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Chapter 1: Introduction
Chapter 1: Introduction
 What Operating Systems Do
 Computer-System Organization
 Computer-System Architecture
 Operating-System Structure
 Operating-System Operations
 Process Management
 Memory Management
 Storage Management
 Protection and Security
 Distributed Systems
 Special-Purpose Systems
 Computing Environments
Operating System Concepts
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Four Components of a Computer System
Operating System Concepts
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What is an Operating System?
 A program that acts as an intermediary between a user of a
computer and the computer hardware.

The job of the OS is to adapt to hardware. Examples:
MS-DOS/Windows, MacOS, Unix, and many more
 Operating system goals:

Execute user programs and make solving user
problems easier.

Make the computer system convenient to use.
 Use the computer hardware in an efficient manner.
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Computer System Structure
 Computer system can be divided into four components

Hardware – provides basic computing resources


Operating system


Controls and coordinates use of hardware among
various applications and users
Application programs – define the ways in which the
system resources are used to solve the computing
problems of the users


CPU, memory, I/O devices
Word processors, compilers, web browsers, database
systems, video games
Users

Operating System Concepts
People, machines, other computers
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Operating System Definition
 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
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Operating System Definition (Cont.)

No universally accepted definition

“Everything a vendor ships when you order an operating system” is
good approximation (but varies wildly)


A Provider of Abstractions

A Resource Coordinator

A Magician: makes your system appear to be more than it is
(more than one processor, more memory)
“The one program running at all times on the computer” is the
kernel. Everything else is either a system program (ships with the
operating system) or an application program
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What is in the OS?
 What is included in the OS?
 Given that very few people write operating systems, why study
them?

OS concepts are relevant throughout computer science.
An understanding of OS concepts gives you a firm
foundation for work building large software systems.

Understanding the OS is key in achieving a deep
understanding of the operation of a computer system. This
is useful even if you don’t want to write your own OS.
Operating System Concepts
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운영체제의 기능
 자원관리자

처리기 – 병행성 (프로세스)

저장장치 – 메모리

입출력장치 – 인터럽트

통신장치 – 통신 프로토콜, 분산 시스템

데이터 – 파일시스템
 기타기능

사용자 인터페이스

하드웨어의 공유

데이터 공유

에러 회복

계정관리
Operating System Concepts
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Computer System Organization
 Computer-system operation

One or more CPUs, device controllers connect through
common bus providing access to shared memory

Concurrent execution of CPUs and devices competing for
memory cycles
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OS and Architectures
 What an OS can do is dictated, at least in part, by the
architecture
 Architecture support can greatly simplify (or complicate) OS
tasks
 Example: PC operating systems have been primitive, in part
because PCs lacked hardware support (e.g., for VM)
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Architectural Features for OS
 Features that directly support OS needs include:

1. Timer (clock) operation

2. Synchronization (atomic instructions)

3. Memory protection

4. I/O control and operation

5. Interrupts and exceptions

6. OS protection (kernel/user mode)

7. Protected instructions

8. System calls
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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.
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Common Functions of Interrupts
 Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the
addresses of all the service routines.
 Interrupt architecture must save the address of the interrupted
instruction.
 Incoming interrupts are disabled while another interrupt is
being processed to prevent a lost interrupt.
 A trap is a software-generated interrupt caused either by an
error or a user request.
 An operating system is interrupt driven.
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Interrupt Handling
 The operating system preserves the state of the CPU by
storing registers and the program counter.
 Determines which type of interrupt has occurred:
 Separate segments of code determine what action should be
taken for each type of interrupt
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Interrupt Timeline
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Exceptions
 Hardware must detect special conditions: page fault, write to a
read-only page, overflow, trace trap, odd address trap,
privileged instruction trap, syscall …
 Must transfer control to handler within the OS
 Hardware must save state on fault (PC, etc) so that the faulting
process can be restarted afterwards
 Modern operating systems use VM traps for many functions:
debugging, distributed VM, garbage collection, copy-on-write
…
 Exceptions are a performance optimization,
i.e., conditions could be detected by inserting extra
instructions in the code (at high cost)
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I/O Control
 I/O issues:

