Introduction to Operating Systems

1. Introduction to Operating Systems CS-2301, System Programmingfor Non-Majors (Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel) Introduction to Operating Systems

2. Why an Intro to Operating Systems? • This is a System Programming Course • For people who are not CS majors • (Nearly) every programming task in real-life includes working with an OS • Inevitably will have to deal with principle OS features • Even if not knowledgeable in their designs Introduction to Operating Systems

3. Class Discussion What is an Operating System?(Laptops closed, please!) Introduction to Operating Systems

4. What Operating Systemshave you Used? (Other than Windows, Linux, Mac-OS, Unix) Introduction to Operating Systems

5. Characteristics Large, complex set of programs Long-lived, evolving Worked on by many people for many years Functions Creates abstractions Multiplexes concurrent activities Manages resources Mediates access to hardware devices Provides a variety of services to users and applications … Large = 108–109 lines of code (Windows and Linux) 107 line of code for a real-time OS. Some systems smaller What is an Operating System? Introduction to Operating Systems

6. We have already had this notion in thedefinition of function in C Abstraction is a collection of functionsand data for creating a simple model Definition – Abstraction • The distillation of a complex mechanism into a simple, conceptual model • User of abstraction does not need to worry about details • Implementer of abstraction does not need to worry about how user will use it • within limits Introduction to Operating Systems

7. Characteristics Large, complex set of programs Long-lived, evolving Worked on by many people for many years Functions Creates abstractions Multiplexes concurrent activities Manages resources Mediates access to hardware devices Provides a variety of services to users and applications … What operating system services have we used already in this course? What is an Operating System? Introduction to Operating Systems

8. Operating Systems • Typically • Long-lived • Frequently extended and updated • Worked on by many developers • Used and, maybe, abused by a variety of users with varying expertise and expectations • Essential to create an acceptable computing environment to create and execute other programs that achieve business or personal goals Introduction to Operating Systems

9. Kinds of operating systems • Mainframe Operating Systems • Server Operating Systems • Multiprocessor Operating Systems • Personal Computer Operating Systems • Mobile Phone Operating Systems • Handheld Computer Operating Systems • Embedded Operating Systems • Sensor Node Operating Systems • Real-time Operating Systems • Smart-card Operating Systems • … E.g.:– WindowsLinuxUnix Important new category Introduction to Operating Systems

10. z/OS VMS/Open VMS VxWorks RT Linux QNX Neutrino eCOS BrickOS/LeJos TinyOS Arduino … iPhone OS Android Symbian Blackberry OS PalmOS/Garnet Windows Mobile … Some operating systems you may(or may not) have heard about Introduction to Operating Systems

11. OS and Hardware • OS mediates programs’ access to hardware • Computation – CPU • Storage – volatile (memory) and persistent (disk) • Networks – NIC, protocols • I/O devices – sound cards, keyboards, displays Introduction to Operating Systems

12. Four Fundamental Abstractions • Processes & threads • Multiplexing of processor(s) to create the illusion of many of them • Virtual memory • Multiplexing of physical memory and disk blocks to create illusion of own memory per process • Files – i.e., persistent storage • Organizing principles about long-term data storage • Sockets & Connections • Organizing principles about network communication Introduction to Operating Systems

13. Four Fundamental Abstractions • Processes & threads • Multiplexing of processor(s) to create the illusion of many of them • Virtual memory • Multiplexing of physical memory and disk blocks to create illusion of own memory per process • Files – i.e., persistent storage • Organizing principles about long-term data storage • Sockets & Connections • Organizing principles about network communication Introduction to Operating Systems

14. Definition – Process • A particular execution of a program • Different from all other executions of that program • Different from executions of other programs • The OS uses one or more CPUs to make it seem like each process has its own CPU • Can execute at same time! • Uses interrupts to manage and enforce multiplexing of CPU Introduction to Operating Systems

15. Definition – Interrupt A mechanism by which the processor suspends execution of the current, running program and gives control to the OS OS saves the state of the interrupted program so that it can be restarted later OS then takes appropriate action Introduction to Operating Systems

