Transcript What is an Operating System?

个人简介
 杨公平 工学博士、副教授、硕士生导师
中国计算机学会
多值逻辑和模糊逻辑
专委会委员
中国人工智能学会
机器学习专委会委员
 智能信息处理研究中心—机器学习与数据挖
掘实验室 http://mla.sdu.edu.cn/ygp.html
 Email: [email protected]
 Phone:15865251995
 Office:学科楼304
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关于课程
 接触OS的四类人员：
OS设计者，OS管理员，开发人员，一般用户
 操作系统漫谈
 关于实验、作业 、考勤
 关于考核：考试75%+实验15%+(作业+考勤)10%
 参考书：

西电出版社 汤子灜 计算机操作系统

该书中文版 郑扣根译
 上课的重要性！
 访问教务处课程中心！！！！！！
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教学内容
 Part one：Overview
 Part two：Process Management
 Part three：Memory Management
 Part four: Storage Management
 Part five: Protection and Security
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Part one：Overview
 An operating system acts as an intermediary between the
user of a computer and the computer hardware.
 The purpose of an operating system is to provide an
environment in which a user can execute programs in a
convenient and efficient manner.
 An operating system is software that manages the
computer hardware. The hardware must provide
appropriate mechanisms to ensure the correct operation
of the computer system and to prevent user programs
from interfering with the proper operation of the system.
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Chapter 1: Introduction
Chapter 1: Introduction
 What Operating Systems Do
 Computer-System Organization
 Operating-System Structure
 Operating-System Operations
 Process Management
 Memory Management
 Storage Management
 Protection and Security
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Objectives
 To provide a grand tour of the major operating
systems components
 To provide coverage of basic computer system
organization
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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 purpose of an operating system is to
provide an environment in which a user can
execute programs in a convenient and efficient
manner.
操作系统的总体目标：
使计算机系统方便使用 convenient
使硬件能高效利用 efficient
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不同类型的操作系统设计目标不同
 An amazing aspect of operating system systems is how
varied they are in accomplishing these tasks
 Mainframe operating systems are designed primarily to
optimize utilization of hardware
 Personal Computer operating systems support complex
games, business applications, and everything in between.
 Operating systems for handheld computers are designed to
provide an environment in which a user can easily interface
with the computer to execute programs
 Thus, some operating systems are designed to be
convenient, others to be efficient, and others some
combination of the two.
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Four Components of a Computer System
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Computer System Structure
 Computer system can be divided into four
components

Hardware – provides basic computing
resources

Operating system

Application programs – define the ways in
which the system resources are used to solve
the computing problems of the users

Users
People,
machines, other computers
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OS in computer system
 Controls and coordinates use of hardware among
various applications and users
 We also view a computer system as consisting of
hardware , software, and data. The operating
system provides the means for proper use of
these resources in the operation of the computer
system
 An operating system is similar to a government. It
performs no useful function by itself. It simply
provides an environment within which other
programs can do useful work.
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研究操作系统的两种观点
 User View

the user’s view of the computer varies
according to the interface being used.
 System View

From the computer’s point of view, the
operating system is the program mostly
intimately involved with hardware
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User View
 PC: the operating system is designed mostly for ease of
use, with some attention paid to performance and none paid
to resource utilization
 Mainframe or minicomputer: The operating system in
such cases is designed to maximize resource utilization to
assure that all available CPU time, memory, and I/O are
used efficiently and that no individual user takes more than
her fair share.
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User View
 Workstation: their operating system is designed to
compromise between individual usability and resource
utilization.
 Handheld computers: Their operating systems are
designed mostly for individual usability, but performance per
amount of battery life is important as well.
 Embedded computers: have little or no user view, are
designed primarily to run without user intervention
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System View
 OS is a resource allocator

Manages all resources

Decides between conflicting requests for efficient and
fair resource use

the operating system must decide how to allocate
them to specific programs and users so that it can
operate the computer system efficiently and fairly.
 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
 No universally accepted definition
 Operating systems exist because they offer a reasonable
way to solve the problem of creating a usable computing
system.
 The fundamental goal of computer systems is to execute
user programs and to make solving user problems easier.
 Toward this goal, computer hardware is constructed. Since
bare hardware alone is not particularly easy to use,
application programs are developed.
 These programs require certain common operations, such
as those controlling the I/O devices. The common functions
of controlling and allocating resources are then brought
together into one piece of software: the operating system.
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Operating System Definition (Cont.)
 “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-A more common definition
 The matter of what constitutes an operating system has
become increasingly important. In 1998, the United States
Department of Justice filed suit against Microsoft, in essence
claiming that Microsoft included too much functionality in its
operating systems and thus prevented application vendors
from competing. For example: IE web browser.
 操作系统是一组控制和管理计算机硬件和软件资源、合理地对
各类作业进行调度以及方便用户的程序的集合。
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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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Computer Startup
 bootstrap program is loaded at power-up or
reboot