How to start an I/O (special instructions or memorymapped I/O

I/O completion (interrupts)
 Interrupts are the basis for asynchronous I/O

Device controller performs an operation asynch to CPU

Device sends an interrupt signal on bus when done

In memory is a vector table containing a list of addresses
of kernel routines to handle various events

CPU switches to address indicated by vector specified by
the interrupt signal
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I/O Control
Device interrupts
CPU stops current operation, switches to kernel mode,
and saves current PC and other state on kernel stack
CPU fetches proper vector from vector table and branches
to that address (to routine to handle interrupt)
Interrupt routine examines device database and performs
action required by interrupt
Handler completes operation, restores saved (interrupt
state) and returns to user mode (or calls scheduler to
switch to another program)
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I/O Structure


After I/O starts, control returns to user program only upon I/O
completion.

Wait instruction idles the CPU until the next interrupt

Wait loop (contention for memory access).

At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
After I/O starts, control returns to user program without waiting for
I/O completion.

System call – request to the operating system to allow user to
wait for I/O completion.

Device-status table contains entry for each I/O device
indicating its type, address, and state.

Operating system indexes into I/O device table to determine
device status and to modify table entry to include interrupt.
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Two I/O Methods
Synchronous
Operating System Concepts
Asynchronous
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Device-Status Table
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Timer Operation
 How does the OS prevent against runaway user programs
(infinite loops) ?
 A timer can be set to generate an interrupt in a given time
 Before it transfers to a user program, the OS loads the timer
with a time to interrupt
 When the time arrives, the executing program is interrupted
and the OS regains control
 This ensures that the OS can get the CPU back even if a user
program erroneously or purposely continues to execute past
some allotted time
 The timer is privileged: only the OS can load it
Operating System Concepts
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Synchronization
 Interrupts cause potential problems because an interrupt can
occur at any time – causing code to execute that interferes
with code that was interrupted.
 OS must be able to synchronize concurrent processes.
 This involves guaranteeing that short instruction sequences
(read-modify-write) execute atomically.
 One way to guarantee this is to turn off interrupts before the
sequence, execute it, and re-enable interrupts; CPU must have
a way to disable interrupts.
 Another is to have special instructions that can perform a
read/modify/write in a single cycle, or can atomically test and
conditionally set a bit, based on its previous value.
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Direct Memory Access Structure
 Used for high-speed I/O devices able to transmit information
at close to memory speeds.
 Device controller transfers blocks of data from buffer storage
directly to main memory without CPU intervention.
 Only on interrupt is generated per block, rather than the one
interrupt per byte.
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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

Disk surface is logically divided into tracks, which are
subdivided into sectors.

The disk controller determines the logical interaction
between the device and the computer.
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Storage Hierarchy
 Storage systems organized in hierarchy.

Speed

Cost

Volatility
 Caching – copying information into faster storage system;
main memory can be viewed as a last cache for secondary
storage.
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Storage-Device Hierarchy
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Caching
 Use of high-speed memory to hold recently-accessed data.
 Requires a cache management policy.
 Caching introduces another level in storage hierarchy. This
requires data that is simultaneously stored in more than one
level to be consistent.
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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

If it is, information used directly from the cache (fast)

If not, data copied to cache and used there
 Cache smaller than storage being cached

Cache management important design problem

Cache size and replacement policy
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Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit
or implicit
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Migration of Integer A from Disk to Register
 Multitasking environments must be careful to use most recent
value, not 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

Several copies of a datum can exist

Various solutions covered in Chapter 17
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Computer Startup
 bootstrap program is loaded at power-up or reboot

Typically stored in ROM or EEPROM, generally known as
firmware

Initializates all aspects of system

Loads operating system kernel and starts execution
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Operating System Structure


Multiprogramming needed for efficiency

Single user cannot keep CPU and I/O devices busy at all times

Multiprogramming organizes jobs (code and data) so CPU always
has one to execute

A subset of total jobs in system is kept in memory

One job selected and run via job scheduling

When it has to wait (for I/O for example), OS switches to another
job
Timesharing (multitasking) is logical extension in which CPU switches
jobs so frequently that users can interact with each job while it is
running, creating interactive computing

