1. Chapter 10 Operating Systems

2. 2 Chapter Goals Describe the two main responsibilities of an operating system Define memory and process management Explain how timesharing creates the virtual machine illusion Explain the relationship between logical and physical addresses Compare and contrast memory management techniques

3. 3 Chapter Goals Distinguish between fixed and dynamic partitions Define and apply partition selection algorithms Explain how demand paging creates the virtual memory illusion Explain the stages and transitions of the process life cycle Explain the processing of various CPU scheduling algorithms

4. What is an Operating System? A collection of programs that carry out six basic functions –Booting the Computer –Provides the User Interface –Provides File Management –Handles Input and Output Operations –Manages Memory –Provides Context Switching & Multitasking Examples of Operating Systems: Windows (98, NT, ME, XP), MS-DOS, CPM, Mac OS X, UNIX, OS/2, LINUX, VMS, PALM OS, SOLARIS

5. What is an Operating System? A collection of programs that manages and controls software, and coordinates the various hardware components to perform tasks requested by the user Isolate the hardware from users and direct manipulation by applications

6. What if the OS wasn’t there? Programs “poked” into RAM via switches First program loaded “read” other programs Next program added “device support” Devices could then be used to read in larger programs like BASIC Overall process was tedious and slow (could take up to 30 minutes) You wouldn’t want to shut off the computer

7. BIOS to the rescue BIOS is the Basic Input Output System Collection of programs Provides the capability of communicating with peripheral devices –Keyboard –Disk drives –Monitor Allows the rest of the operating system to be read in from the disk drive Stored in ROM so it is always there

8. Booting the Computer Process of automatically running a program in ROM which in turn loads the operating system into RAM and turns control over to it. Cold Boot – booting up when the power was originally off Warm Boot – booting up when the power was originally on (re-booting) –Gets a fresh copy of the operating system Multiple Operating Systems – partitioning the hard drive for each OS

9. User Interfaces The user interface is the part of the Operating System that the user sees and interacts with Different types of interfaces for different purposes –GUI –CLI GUI – Graphical user interface –Mouse oriented and utilizes icons and buttons –Allows symbolic manipulation of programs and files CLI – Command line interface –Instructions are given by typing commands on the keyboard –Requires technical understanding of commands –Not user friendly or intuitive

10. Files and File Management A file is a collection of [data] that is treated as a single unit –A picture –A spread sheet –A document –A program A file has a unique name –Filename. Extension Extensions are used to associate files with programs

11. Files and File Management There are two basic types of file systems –Flat file structure –Hierarchical file structure Flat file structure –Organizes files in one flat list –No subdivision or grouping possible –Like dumping all of your papers in a pile on the top of your desk –Difficult to manage and keep track of files

12. Files and File Management Hierarchical file structure –Organizes files in a tree like structure –Subdivision is achieved via subdirectories (usually depicted as folders) –Similar to filing cabinets with drawers and folders within them –Easy to manage and keep track of files –The folders become part of the naming convention for the file C:\Windows\System32\Regedit.exe

13. Input and Output Support I/O support provides basic support to read and write information to peripheral devices Specialized devices may require special software called a device driver –Device drivers are usually provided by the manufacturer of the specialized hardware –Device drivers provide enhanced features that the operating system doesn’t know about I/O support also provides error handling in case of device failures

14. Memory Management Real vs Virtual Memory Computers have a physical amount of RAM All memory in the computer is “owned” by the operating system The operating system allocates memory to programs as they need it The operating system reclaims memory when programs are done with it

15. Memory Management Memory “some” PC after boot up

16. Memory Management The user starts up a web browser

17. Memory Management The user starts up several more programs

18. Memory Management The user wants to start a program that is 35M, but only 20M is free

19. Memory Management Windows will clear out 15M of memory that hasn’t been used in a while and load the program

20. Memory Management The process of swapping memory in and out creates the illusion that you have more than there really is (Virtual Memory) –Too much swapping is bad (poor performance) –Windows hides the disk file from the user Pagefile.sys It is usually around 1.5 times the size of real memory Other uses of memory –Disk cache –RAM disk –I/O buffers

