1. 23/06/2016 03:54:20 CSC Alliance — 1 Kimera Richard E-mail: rkimera@must.ac.ugrkimera@must.ac.ug Phone: 0701 437989 INSTITUTE OF COMPUTER SCIENCE DEPARTMENT OF INFORMATION TECHNOLOGY Kimera Richard E-mail: rkimera@must.ac.ugrkimera@must.ac.ug Phone: 0701 437989 INSTITUTE OF COMPUTER SCIENCE DEPARTMENT OF INFORMATION TECHNOLOGY Introduction to operating systems: BIT 2104 Chapter 3: Computer System structures Thursday 19 th Sept 2013 http://kimrichies.blogspot.com

2. MUST- ICS rkimera@must.ac.ug 2  System Components  OS Services  System Calls  System Programs  System Structure  Virtual Machines  System Design and Implementation  System Generation overview

3. MUST- ICS rkimera@must.ac.ug 3 Common System Components  Process Management  Main Memory Management  File Management  I/O System Management  Secondary Management  Networking  Protection System  Command-Interpreter System

4. MUST- ICS rkimera@must.ac.ug 4 Process Management  A process is a program in execution.  A time-shared user program such as a compiler is a process.  A word-processing program being run by an individual user on a PC is a process.  A system task, such as sending output to a printer, also is a process.

5. MUST- ICS rkimera@must.ac.ug 5 Process Management (cont.)  A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task.  These resources are either given to the process when it is created, or allocated to it while it is running.  In addition to the various physical and logical resources that a process obtains when it is created, various initialization data (input) may be passed along.  A program by itself is not a process; a program is a passive entity, such as the contents of a file stored on disk, with a program counter specifying the next instruction to execute.  The execution of a process must be sequential.  A process is the unit of work in a system.  Operating-system process  User process

6. MUST- ICS rkimera@must.ac.ug 6 Process Management (cont.)  The OS is responsible for the following activities in connection with process management.  Process creation and deletion.  process suspension and resumption.  Provision of mechanisms for:  process synchronization  process communication  Deadlock handling

7. MUST- ICS rkimera@must.ac.ug 7 Main-Memory Management  The main memory is central to the operation of a modern computer system.  Main memory is a large array of words or bytes, each with its own address. It is a repository of quickly accessible data shared by the CPU and I/O devices.  Main memory is a volatile storage device. It loses its contents in the case of system failure.  The OS is responsible for the following activities in connections with memory management:  Keep track of which parts of memory are currently being used and by whom.  Decide which processes to load when memory space becomes available.  Allocate and deallocate memory space as needed.

8. MUST- ICS rkimera@must.ac.ug 8 File Management  A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.  The OS is responsible for the following activities in connections with file management:  File creation and deletion.  Directory creation and deletion.  Support of primitives for manipulating files and directories.  Mapping files onto secondary storage.  File backup on stable (nonvolatile) storage media.

9. MUST- ICS rkimera@must.ac.ug 9 I/O System Management  The I/O system consists of:  A buffer-caching system  A general device-driver interface  Drivers for specific hardware devices

10. MUST- ICS rkimera@must.ac.ug 10 Secondary-Storage Management  Since main memory (primary storage) is volatile and too small (about 1GB now) to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.  Most modern computer systems use disks (usu. HD) as the principle on-line storage medium, for both programs and data.  The OS is responsible for the following activities in connection with disk management:  Free space management  Storage allocation  Disk scheduling

11. MUST- ICS rkimera@must.ac.ug 11 Networking (Distributed Systems)  A distributed system is a collection processors that do not share memory or a clock. Each processor has its own local memory.  The processors in the system are connected through a communication network.  Communication takes place using a protocol (TCP/IP).  A distributed system provides user access to various system resources.  Access to a shared resource allows:  Computation speed-up  Increased data availability  Enhanced reliability

12. MUST- ICS rkimera@must.ac.ug 12 Protection System  Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.  The protection mechanism must:  distinguish between authorized and unauthorized usage.  specify the controls to be imposed.  provide a means of enforcement.