Response time should be < 1 second

Each user has at least one program executing in memory
process

If several jobs ready to run at the same time  CPU scheduling

If processes don’t fit in memory, swapping moves them in and out
to run

Virtual memory allows execution of processes not completely in
memory
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Memory Layout for Multiprogrammed System
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OS Features Needed for Multiprogramming
 I/O routine supplied by the system.
 Memory management – the system must allocate the
memory to several jobs.
 CPU scheduling – the system must choose among several
jobs ready to run.
 Allocation of devices.
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Operating-System Operations

Interrupt driven by hardware

Software error or request creates exception or trap

Division by zero, request for operating system service

Other process problems include infinite loop, processes modifying
each other or the operating system

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
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Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources

Set interrupt after specific period

Operating system decrements counter

When counter zero generate an interrupt

Set up before scheduling process to regain control or
terminate program that exceeds allotted time
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Process Management

A process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.

Process needs resources to accomplish its task

CPU, memory, I/O, files

Initialization data

Process termination requires reclaim of any reusable resources

Single-threaded process has one program counter specifying location
of 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

Concurrency by multiplexing the CPUs among the processes /
threads
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Process 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
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Memory Management
 All data in memory before and after processing
 All instructions in memory in order to execute
 Memory management determines what is in memory when

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 deallocating memory space as needed
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Storage 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
Operating System Concepts

Creating and deleting files and directories

Primitives to manipulate files and dirs

Mapping files onto secondary storage

Backup files onto stable (non-volatile) storage media
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Mass-Storage Management

Usually disks used to store data that does not fit in main memory or
data that must be kept for a “long” period of time.

Proper management is of central importance

Entire speed of computer operation hinges on 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)
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I/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
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Hardware Protection
 Dual-Mode Operation
 I/O Protection
 Memory Protection
 CPU Protection
Operating System Concepts
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Use of A System Call to Perform I/O
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Memory Protection
 Must provide memory protection at least for the interrupt
vector and the interrupt service routines.
 In order to have memory protection, add two registers that
determine the range of legal addresses a program may
access:

Base register – holds the smallest legal physical memory
address.

Limit register – contains the size of the range
 Memory outside the defined range is protected.
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Use of A Base and Limit Register
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Hardware Address Protection
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Hardware Protection
 When executing in monitor mode, the operating system has
unrestricted access to both monitor and user’s memory.
 The load instructions for the base and limit registers are
privileged instructions.
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CPU Protection
 Timer – interrupts computer after specified period to ensure
operating system maintains control.

Timer is decremented every clock tick.

When timer reaches the value 0, an interrupt occurs.
 Timer commonly used to implement time sharing.
 Time also used to compute the current time.
 Load-timer is a privileged instruction.
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Protection and Security

Protection – any mechanism for controlling access of processes or
users to resources defined by the OS

Security – defense of the system against internal and external attacks


Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
Systems generally first distinguish among users, to determine who
can do what

User identities (user IDs, security IDs) include name and
associated number, one per user

User ID then associated with all files, processes of that user to
determine access control

Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file

Privilege escalation allows user to change to effective ID with
more rights
Operating System Concepts
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운영체제의 발전
1940 - 1950
Batch processing
1960
Multi programming (System 360)
Software 공학의 출현
1970
Multimode timesharing system
1980
Personal computer, Workstation
Computer 통신망
1990 년대와 이후
Operating System Concepts
Distributed Computing
Open Systems
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A Social History of Operating Systems
 Phase 0: Computers are exotic experimental science: No
operating system.


Use plug-boards to direct computer.

User sits at console. All activity is sequential: no
overlap between computation, I/O, and user think or
response time.

People manually load card decks to run programs.

Eventually, people developed libraries, used by all
users. This was the precursor to the operating system.
Problem: Too much waiting.
Operating System Concepts

The user waits for the machine.

The machine waits for user.

Everybody waits for card reader.
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A Social History of Operating Systems
 Phase 1: Computers are expensive: People are cheap.

Make more efficient use of the computer by decoupling
the people’s activity from the computer’s activity.

The OS functions as a batch monitor, continually
loading a job, running it, and moving on to the next job.
If program failed, the OS saves a copy of the memory
contents for debugging (circa 1955 – 1965)

Better use of hardware, but harder to debug!