21. Context Switching & Multitasking Context switching allows several application programs to be in RAM at one time. The user controls which program is the current one. –The user switches back and forth between programs Multitasking allows several application programs to be in RAM at one time. Each is allowed CPU time as needed, all under the control of the CPU. –Allows programs to continue to run in the back ground MS-DOS / Windows 3.1 / Windows XP

22. OS for the Networked World Parallel processing / multiprocessing takes place on a computer with more than 1 CPU –Requires more complex operating system –Utilizes special programming techniques Distributed processing utilizes a network to decentralize and distribute the computing needs over several dispersed computers –SETI screen saver Client / server computing – the server provides data and programs for the clients –Web server and web browsers

23. OS for the Networked World Real Time processing – involves human interaction with the computer in a situation in which quick or timely return of results is important –ATM –Airline reservation system Process control – is the control of a process by a computer. It usually requires input to be accepted by the CPU, processed, and then a task is executed based upon the processed input. –Automobile electronic ignition system –Missile guidance system –Usually require “feed back”

24. 24 Software Categories Application software Software written to address specific needs—to solve problems in the real world System software Software that manages a computer system at a fundamental level Can you name examples of each?

25. 25 Roles of an Operating System Operating system System software that –manages computer resources, such as memory and input/output devices –provides an interface through which a human can interact with the computer –allows an application program to interact with these other system resources

26. 26 Roles of an Operating System The various roles of an operating system generally revolve around the idea of “sharing nicely” An operating system manages resources, and these resources are often shared in one way or another among programs that want to use them

27. 27 Resource Management Multiprogramming The technique of keeping multiple programs that compete for access to the CPU in main memory at the same time so that they can execute Memory management The process of keeping track of what programs are in memory and where in memory they reside

28. 28 Resource Management Process A program in execution Process management The act of carefully tracking the progress of a process and all of its intermediate states CPU scheduling Determining which process in memory is executed by the CPU at any given point

29. 29 Batch Processing The first operating system was a human operator, who organized various jobs from multiple users into batches of jobs that needed the same resources

30. 30 Timesharing Timesharing system A system that allows multiple users to interact with a computer at the same time Virtual machine The illusion created by a time-sharing system that each user has his/her own machine As computer speed increased, the human operator became the bottleneck

31. 31 Other Factors Real-time System A system in which response time is crucial given the nature of the application Response time The time delay between receiving a stimulus and producing a response Device driver A small program that “knows” the way a particular device expects to receive and deliver information

32. 32 Memory Management Operating systems must employ techniques to –Track where and how a program resides in memory –Convert logical addresses into actual addresses Logical address Reference to a stored value relative to the program making the reference Physical address Actual address in main memory

33. 33 Memory Management Program 1: sum is assigned memory location 23, a location relative to Program 1 Logical address for sum (23) is bound to a physical address in memory before the program runs OS must map sum (relative location 23) to a specific physical address

34. 34 Single Contiguous MM There are only two programs in memory The operating system The application program This approach is called single contiguous memory management

35. 35 Single Contiguous MM In concrete terms: A logical address is simply an integer value relative to the starting point of the program A physical address is a logical address added to the starting location of the program in main memory

36. 36 Single Contiguous MM If A is location 100, and the application program is Program 1, then sum is stored at location 123.

37. 37 Partition Memory Management Single contiguous MM has only the OS and one other program in memory at one time Partition MM has the OS and any number of other programs in memory at one time There are two schemes for dividing up memory for programs: –Fixed partitions Main memory is divided into a fixed number of partitions into which programs can be loaded –Dynamic partitions Partitions are created as needed to fit the programs waiting to be loaded

38. 38 Partition Memory Management Memory is divided into a set of partitions, some empty and some allocated to programs Base register A register that holds the beginning address of the current partition (the one that is running) Bounds register A register that holds the length of the current partition

39. 39 Partition Memory Management Why check?

