Chapter 2: Operating System Overview

1. 1 Chapter 2: Operating System Overview CS 472 Operating Systems Indiana University – Purdue University Fort Wayne

2. 2 Operating system overview An example of computing without an operating system Session scheduling and set-up time Front panel lights and toggle switches Bootstrap loader on paper tape Compiler on cards Source deck Paper tape object file Data deck Execution An early computer system Argonne National Lab Late 1960sAn early computer system Argonne National Lab Late 1960s

3. 3 Operating system objectives Convenience Makes the computer more convenient to use Efficiency Allows computer system resources to be used in an efficient manner Ability to evolve Permit effective development, testing, and introduction of new system functions without interfering with service

4. 4 Layers of computer system

5. 5 Operating system kernel The kernel is a portion of operating system that is always in main memory It contains most frequently used functions Also called the nucleus

6. 6 Convenience Services provided by the operating system Program development Editors and debuggers Program execution Access to I/O devices Controlled access to files System access Login and passwords Error detection and response Accounting

7. 7 Error detection and response Internal and external hardware errors Memory error Device failure Software errors Arithmetic overflow Access forbidden memory locations Operating system cannot grant a request made by an application

8. 8 Accounting Collect usage statistics Monitor performance This information can be used . . . to anticipate future enhancements for billing purposes

9. 9 Efficiency The OS promotes efficiency by managing resources Processor(s) Memory Devices Files The OS functions same way as ordinary computer software It is program that is executed The OS frequently relinquishes control of the processor and relies on the processor to regain control

10. 10 Ability to evolve A major OS should be able to evolve over time in response to . . . Hardware upgrades and new types of hardware such as Paging hardware for virtual memory Multiple processors New services such as Overlapping windows Client / server computing Errors in the OS

11. 11 Evolution of operating systems Serial processing Simple batch systems Multiprogrammed batch systems Time-sharing systems

12. 12 Serial processing (discussed earlier) No operating system Manual scheduling of use Computer operated from a console with display lights, toggle switches, input device, and printer Setup included . . . Loading the compiler Loading the source program Saving compiled program Loading and linking object files

13. 13 Simple batch systems Developed in the 1950s by GM for the IBM 701 Simple batch OS called a monitor Monitor software controlled a sequence of jobs A batch of sequential jobs was loaded into a card tray The monitor called the first (next) job Each job program branched back to monitor when finished Only one user job could run at a time Processor idle waiting for I/O But idle time between jobs and within jobs eliminated

14. 14 Job Control Language (JCL) A special type of programming language Provides commands to the monitor Identifies new jobs Specifies what compiler to use Specifies which object files to load and link Specifies what data to use Etc.

15. 15 Job Control Language (JCL) Example of JCL cards (with / /) in a deck

16. 16 Hardware support for simple batch Memory protection Does not allow the memory area containing the monitor to be altered by a job Timer Prevents a job from monopolizing the system Privileged instructions Certain machine level instructions can only be executed by the monitor E.g. - I/O instructions (a program should not read cards of next job) Note: a program “requests” that the monitor perform the instruction Interrupts Allow processor to do something else while waiting for I/O Early computer models did not have this capability

17. 17 Hardware support for simple batch The need for memory protection and privileged instructions led to the concept of processor modes A bit in the PSW register toggles the processor between user mode and kernel mode Each user program executes in user mode Certain privileged instructions may not be executed Only the program area may be referenced The monitor executes in kernel mode Privileged instructions may be executed Protected areas of memory may be accessed

18. 18 Multiprogrammed batch systems Several jobs resident in memory simultaneously Gives processor something to do while one job is waiting for I/O

19. 19 Multiprogrammed batch systems

20. 20 Multiprogrammed batch systems

21. 21 Multiprogrammed batch systems In these systems . . . Interrupt-driven DMA emerged This was the start of multiprocessing Concept of logical (vrs. physical) devices emerged