16. Why Processes? • Enables programmers • to completely disengage from issues of concurrent execution of independent programs • to build applications with more than one concurrent activity • Enables independent applications to share a computing system • Safely! Introduction to Operating Systems

17. Modern limitation due to power dissipation. Why Processes (continued)? • Exploit modern processors • Capable of executing multiple, simultaneous, program executions • Interleaved at instruction level or even memory access level • Moore’s Law:– • Integrated circuit components shrink in size by 50% every 18 months • Double in speed every 18 months Introduction to Operating Systems

18. Resources Assigned to a Process • Memory • Virtual or real • Processor time • Priorities • Deadlines for real-time activities • Privileges • Security, authentication, etc. • Files and file space • For long-term storage, temporary storage • Devices • For input and output activity, sensors, etc. Introduction to Operating Systems

19. Resources (continued) • Managed by OS • Protection and isolation from other processes • Allocation according to defined policies • Enforcement of limits, etc. • … Introduction to Operating Systems

20. Shell / Command Prompt • Linux Shell is a process • Windows Command Prompt is a process • Created when • you log on or connect to system (e.g., via PuTTY) • Open Command Prompt, konsole, xterm , etc., window • Reads what you type (and displays it in your window) • Interprets lines as commands and arguments • Creates a process for each command, passes args • (Typically) waits for process to complete before interpreting next line Introduction to Operating Systems

21. Window Manager • Window Manager is a process • Tracks mouse movements, key clicks, menu actions • “Open” an application means … • Create process for that application; give it a window • “Open” a document means … • If application is not open, create process for it • Pass document as argument to application Introduction to Operating Systems

22. Creating and Deleting Processes A process is created … • … at system boot time • The first process • Built-in processes – e.g., in embedded systems OR • … by another process • Possibly in response to an action by a (human) user A process is deleted … • When its program exits • By another process – killed or paused (by debugger) • When system crashes or shuts down Introduction to Operating Systems

23. Questions about Processes? Introduction to Operating Systems

24. Four Fundamental Abstractions • Processes & threads • Multiplexing of processor(s) to create the illusion of many of them • Virtual memory • Multiplexing of physical memory and disk blocks to create illusion of separate memory per process • Files – i.e., persistent storage • Organizing principles about long-term data storage • Sockets & Connections • Organizing principles about network communication Introduction to Operating Systems

25. Virtual Memory • Definition:– the illusion that a process has its own, independent memory • (Often) more memory than machine has installed • May be implemented using interrupts, pages, and disk blocks • Swapping fast enough so process is unaware • May be implemented by partitioning • Swapping not necessary for real-time activities Introduction to Operating Systems

26. Independence of Virtual Memories • A process cannot even see the virtual memory of another process • A process cannot even see the memory used by the OS • I.e., no possible pointer value can point to something in a different virtual memory • Separate, parallel universes • Except where explicitly linked together Introduction to Operating Systems

27. 0xFFFFFFFF stack (dynamically allocated) SP heap (dynamically allocated) address space static data program code (text) PC 0x00000000 Typical Virtual Memory for Process(Windows & Linux) Introduction to Operating Systems

28. 0xFFFFFFFF stack (dynamically allocated) Every process has one of these. Independent of all others. SP heap (dynamically allocated) address space static data program code (text) PC 0x00000000 Typical Virtual Memory for Process(Windows & Linux) Introduction to Operating Systems

29. Virtual Memory (continued) Typical virtual memory size – 4 GBytes Per process — even in a 1 GByte computer! “Inactive” pages are not resident in RAM Reference results in interrupt called a page fault OS brings in page from disk, (maybe) swaps one out Unused pages not filled in by OS Dereferencing pointer results in Segmentation Fault New pages created by OS as needed When stack or heap grows or programs are loaded Introduction to Operating Systems

30. Virtual Memory in Embedded Systems Implemented by partitioning RAM No swapping in or out May not even have a disk! Also in smart-phoneoperating systems Introduction to Operating Systems

31. Virtual Memories in Embedded System 0x0000FFFF Process 3 stack stack stack Process 2 Physical memory Process 1 0x00000000 OS Kernel Introduction to Operating Systems

