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Operating System Overview

Chapter 2

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Operating System

A program that controls the execution of application programs

An interface between applications and hardware

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Operating System Objectives

Convenience( 方便 ) Makes the computer more convenient to use

Efficiency( 有效 ) Allows computer system resources to be used

in an efficient mannerAbility to evolve( 扩展的能力 )

It should have the capability to permit the effective development, testing & introduction of new system functions.

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Layers of Computer System

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Services Provided by the Operating System

Program development （程序开发） Editors （编辑器） and debuggers

（调试器） Program execution （程序运行） Access to I/O devices （访问 I/O 设备） Controlled access to files （控制访问文

件） System access （系统访问）

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Services Provided by the Operating System

Error detection and response （错误检测和响应） Internal and external hardware errors

Memory errorDevice failure

Software errorsArithmetic overflowAccess forbidden memory locationsOperating system cannot grant request of

application

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Operating System as Resource Manager

Responsible for managing resources Functions same way as ordinary computer

software

It is a program executed by the processor Operating system relinquishes control of

the processor

In fact, the OS is a computer program.

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The OS directs the processor in the use of the other system resources and in the timing of its execution of other programs.

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Kernel （核心程序）

Portion of operating system that is in main memory

Contains most frequently used functions

Also called the nucleus

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Evolution of an Operating System

Hardware upgrades plus new types of hardware

New servicesFixes （修正）The system should be modular in

construction, with clearly defined interfaces between the modules.

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Evolution of Operating Systems

Serial Processing （串行处理） No operating system Machines run from a console with display lights

( 显示灯 ), toggle switches （触发器） , input device, and printer

Computer systems were used in a serial fashion, meaning access to the computer was available in series.

Programmers were directly interacting with the computer system.

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Evolution of Operating Systems

Schedule time( 调度 )imagine if some body has schedual to use it

for 3 hours and some how finished in 30 minute. Therefore, the remaining 2 hours 30 minute are wasted as nothing is schedual for that period.

Setup included loading the compiler, source program, saving compiled program （目标程序） , and loading and linking

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Evolution of Operating Systems

Simple Batch Systems （简单批处理）（ IBSYS ）

Monitors（监控程序）Software that controls the sequence of eventsUser no longer has direct access to the machineBatch jobs together sequentially, and used by

the monitorProgram branches back to monitor when

finished

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Two points to Simple Batch Systems

Monitor point The monitor is the one controlling every

thing. Much of the monitor must always be in

main memory and available for execution, it is called resident monitor.

When read in job, the control is passed to this job; when it is completed, it returns control to the monitor.

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Two points to Simple Batch Systems

Processor point The processor is executing instructions from the

portion of main memory containing the monitor. When a job has been read in, the processor will

encounter a branch instruction in the monitor that instructs the processor to continue execution at the start of the user program.

When complete the job, the processor fetch its next instruction from the monitor program.

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Hardware Features( 硬件功能 )

The batch OS relies on the ability of processor to fetch instructions from various portions of main memory.

Again, there are other hardware features: Memory protection （存储器保护）

Do not allow the memory area containing the monitor to be altered

Timer （计时器） Prevents a job from monopolizing （独

占） the system

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Hardware Features

Privileged instructions （特权指令） Certain machine level instructions can

only be executed by the monitor. For example, I/O instructions

Interrupts Early computer models did not have

this capability

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Memory Protection

User program executes in user mode Certain instructions may not be

executed Monitor executes in system mode

Kernel mode Privileged instructions are executed Protected areas of memory may be

accessed

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Batch operating system

Machine time alternates between execution of user programs and execution of the monitor

Two sacrifices: Some main memory is now given over to

the monitor Some machine time is consumed by the

monitor

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I/O Devices Slow

The computer spends over 96% of its time waiting for I/O devices to finish transferring

data

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Uniprogramming( 单道程序 )

Processor must wait for I/O instruction to complete before preceding

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Multiprogramming( 多道程序 )

When one job needs to wait for I/O, the processor can switch to the other job

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Multiprogramming

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Example

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Utilization Histograms(利用率 )

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Multiprogramming

The most notable additional feature that is useful for Multiprogramming is the hardware that supports I/O interrupts and DMA.

To have several jobs ready to run, they must be kept in main memory, requiring some form of memory management.

It also requires some algorithm for scheduling.

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Time-Sharing Systems( 分时系统 )

Using multiprogramming to handle multiple interactive( 交互式 ) jobs

Processor’s time is shared among multiple users

Multiple users simultaneously access the system through terminals

The OS interleave the execution of each user program in a short time

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Compatible Time-Sharing System (CTSS)

First time-sharing system developed at MIT

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Some new problems

If multiple jobs are in memory, then they must by protected from interfering with each other

With multiple interactive users, the file system must be protected so that only authorized users have access to a particular file

Handle the contention for resources

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Major Achievements

Processes （进程） Memory Management （存储器管理） Information protection and security

（信息保护和安全） Scheduling and resource

management （调度和资源管理） System structure （系统结构）

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Difficulties with Designing System Software

Improper synchronization （不正确的同步） Ensure a process waiting for an I/O device

receives the signal Failed mutual exclusion （失败的互斥） Nondeterminate program operation （不确定的

程序操作） Program should only depend on input to it,

not on the activities of other programs Deadlocks （死锁）

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Processes

A program in execution An instance of a program running on a

computer The entity that can be assigned to and

executed on a processor A unit of activity characterized by a single

sequential thread of execution （顺序的执行线程） , a current state （当前状态） , and an associated set of system resources （相关的系统资源）

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Process

Consists of three components ： An executable program Associated data needed by the

program Execution context of the program

All information the operating system needs to manage the process

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Process

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Memory Management

Process isolation （进程隔离） Automatic allocation and management

（自动分配和管理） Support of modular programming （支持

模块化程序设计） Protection and access control （保护和访

问控制） Long-term storage （长期存储）

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Virtual Memory

Allows programmers to address memory from a logical point of view

Meet the requirement of having multiple user jobs reside in main memory concurrently

No hiatus （缝隙） between the execution of successive processes while one process was written out to secondary store and the successor （后继） process was read in

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Virtual Memory

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Virtual Memory Addressing

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File System

Implements long-term storeInformation stored in named

objects called files

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Information Protection and Security

Availability( 可用性 ) Concerned with protecting the system

against interruption

Confidentiality( 机密性 ) Assuring that users cannot read data

for which access is unauthorized

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Information Protection and Security

Data integrity( 数据完整性 ) Protection of data from

unauthorized modificationAuthenticity( 认证 )

Concerned with the proper verification of the identity of users and the validity of messages or data

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Scheduling and Resource Management

Fairness （公平性） Give equal and fair access to resources

Differential responsiveness （有差别的响应性） Discriminate among different classes of jobs

Efficiency （有效性） Maximize throughput, minimize response

time, and accommodate as many users as possible

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Key Elements ofOperating System

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System Structure

View the system as a series of levels Each level performs a related subset of

functions Each level relies on the next lower level to

perform more primitive functions This decomposes a problem into a number

of more manageable subproblems

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Modern Operating Systems

Microkernel architecture （微内核结构） Assigns only a few essential functions to the

kernel Multithreading （多线程）

Process is divided into threads that can run concurrently, it is a collection of one or more threads

Thread:Dispatchable unit of workexecutes sequentially and is interruptable

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Modern Operating Systems

Symmetric multiprocessing (SMP, 对称多处理 ) There are multiple processors These processors share same main

memory and I/O facilities All processors can perform the

same functions

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Multiprogramming and Multiprocessing