22. 22 Time-sharing systems Adds an interactive computing capability to a multiprogrammed batch system Processor’s time is shared among multiple interactive users Multiple users simultaneously access the system through terminals Essential for transaction processing systems Example: Compatible Time-Sharing System (CTSS) First time-sharing system Developed at MIT in 1961 for the IBM 709

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24. 24 Problems Multiprogramming and time sharing led to the identification of new problems Memory protection File security Contention for resources E.g. - printers

25. 25 Major achievements Process concept Focus of chapters 3, 4, 5, and 6 Memory management Focus of chapters 7 and 8 Information protection and security Focus of chapters 12, 14, and 15 Scheduling and resource management Focus of chapters 9, 10, and 11 System structure

26. 26 Major achievement – process concept A process is . . . A program in execution An instance of a program running on a computer An entity that can be assigned to and executed on a processor Activity characterized by a current state and an associated set of system resources

27. 27 A process consists of . . . Activity An executable program Data Variables Work space (stack) Buffers Etc. Execution context (process state) All information the operating system needs to manage the process Registers (PC, PSW, registers) Pending I/O Allocated memory Priority and privilege

28. 28 Context switch A context switch is where . . . The hardware context registers (hardware Process Control Block or hardware PCB) of the old process are saved in its context area in memory The hardware PCB of the new process is restored from its context area of memory This is similar to a subprogram call But includes the PSW and stack pointer Implemented in hardware An interrupt, for example, results in a context switch From the interrupted program to the interrupt handler

29. 29 A context switch

30. 30 VAX computer systemhardwareprocess control block (PCB)

31. 31 Problems leading to process concept Multiprogrammed batch Many jobs in memory simultaneously Need to isolate one job from the others Need to switch among jobs at I/O start and end Time sharing Need to switch rapidly due to a timer interrupt Need for responsiveness Real-time transaction processing Many jobs share information and memory

32. 32 Problems leading to process concept See the paragraph before the bullets, text p. 66 Four main causes of errors Improper synchronization Failed mutual exclusion Deadlock Nondeterminate program operation

33. 33 Problems leading to process concept Improper synchronization Data is lost or duplicate data is received A process initiating I/O must wait until the handler process detects completion Failed mutual exclusion Two process must not book the same seat on an airplane Example from transaction processing

34. 34 Problems leading to process concept Deadlock Common for two processes to hang up waiting for each other For example, one process may want to copy disk to tape while another copies the tape to disk Nondeterminate program operation Outputs or success of an operation depends on the scheduling of unrelated programs Hard to debug because the condition that produced the error is difficult to reproduce

35. 35 Major achievement – memory management Responsibilities of memory management Process isolation Transparent allocation and management of memory within a process across the memory hierarchy Virtual memory Real (main) memory Cache memory Support of modular programming Dynamic creation, resizing, and deallocation of program modules Protection and access control Controlled sharing of memory among processes Long-term storage in files

36. 36 Major achievement – information protection and security Availability Concerned with protecting the system against interruption Confidentiality Assuring that users cannot read data for which access is unauthorized Data integrity Protection of data from unauthorized modification Authenticity Concerned with the proper verification of the identity of users and the validity of messages or data

37. 37 Major achievement – scheduling and resource management Fairness Give equal and fair access to resources within the same class Differential responsiveness Discriminate among different classes of jobs Efficiency Maximize throughput, minimize response time, and accommodate as many users as possible

38. 38 Major achievement – system structure In a large OS, modularity is necessary but not sufficient View the system as a series of layers Each layer performs a related subset of functions Each layer relies on lower levels to perform more primitive functions Each layer provides services to higher levels Think client / server computing The text identifies 13 levels

39. 39 Hardware levels (not part of OS) Level 1 Electronic circuits Objects are registers, memory cells, and logic gates Operations are clearing a register or reading a memory location Level 2 Processor’s instruction set Operations such as add, subtract, load, and store Level 3 Adds the concept of a procedure or subroutine, plus call/return operations Level 4 Interrupts Interrupt handling routines are part of the OS

