Chapter 2: Operating System Overview

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

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

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. Layers of computer system

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

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

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. 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. 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. Job Control Language (JCL) • Example of JCL cards (with / /) in a deck / / JOB / / FORT    < source program cards >    / / LOAD / / RUN    < data cards >    / / END Protection After an //END card or an error, the monitor flushes cards until the next //JOB card

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

19. Multiprogrammed batch systems

20. Multiprogrammed batch systems

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. 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

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

25. Major achievements • Processes • Focus of chapters 3, 4, 5, and 6 • Memory management • Focus of chapters 7 and 8 • Information protection and security • Focus of chapters 12 and 16 • Scheduling and resource management • Focus of chapters 9, 10, and 11 • System structure

26. Major achievement – process • 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. 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. A 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. A context switch processor hardware process control block registers context context switch registers data process data structure activity memory

30. VAX computer systemhardwareprocess control block (PCB)

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. Problems leading to process concept • See the paragraph before the bullets, text p. 67 • Four main causes of errors • Improper synchronization • Failed mutual exclusion • Deadlock • Nondeterminate program operation

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 • Mutual exclusion • Two process must not book the same seat on an airplane • Example from transaction processing

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

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. 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. Multiprogramming and multiprocessing

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. Example systems • Microsoft Windows • UNIX • Traditional • Modern • Linux

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