Data channels and interrupts => overlap I/O and
computation.
Operating System Concepts
–
Buffering and interrupts handling is done by OS.
–
Spool jobs onto drum.
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A Social History of Operating Systems

Phase 1 (Cont’d)

Problems:

Hardware to the rescue (again): Add memory protection and
relocation



Multiprogramming: many users can share the system.

Small jobs can complete quickly.

OS must manage the interaction between concurrent jobs.
IBM OS/360: The first OS designed for a family of computers.

Use the same operating system on all machines, smallest to
largest. OS becomes a field of study in computer science.

People began to study operating systems because they didn’t
work. OS/360 is an example. It was announced in 1963 but
didn’t really work until 1968.
New Problems:
Operating System Concepts

The OS was enormously complicated.

It was all in assembly code.

People are still waiting for computers. This motivates Phase
2.
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A Social History of Operating Systems
 Phase 2: Computers are fast; People are slow; both are
expensive. Need to help people be more productive.

Operating System Concepts
Interactive timesharing: let many users use the same
machine at once.

Give everybody a terminal.

Keep data on line: use a structured file systems.

Provide reasonable response time (or not:
thrashing).
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A Social History of Operating Systems
 Phase 3: Computers are cheap; People are expensive. Give
everybody a computer.

The personal workstation
–
The PERQ.
–
The Sun workstation (Stanford University Network)
–
The Xerox Alto

The Apple II

The IBM PC

The Macintosh
Operating System Concepts
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A Social History of Operating Systems
 Phase 4: Personal Computers take over the planet.
Software vendors produce high quality personal
software with limited OS support.
 Protection and multiprogramming are of secondary
importance.
 Networks (an offshoot of OS research) make electronic
mail, bboard, WWW important applications.
Client/server architecture.
 Server machines motivate more advanced OS
technology.
 Problems
 People are still waiting for computers (see Phase 1).

Viruses.
 Hackers.

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A Technical Perspective
 Timesharing Systems

CTS (1962)
 MIT
 One of the first timesharing systems
 Very successful: Motivated MULTICS
 MULTICS (1965)
Joint project between MIT, Bell Labs, and General
Electric.
 Envisioned a computing “utility”. People would buy
computing services like electricity.
 Many fundamental ideas: (protection rings, treestructured file system)
 Building it was more difficult than expected.

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A Technical Perspective

Timesharing Systems (Cont’d)

UNIX (1970)
Operating System Concepts

Ken Thompson (from Multics project) found on old PDP-7
lying around Bell Labs.

He and Dennis Ritchie built a system designed by
programmers for programmers.

First in assembly language, then in C

Source code made available to universities.

Berkeley added virtual memory support.

UNIX becomes a commercial operating system.

Important ideas popularized by UNIX:
–
High level language
–
Portable operating system
–
Tree structured file system
–
Many others
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Personal Computers and Workstations
 Computers are cheap, so don’t bother with terminal.

CP/M first PC operating system.
 IBM needed software for their new PC. CP/M was behind
schedule.
 Asked Microsoft (Bill Gates, BASIC, Unix) to build one.
 Gates bought 86-DOS from a memory board manufacturer.

Primary goal: finish quickly and be compatible with CP/M.

OS becomes a subroutine library and command executive.
 Today: Networks
 Computing system are made up of personal as well as
shared resources.

People don’t want to share their processor or memory, but
they do want to share electronic mail, news, data bases,
programs.
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Operating Systems Today

Today’s operating systems are:



Enormous: if your operating system kernel is under 1 megabyte,
it’s considered small.

Typically hundreds of thousands of lines of code

100-1000 man years of development effort
Complex:

Asynchronous.

Hardware idiosyncracies (must run on every platform).

Different classes of users have different needs.

Performance is crucial!
Poorly understood:
Operating System Concepts

The system outlives its creators.

Too large for a single person to comprehend.

Never fully debugged (OS/360 releases with 1000 bugs).