40. 40 Multiprogramming levels Level 5 Concept of the process is supported Ability to suspend and resume processes (context switch) Synchronization primitives implemented Level 6 Secondary storage devices Transfer of blocks of data Level 7 Creates logical address space for processes Organizes virtual address space into blocks

41. 41 Support for external devices / networks Level 8 Communication of information and message passing Level 9 Supports long-term storage of named files Level 10 Provides access to external devices using standardized interfaces

42. 42 Support for external devices / networks Level 11 Responsible for maintaining the association between the external and internal identifiers Level 12 Provides full-featured facility for the support of processes Level 13 Provides an interface to the operating system for the user Called a shell or an Application Programming Interface (API)

43. 43 Modern OS characteristics Microkernel architecture There is no monolithic OS kernel with most OS features Kernel has only a few essential functions Address space Interprocess communication (IPC) Basic scheduling Other traditional OS features are distributed among user mode processes

44. 44 Modern OS characteristics Multithreading A process is divided into threads that can run concurrently Thread Dispatchable unit of work Includes registers and a stack Executes sequentially and is interruptable Process is a collection of one or more threads A thread switch has less overhead than a process switch

45. 45 Modern OS characteristics Symmetric MultiProcessing (SMP) There are multiple processors These processors share same main memory and I/O facilities All processors are equivalent and can perform the same functions True parallel processing is possible Processors may be added and removed The number of processors is transparent to applications

46. 46 Multiprogramming and multiprocessing

47. 47 Modern OS characteristics Distributed operating system This is an OS for a cluster of separate computers (multicomputer system) Separate main memories, disks, and devices Provides the illusion of a single main memory space, a single secondary memory space, and unified access to all devices

48. 48 Example systems Microsoft Windows UNIX Traditional Modern Linux

49. 49 Microsoft Windows Impressive OS Single-user multitasking OS that evolved out of MS-DOS Modular structure for flexibility Any module can be removed, upgraded, or replaced without rewriting the entire system Executes on a variety of hardware platforms Pentium, Itanium, PowerPC, Alpha, etc. Provided by the Hardware Abstraction Layer (HAL) This isolates the operating system from platform-specific hardware differences Supports applications written for other operating systems This is provided by various environment subsystems

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51. 51 Windows organization Modified microkernel architecture Not a pure microkernel Many system functions outside of the microkernel run in kernel mode Kernel Consists of the most used components Thread scheduling Process switching Interrupt handling SMP Does not run in threads and is not preemptible Executive Contains base operating system services Memory management Process and thread management Security I/O Interprocess communication

52. 52 Windows client-server model Simplifies the Executive Possible to construct a variety of APIs Improves reliability Each service runs on a separate process with its own partition of memory Clients cannot not directly access hardware Provides a uniform means for applications to communicate via Local Procedure Call (LPC) Provides base for distributed computing

53. 53 Windows threads and SMP Operating system routines can run on any available processor or simultaneously on different processors Multiple threads of execution within a single process may execute on different processors simultaneously Server processes may use multiple threads to process requests from multiple processes simultaneously Mechanisms provided to share data and resources between processes

54. 54 Traditional UNIX Hardware is surrounded by the operating system software Operating system is called the system kernel Comes with a number of user services and interfaces Shell Components of the C compiler

55. 55 Traditional UNIX architecture

56. 56 Traditional UNIX kernel

57. 57 Modern UNIX kernel

58. 58 Modern UNIX systems System V Release 4 (SVR4) Platform for commercial UNIX deployment Solaris 9 Widely used Preemptable, multithreaded kernel Full support for SMP 4.4BSD (Berkeley Software Distribution) Widely used in universities Mac OS X is based on this Linux Began as a UNIX variant for the Intel X86 architecture (1991) Supported by the Free Software Foundation (FSF) High quality kernel Highly modular