13. MUST- ICS rkimera@must.ac.ug 13 Command-Interpreter System  Many commands are given to the OS by control statements which deal with:  process creation and management  I/O handling  secondary-storage management  main-memory management  file-system access  protection  networking

14. MUST- ICS rkimera@must.ac.ug 14 Command-Interpreter System (Cont.)  The program that reads and interprets control statements is called variously:  command-line interpreter  shell (in UNIX) http://en.wikipedia.org/wiki/List_of_comm and-line_interpreters Its function is to get and execute the next command statement.

15. MUST- ICS rkimera@must.ac.ug 15 OS Services  Program execution – system capability to load a program into memory and to run it.  I/O operations – since user programs cannot execute I/O operations directly, the OS must provide some means to perform I/O.  File-system manipulation – program capability to read, write, create, and delete files.  Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.  Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.

16. MUST- ICS rkimera@must.ac.ug 16 Additional OS Functions Additional functions exist not for helping the user, but rather for ensuring efficient system operations. Resource allocation – allocating resources to multiple users or multiple jobs running at the same time. Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics. Protection – ensuring that all access to system resources is controlled.

17. MUST- ICS rkimera@must.ac.ug 17 System Calls  System calls provide the interface between a running program and the OS.  Generally available as assembly language instructions.  Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, C++)  3 general methods are used to pass parameters between a running program and the OS.  Pass parameters in registers.  Store the parameters in a table in memory, and the table address is passed as a parameter in a register.  Push (store) the parameters onto the stack by the program, and pop off the stack by OS.

18. MUST- ICS rkimera@must.ac.ug 18 Passing of Parameters As A Table

19. MUST- ICS rkimera@must.ac.ug 19 Types of System Calls  Process control  File management  Device management  Information maintenance  Communications

20. MUST- ICS rkimera@must.ac.ug 20 MS-DOS Execution At System Start-up Running a Program

21. MUST- ICS rkimera@must.ac.ug 21 UNIX Running Multiple Programs

22. MUST- ICS rkimera@must.ac.ug 22 Communication Models Message Passing Shared Memory Communication may take place using either message passing or shared memory.

23. MUST- ICS rkimera@must.ac.ug 23 System Programs  System programs provide a convenient environment for program development and execution. They can be divided into:  File manipulation  Status information  File modification  Programming language support  Program loading and execution  Communications  Application programs  Most users’ view of the OS is defined by system programs, not the actual system calls.

24. MUST- ICS rkimera@must.ac.ug 24 MS-DOS System Structure  MS-DOS – written to provide the most functionality in the least space  not divided into modules  Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

25. MUST- ICS rkimera@must.ac.ug 25 MS-DOS Layer Structure

26. MUST- ICS rkimera@must.ac.ug 26 UNIX System Structure  UNIX – limited by hardware functionality, the original UNIX OS had limited structuring. The UNIX OS consists of two separable parts.  Systems programs  The kernel  Consists of everything below the system-call interface and above the physical hardware  Provides the file system, CPU scheduling, memory management, and other OS functions; a large number of functions for one level.

27. MUST- ICS rkimera@must.ac.ug 27 UNIX System Structure

28. MUST- ICS rkimera@must.ac.ug 28 Layered Approach  The OS is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.  With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers.

29. MUST- ICS rkimera@must.ac.ug 29 An OS Layer

30. MUST- ICS rkimera@must.ac.ug 30 OS/2 Layer Structure

31. MUST- ICS rkimera@must.ac.ug 31 Microkernel System Structure  Moves as much from the kernel into “user” space.  Communication takes place between user modules using message passing.  Benefits: - easier to extend a microkernel - easier to port the OS to new architectures - more reliable (less code is running in kernel mode) - more secure

32. MUST- ICS rkimera@must.ac.ug 32 Virtual Machines  A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the OS kernel as though they were all hardware.  A virtual machine provides an interface identical to the underlying bare hardware.  The OS creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.