Behavior is hard to predict. Performance tuning often based
on hunch. Intuition is often wrong.
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UNIX 의 발전
 1965 년 : Bell Lab + GE + MIT (project MAC)





MULTICS 개발 착수.
1969 년 : Bell Lab team quit.
Thompson 과 Ritchie 가 PDP-7 에
초기 시스템 구현. (어셈블리어)
1971 년 : PDP-11으로 porting.
1973 년 : C 로 다시 작성. (크기가 1/3 증가)
대학에 보급 시작
1974 년 : CACM 에 발표
1977 년 : Interdata 8/32 에 이식
1 BSD
 1980 년 : XENIX (Microsoft)
 1983 년 : UNIX 시스템 V 지원 개시
4.2 BSD, 4.2 on SUN
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UNIX 의 발전
 1984 년 : System V Release 2
 1989 년 : SVR 4.0
 1991 년 : SVR 4.0 MP, LINUX v0.01
 1992 년 : SVR 4.0 ES/MP
 1993 년 : Novell UNIXWARE
 1994 년 : LINUX v1.0
 1995 년 : SCO UNIX
 1996 년 : LINUX v2.0
 1999 년 : LINUX v2.2
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DOS 의 발전
 1981 년 : DOS 1.0

CP/M, MP/M

$ 40

96 % 차지

1982 (DOS 2.0), 1984 (DOS 3.0), 1986 (DOS 3.2),
1988 (DOS 4.0)
 1987 년 : OS/2

80286 용 다중 태스킹 OS

DOS 또는 OS/2 dual mode 로 동작
 1989 년 : OS/386 (= OS/3)

Concurrent DOS session 지원
 DOS 는 다양한 S/W 가 존재, OS/2 는 S/W 부족
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DOS 의 발전
 1990 년 : Windows 3.0
 1991 년 : DOS 5.0
Windows 3.1
 1993 년 : DOS 6.0
Windows NT 3.1
 1994 년 : Windows NT 3.5 (Daytona)
 1995 년 : DOS 7.0
Windows 95 (Chicago)
Windows NT (Cairo)
 1998 년 : Windows 98
 2000 년 : Windows 2000
 2001 년 : Windows XP
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기종별 운영체제
 IBM-PC
Windows 98/2000
 매킨토시
맥 OS
 SUN
Solaris
 HP
HP-UX
 IBM
MVS, VM, AIX
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Computing Environments
 Traditional computer

Blurring over time

Office environment
 PCs
connected to a network, terminals attached to
mainframe or minicomputers providing batch and
timesharing
 Now
portals allowing networked and remote
systems access to same resources

Home networks
 Used
 Now
Operating System Concepts
to be single system, then modems
firewalled, networked
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Computing Environments (Cont.)

Client-Server Computing
 Dumb terminals supplanted by smart PCs
 Many systems now servers, responding to requests generated by
clients
 Compute-server provides an interface to client to request
services (i.e. database)
 File-server provides interface for clients to store and retrieve
files
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Peer-to-Peer Computing
 Another model of distributed system
 P2P does not distinguish clients and servers

Instead all nodes are considered peers

May each act as client, server or both

Node must join P2P network


Registers its service with central lookup service on
network, or

Broadcast request for service and respond to requests
for service via discovery protocol
Examples include Napster and Gnutella
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Web-Based Computing
 Web has become ubiquitous
 PCs most prevalent devices
 More devices becoming networked to allow web access
 New category of devices to manage web traffic among similar
servers: load balancers
 Use of operating systems like Windows 95, client-side, have
evolved into Linux and Windows XP, which can be clients and
servers
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Network Structure
 Local Area Networks (LAN)
 Wide Area Networks (WAN)
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Local Area Network Structure
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Wide Area Network Structure
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Real-Time Systems
 Often used as a control device in a dedicated application
such as controlling scientific experiments, medical imaging
systems, industrial control systems, and some display
systems.
 Well-defined fixed-time constraints.
 Real-Time systems may be either hard or soft real-time.
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Real-Time Systems (Cont.)
 Hard real-time:

Secondary storage limited or absent, data stored in
short term memory, or read-only memory (ROM)

Conflicts with time-sharing systems, not supported by
general-purpose operating systems.
 Soft real-time

Limited utility in industrial control of robotics

Useful in applications (multimedia, virtual reality)
requiring advanced operating-system features.
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Handheld Systems
 Personal Digital Assistants (PDAs)
 Cellular telephones
 Issues:

Limited memory

Slow processors

Small display screens.
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End of Chapter 1