40. 40 Partition Selection Algorithms Which partition should we allocate to a new program? First fit Allocate program to the first partition big enough to hold it Best fit Allocated program to the smallest partition big enough to hold it Worst fit Allocate program to the largest partition big enough to hold it Can you give a rationale for each?

41. 41 Partition Selection Algorithms A: 1000 B: 700 C: 750 D: 1500 E: 300 F: 350 Requests come in for blocks of the following sizes: 1000, 25, 780, 1600, and 325 What block will be assigned to each request if the first-fit algorithm is used? best-fit algorithm is used? worst-fit algorithm is used? (Treat each request as an independent event)

42. 42 Paged Memory Management Paged memory technique A technique in which processes are divided into fixed-size pages and stored in memory frames when loaded Frame A fixed-size portion of main memory that holds a process page Page A fixed-size portion of a process that is stored into a memory frame We assume that a frame and a page are the same size

43. 43 Paged Memory Management Prog. 1, Page 3 Prog. 2, Page 2 If Prog. 1 is running and needs logical address 2566, how is the actual address calculated?

44. 44 Paged Memory Management Integer logical address is mapped into a logical address Page number Address divided by the page size (say 1024) Offset The remainder of the address divided by the page size 2566 DIV 1024 = 2 2566 MOD 1024 = 518 ==> And???

45. 45 Paged Memory Management This new logical address is mapped to a physical address with the help of a page-map table (PMT) Every program has a PMT that shows into which frame each page of the program is stored What is the physical address of ?

46. 46 Paged Memory Management Demand paging An extension of paged memory management in which pages are brought into memory on demand Page swap The act of bringing in a page from secondary memory, which often causes another page to be written back to secondary memory

47. 47 Paged Memory Management Virtual memory The illusion that there are no restrictions on the size of a program because an entire process doesn't have to be in memory at the same time Thrashing Inefficient processing caused by constant page swaps Relate the expression "all computing is a tradeoff" to this process

48. 48 Process Management Process management The act of managing the use of the CPU by individual processes Recall that a process is a program in execution What stages does a process go through?

49. 49 Process Management The Process States What can cause a process to move to the Waiting state?

50. 50 Process Management Process control block (PCB) A data structure used by the OS to manage information about a process, including –current value of the program counter –values of all CPU registers for the process –base and bound register values (or page tables) –accounting information Each state is represented by a list of PCBs, one for each process in that state

51. 51 Process Management There is only one CPU and therefore only one set of CPU registers, which contain the values for the currently executing process Each time a process is moved to the running state: –Register values for the currently running process are stored into its PCB –Its PCB is moved to the list of the state into which it goes –Register values of the new process moving into the running state are loaded into the CPU –This exchange of register information is called a context switch

52. 52 CPU Scheduling The act of determining which process in the ready state should be moved to the running state –Many processes may be in the ready state –Only one process can be in the running state, making progress at any one time Which one gets to move from ready to running?

53. 53 CPU Scheduling Nonpreemptive scheduling The currently executing process gives up the CPU voluntarily Preemptive scheduling The operating system decides to favor another process, preempting the currently executing process Turnaround time The amount of time between when a process arrives in the ready state the first time and when it exits the running state for the last time

54. 54 CPU Scheduling Algorithms First-Come, First-Served Processes are moved to the CPU in the order in which they arrive in the running state Shortest Job Next Process with shortest estimated running time in the ready state is moved into the running state first Round Robin Each process runs for a specified time slice and moves from the running state to the ready state to await its next turn if not finished

55. 55 First-Come, First-Served What is the average turn- around time?

56. 56 Shortest Job Next What is the average turn- around time?

57. 57 Round Robin Every process is treated the same! Time slice (quantum) The amount of time each process receives before being preempted and returned to the ready state to allow another process its turn

58. 58 Round Robin Suppose the time slice is 50 What is the average turnaround time?

59. 59 CPU Scheduling Algorithms Are these scheduling algorithms preemptive or non-preemptive? Explain First-Come, First-Served? Shortest Job Next? Round Robin?

60. Questions?? There should be.....