CS1356 資訊工程導論
Operating Systems
國立清華大學資訊工程學系
2016/3/22
How does a computer start
executing?
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)
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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?
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 blackboard
a speaker can write?
 memory management
Chairperson is OS
• Who can use 幻燈機 (projector)?
– Why not operated by chairperson?
– Device driver and management
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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
User
Software
System
Data
Procedure
Hardware
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Software Classification
(Fig. 3.3)
• Operating system is one kind of software
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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 Page 0
Page 1
address to access
Page 2
data and code
Page 3
– OS does the mapping Page 4
and paging
Page 5
Main memory
Page 6
Page 7
Disk
Virtual address
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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 handler
a. 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 realtime computing constraints.
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Multiprocessor Machines
Processor 1
Processor 3
Processor 2
Processor 4
Task 1
Task 4
Task 7
Task 2
Task 5
Task 8
Task 3
Task 6
Task 9
• How to assign tasks to processors?
– Load balance problem
• How to use processors to handle one task?
– Parallelization, scaling problem
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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:
Process A
if (flag == 0) {
flag = 1;
/*critical region*/
}
Context switch to A
Process B
Context switch to B
if (flag == 0) {
flag = 1;
/*critical region*/
}
– Both processes get into the critical region
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Solutions
• Testing&setting the flag must be
completed w/o interruption (atomic)
1. Use disable_Interrupt() to Diable_Interrupt();
if (flag == 0) {
flag = 1;
prevent context switch
Enable_Interrupt();
/ *critical region*/
during the flag test and
}
set process.
Enable_Interrupt();
2. A machine instruction called “test-and-set”
which cannot be interrupted
• Semaphore: a properly implemented flag
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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
Context switch to A
Process A
if (test_set(flag1)) {
/*critical region 1*/
while(!test_set(flag2));
/*critical region 2*/
}
Process B
Context switch to B
if (test_set(flag2)) {
/*critical region 2*/
while (!test_set(flag1));
/*critical region 1*/
}
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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 process
2. Processes need to request all the
required resources at one time
3. 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
References
• http://www.wikipedia.org/
• Textbook chap3, sec 4.5 (security)
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