33. MUST- ICS rkimera@must.ac.ug 33 Virtual Machines (Cont.)  The resources of the physical computer are shared to create the virtual machines.  CPU scheduling can create the appearance that users have their own processor.  Spooling and a file system can provide virtual card readers and virtual line printers.  A normal user time-sharing terminal serves as the virtual machine operator’s console.

34. MUST- ICS rkimera@must.ac.ug 34 System Models Non-virtual Machine Virtual Machine

35. MUST- ICS rkimera@must.ac.ug 35 Advantages/Disadvantages of Virtual Machines  The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.  A virtual-machine system is a perfect vehicle for OSs research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupted normal system operation.  The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.

36. MUST- ICS rkimera@must.ac.ug 36 Java Virtual Machine  Compiled Java programs are platform-neutral byte codes executed by a Java Virtual Machine (JVM).  JVM consists of - class loader - class verifier - runtime interpreter  Just-In-Time (JIT) compilers increase performance

37. MUST- ICS rkimera@must.ac.ug 37 Java Virtual Machine

38. MUST- ICS rkimera@must.ac.ug 38 System Design Goals  User goals – OS should be convenient to use, easy to learn, reliable, safe, and fast.  System goals – OS should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient.

39. MUST- ICS rkimera@must.ac.ug 39 Mechanisms and Policies  Mechanisms determine how to do something, policies decide what will be done.  The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later.  For instance, the timer construct for ensuring CPU protection is mechanism. On the other hand, the decision of how long the timer is set for a particular user is a policy decision.  Why is the separation of mechanism and policy a desirable property?  Mechanism and policy must be separate to ensure that systems are easy to modify. No two system installations are the same, so each installation may want to tune the operating system to suit its needs. With mechanism and policy separate, the policy may be changed at will while the mechanism stays unchanged. This arrangement provides a more flexible system.

40. MUST- ICS rkimera@must.ac.ug 40 System Implementation  Traditionally written in assembly language, OSs can now be written in higher-level languages.  Code written in a high-level language:  can be written faster.  is more compact.  is easier to understand and debug.  An OS is far easier to port (move to some other hardware) if it is written in a high-level language.

41. MUST- ICS rkimera@must.ac.ug 41 System Generation (SYSGEN)  OSs are designed to run on any of a class of machines; the system must be configured for each specific computer site.  SYSGEN program obtains information concerning the specific configuration of the hardware system.  Booting – starting a computer by loading the kernel.  Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution.

42. MUST- ICS rkimera@must.ac.ug 42 Home Work  What are the five major activities of an operating system in regard to the following?  process management  memory management  file management  Secondary-storage management  What is the purpose of the command interpreter? Why is it usually separate from the kernel?  List five services provided by an operating system. Explain how each provides convenience to the users. Explain also in which cases it would be impossible for user-level programs to provide these services.  Program execution, I/O operation, File system manipulation and communications, error detection

43. MUST- ICS rkimera@must.ac.ug HOME WORK  What is the purpose of system calls?  What is the main advantage of the layered approach to system design?  List several systems-programming languages.  Answer: C, Bliss, PL/360, Pascal, assembly  Describe three methods for passing parameters needed by system calls.  List eight or more functions for file manipulation.  List categories of systems programs.

44. MUST- ICS rkimera@must.ac.ug Course work 1  In groups of Ten (10) 1.Explain the Boot sequence/process of a Windows XP Operating System 2.What are some of the steps taken during the process of selecting an operating system? 1.Identify one type of operating system and list the process of installation. First define the system requirements 2.Document the steps taken in installing a second Operating systems using a virtual machine. 3.Explain how you would uninstall an operating system? 3.Using an example each of the two different categories of operating systems i.e windows and UNIX, state the languages that were used in development.  The coursework is for one week, hand in a hard copy before the end of the working day (26 th.09.2013).  Indicate what each member contributed to the group, this will be used for assessment as members from the same group will not get the same marks