, resources are shared by processes

• Some resources should not be employed by more than one process simultaneously

– E.g., printer

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Handling Competitions

• Define critical regions– Critical Region: A group of instructions that

should be executed by only one process at a time

– Mutual exclusion: Requirement for proper implementation of a critical region

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First Algorithm

• Use a flag (a global memory address)– flag=1: the critical region is occupied – flag=0: no process is in the critical region

• Problem:

– Both processes get into the critical region

if (flag == 0) { flag = 1; /*critical region*/}

Process Aif (flag == 0) { flag = 1; /*critical region*/ }

Process B Context switch to BContext switch to A

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Solutions

• Testing&setting the flag must be completed w/o interruption (atomic)

1. Use disable_Interrupt() to prevent context switch during the flag test and set process.

2. A machine instruction called “test-and-set” which cannot be interrupted

• Semaphore: a properly implemented flag

Diable_Interrupt();if (flag == 0) { flag = 1; Enable_Interrupt(); / *critical region*/ }Enable_Interrupt();

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Another Problem: Deadlock

• Example:– A is in critical region 1, and waits to enter

critical region 2– B is in critical region 2, and waits to enter

critical region 1

if (test_set(flag1)) { /*critical region 1*/ while(!test_set(flag2)); /*critical region 2*/ }

Process Aif (test_set(flag2)) { /*critical region 2*/ while (!test_set(flag1)); /*critical region 1*/ }

Process B Context switch to BContext switch to A

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Conditions for Deadlock

1. Competition for non-sharable resources

2. Resources requested on a partial basis

3. Allocated resources cannot be forcibly retrieved

4. Circular wait

Remove any one of the conditions can resolve the deadlock.

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Solutions

Which condition is removed?1. Kill one of the process2. Processes need to request all the

required resources at one time3. Spooling

• For example, stores the data to be printed and waits the printer available

4. Divide a file into pieces so that it can be altered by different processes

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Exercises

• There is a bridge that only allows one car to pass. When two cars meet in the middle, it causes “deadlock”. The following solutions remove which conditions– Do not let a car onto the bridge until the

bridge is empty.– If cars meet, make one of them back up.– Add a second lane to the bridge.

• What’s the drawback of solution 1?

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Security

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Security

• Attacks– Malware– Spyware and

phishing– Adware and spam– Abnormal behaviors

• Defenses– User management

• Privilege control

– Protections• Antivirus software• Auditing software• Firewall, spam filter

– Encryption

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Malware

• Infect programs/computers, erase data, slowdown performance…

• Types – Virus: attached to an existing program

– Worm: a stand alone program

– Trojan horse: disguised as valid files or programs

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Spyware and Phishing

• Spyware: collects information about users without their knowledge. – Keylogger: log the keys struck on a keyboard– Login sniffing: simulates the login process to

get valid user name and password.– Network sniffing: intercept network messages

• Phishing: acquires information by masquerading as a trustworthy entity in an electronic communication.

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Adware and Spam

• Adware: automatically plays, displays, or downloads advertisements to a computer after the software is installed on it or while the application is being used.

• Spam: sends unsolicited bulk messages indiscriminately.– Email spam

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Abnormal Behaviors

• Dictionary attack: trying passwords derived from a list of words in a dictionary.

• Denial of service attack: overloading a computer (server) with messages to make a computer resource unavailable to its intended users.

• Spoofing attack: masquerading as a party other than one’s self

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User Management

• To protect the computer’s resource from access by unauthorized personnel.

• User authentication process: – Username, password, fingerprint, …

• Super user / administrator / root– A kind of user having higher privilege to

control machines and operating system.

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Privilege Control

• To prevent malicious programs to execute dangerous instructions.

• Privilege levels: – Nonprivilege mode: only “safe” instructions

• For example, to access some part of memory.

– Privilege mode: all kinds of instructions• Those instructions that can be only executed in the

privilege mode are called privilege instructions.

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Protections

• Antivirus software: detecting and removing the presence of known viruses and other infections.

• Auditing software: detecting and preventing abnormal situations

• Firewall: filtering messages passing through computers.– Spam filter: firewall for email spam

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Related Courses

• Operation system – 作業系統，計算機系統管理，平行程式

• Security– 計算機系統管理，密碼與網路安全概論

• http://www.wikipedia.org/• Textbook chap3, sec 4.5 (security)

References