1 Operating System Overview Chapter 2

2 Operating System  A program that controls the execution of application programs  An interface between applications and hardware

3 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 manner  Ability to evolve( 扩展的能力 ) It should have the capability to permit the effective development, testing & introduction of new system functions.

4 Layers of Computer System

5 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）

6 Services Provided by the Operating System  Error detection and response （错误检测 和响应） Internal and external hardware errors Memory error Device failure Software errors Arithmetic overflow Access forbidden memory locations Operating system cannot grant request of application

7 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.

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

9 Kernel （核心程序）  Portion of operating system that is in main memory  Contains most frequently used functions  Also called the nucleus

10 Evolution of an Operating System  Hardware upgrades plus new types of hardware  New services  Fixes （修正） The system should be modular in construction, with clearly defined interfaces between the modules.

11 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.

12 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

13 Evolution of Operating Systems  Simple Batch Systems （简单批处理） （ IBSYS ） Monitors （监控程序） Software that controls the sequence of events User no longer has direct access to the machine Batch jobs together sequentially, and used by the monitor Program branches back to monitor when finished

14 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.

15 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.

16 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

17 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

18 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

19 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

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

21 Uniprogramming( 单道程序 )  Processor must wait for I/O instruction to complete before preceding

22 Multiprogramming( 多道程序 )  When one job needs to wait for I/O, the processor can switch to the other job

23 Multiprogramming

24 Example

25 Utilization Histograms( 利用率 )

26 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.

27 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

28 Compatible Time-Sharing System (CTSS)  First time-sharing system developed at MIT

29 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

30 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）

31 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）

32 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）

33 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

34 Process

35 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）

36 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

37 Virtual Memory

38 Virtual Memory Addressing

39 File System  Implements long-term store  Information stored in named objects called files

40 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

41 Information Protection and Security  Data integrity( 数据完整性 ) Protection of data from unauthorized modification  Authenticity( 认证 ) Concerned with the proper verification of the identity of users and the validity of messages or data

42 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

43 Key Elements of Operating System

44 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

45 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 work executes sequentially and is interruptable

46 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

47 Multiprogramming and Multiprocessing