Typically stored in ROM or EPROM, generally
known as firmware

Initializates all aspects of system

Loads operating system kernel and starts
execution
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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:

polling

vectored interrupt system
 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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Storage Structure
 Computer programs must be in main memory (also called
random-access memory or RAM) to be executed. Main
memory is the only large storage area (millions to billions of
bytes) that the processor can access directly.
 A typical instruction-execution cycle, as executed on a
system with a von Neumann architecture, first fetches an
instruction from memory and stores that instruction in the
instruction register. The instruction is then decoded and
may cause operands to be fetched from memory and stored
in some internal register.
 most computer systems provide secondary storage as an
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extension of main memory. The main requirement for
secondary storage is that it be able to hold large quantities
of data permanently.
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Storage-Device Hierarchy
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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.
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I/O Structure
 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
Asynchronous
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Device-Status Table
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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 one interrupt is generated per block, rather
than the one interrupt per byte.
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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
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Memory Layout for Multiprogrammed System
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Operating System Structure
 Multiprogrammed systems provide an environment in
which the various system resources (for example, CPU,
memory, and peripheral devices) are utilized effectively,
but they do not provide for user interaction with the
computer system.
 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
 As the system switches rapidly from one user to the next,
each user is given the impression that the entire
computer system is dedicated to his use, even though it
is being shared among many users.
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带来的问题
 Job scheduling
 CPU scheduling
 Memory management
 File system/Disk management
 provide a mechanism for protecting resources
from inappropriate use
 provide mechanisms for job synchronization
and communication
 ensure that jobs do not get stuck in a
deadlock, forever waiting for one another
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Operating-System Operations
 Modern operating systems are interrupt driven.
If there are no processes to execute, no I/O
devices to service, and no users to whom to
respond, an operating system will sit quietly,
waiting for something to happen.
 Events are almost always signaled by the
occurrence of an interrupt or a trap.
 A trap (or an exception) is a software-generated
interrupt caused either by an error (for example,
division by zero or invalid memory access) or by a
specific request from a user program that an
operating-system service be performed.
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Operating-System Operations
 Since the operating system and the users share the
hardware and software resources of the computer
system, we need to make sure that an error in a user
program could cause problems only for the one
program that was running.
 With sharing, many processes could be adversely
affected by a bug in one program. For example, if a
process gets stuck in an infinite loop, this loop could
prevent the correct operation of many other
processes.
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Operating-System Operations
 More subtle errors can occur in a multiprogramming
system, where one erroneous program might modify
another program, the data of another program, or
even the operating system itself.
 Without protection against these sorts of errors, either
the computer must execute only one process at a time
or all output must be suspect. A properly designed
operating system must ensure that an incorrect (or
malicious) program cannot cause other programs to
execute incorrectly.
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Dual-mode operation
 In order to ensure the proper execution of the
operating system, we must be able to distinguish
between the execution of operating-system code
and user defined.
 Dual-mode operation allows OS to protect itself
and other system components

kernel mode(0) and User mode(1)

Mode bit provided by hardware

Provides ability to distinguish when system is
running user code or kernel code
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Transition from User to Kernel Mode
 When the computer system is executing on behalf
of a user application, the system is in user mode.
However, when a user application requests a
service from the operating system (via a system
call), it must transition from user to kernel mode to
fulfill the request.
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Transition from User to Kernel Mode
 At system boot time, the hardware starts in kernel
mode. The operating system is then loaded and
starts user applications in user mode. Whenever a
trap or interrupt occurs, the hardware switches
from user mode to kernel mode (that is, changes
the state of the mode bit to 0). Thus, whenever the
operating system gains control of the computer, it
is in kernel mode. The system always switches to
user mode (by setting the mode bit to 1) before
passing control to a user program.
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Transition from User to Kernel Mode
 The dual mode of operation provides us with the
means for protecting the operating system from
errant users-and errant users from one another.
We accomplish this protection by designating
some of the machine instructions that may cause
harm as privileged instructions.
 The hardware allows privileged instructions to be
executed only in kernel mode.
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Transition from User to Kernel Mode
 The instruction to switch to user mode is an
example of a privileged instruction. Some other
examples include I/O control, timer management,
and interrupt management.
 System calls provide the means for a user
program to ask the operating system to perform
tasks reserved for the operating system on the
user program's behalf.
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Timer
 We must ensure that the operating system maintains
control over the CPU. We must prevent a user program
from getting stuck in an infinite loop or not calling system
services and never returning control to the operating
system.
 To accomplish this goal, we can use a timer.
 A timer can be set to interrupt the computer after a
specified period.

The operating system sets the counter.

Every time the clock ticks, the counter is decremented.

When the counter reaches 0, an interrupt occurs.
 we can use the timer to prevent a user program from
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running too long.
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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
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Process Management
 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)
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Storage Management
 File-System management



Files usually organized into directories
Access control on most systems to determine
who can access what
OS activities include
Creating and deleting files and directories
Primitives to manipulate files and 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
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Caching
 Caching is an important principle of computer
systems. Information is normally kept in some
storage system (such as main memory). As it is
used, it is copied into a faster storage system---the cache---on a temporary basis.
 When we need a particular piece of information,
we first check whether it is in the cache. If it is, we
use the information directly from the cache; if it is
not, we use the information from the source,
putting a copy in the cache under the assumption
that we will need it again soon.
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Caching
 in a multitasking environment, where the CPU is
switched back and forth among various
processes, extreme care must be taken to ensure
that, if several processes wish to access A, then
each of these processes will obtain the most
recently updated value of A.
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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
 Only the device driver knows the peculiarities of the
specific device to which it is assigned.
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Protection and Security
 Mechanisms ensure that files, memory segments, CPU,
and other resources can be operated on by only those
processes that have gained proper authorization from
the operating system.
 Protection – any mechanism for controlling access of
processes or users to resources defined by the OS
 A system can have adequate protection but still be prone
to failure and allow inappropriate access. Consider a
user whose authentication information (her means of
identifying herself to the system) is stolen. Her data
could be copied or deleted, even though file and memory
protection are working.
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Protection and Security
 Security – defense of the system against internal and
external attacks
 Huge range, including denial-of-service, worms,
viruses, identity theft, theft of service
 Protection and security require the system to be able to
distinguish among all its users.
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Homework:1,2,3,4,10,17
End of Chapter 1