32. Questions about Processesor Virtual Memory? Not in Kernighan & Ritchie Introduction to Operating Systems

33. Threads A refinement of concept of process Short for “thread of control” Concurrent execution of a function within the context of a process Needs own stack in order to call other functions Shares heap, static data, and code with other threads of same process Reason Application may need to manage concurrency of its own computation with external events Introduction to Operating Systems

34. Virtual Memory for Multiple Threads 0xFFFFFFFF thread 1 stack SP (T1) thread 2 stack SP (T2) thread 3 stack SP (T3) Virtual address space heap static data code (text) PC (T2) PC (T1) 0x00000000 PC (T3) Introduction to Operating Systems

35. Why Threads? • To enable development of applications with concurrent activities inside them • Need to share data (difficult with separate processes) • Examples • Web server over common data pages • Transaction processor over common data base • Applications within a mobile phone or PDA • Applications with different speeds of devices • … Introduction to Operating Systems

36. Thread Interface – POSIX standard #include <pthread.h> int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void*(*start_routine)(void), void *arg) creates a new thread of control new thread begins executing at start_routine pthread_exit(void *value_ptr) terminates the calling thread pthread_join(pthread_t thread, void **value_ptr) blocks the calling thread until the thread specified terminates pthread_t pthread_self() Returns the calling thread's identifier Introduction to Operating Systems

37. Thread Interface – POSIX standard #include <pthread.h> int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void*(*start_routine)(void), void *arg) creates a new thread of control new thread begins executing at start_routine pthread_exit(void *value_ptr) terminates the calling thread pthread_join(pthread_t thread, void **value_ptr) blocks the calling thread until the thread specified terminates pthread_t pthread_self() Returns the calling thread's identifier Pointer to a function– see §5.7 of K & R Introduction to Operating Systems

38. Thread Interface – POSIX standard #include <pthread.h> int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void*(*start_routine)(void), void *arg) creates a new thread of control new thread begins executing at start_routine pthread_exit(void *value_ptr) terminates the calling thread pthread_join(pthread_t thread, void **value_ptr) blocks the calling thread until the thread specified terminates pthread_t pthread_self() Returns the calling thread's identifier Arguments to that function Introduction to Operating Systems

39. Thread Example pthread_create(tp,&f,&args) f main function g main function f h main function f pthread_join(tp) Introduction to Operating Systems

40. Thread Example pthread_create(tp,&f,&args) Two threads run at the same time f main function g main function f h main function f pthread_join(tp) Introduction to Operating Systems

41. Why Threads? To enable development of applications with concurrent activities inside them Need to share data (difficult with separate virtual memories) Examples Web server over common data pages Transaction processor over common data base Applications within a mobile phone or PDA Applications with different speeds of devices … Introduction to Operating Systems

42. Example Thread Application One or more threads to get data from sensor 1000 times per second; cannot afford to miss a reading Places data on queue Another thread processes and displays data Removes and processes each data item from queue Displays system state on control panel User thread Allows operator to adjust system parameters While other threads are still running Introduction to Operating Systems

43. Additional Functions & Data Required pthread_mutex_t— mutual exclusion lock Enables a thread to “lock” some data so that other threads do not touch in mid-operation pthread_cond_t — condition variable Enables a thread to wait for an event to happen Signaled by another thread See man pages for details Introduction to Operating Systems

44. Questions about Threads? Not in Kernighan & Ritchie Introduction to Operating Systems

45. Four fundamental Abstractions Processes & threads Multiplexing of processor(s) to create the illusion of many of them Virtual memory Multiplexing of physical memory and disk blocks to create illusion of own memory per process Files & persistent storage Organizing principles about long-term data storage Sockets & connections Organizing principles about network communication Introduction to Operating Systems

46. Definition – File A (potentially) large amount of information or data that lives a (potentially) very long time Often much larger than the memory of the computer Often much longer than any computation Sometimes longer than life of machine itself (Usually) organized as a linear array of bytes or blocks Internal structure is imposed by application (Occasionally) blocks may be variable length (Often) requiring concurrent access by multiple threads or processes Even by processes on different machines! Introduction to Operating Systems

47. Implementations of Files Usually on disks (or devices that mimic disks) Magnetic – hard drive or floppy Optical – CD, DVD Flash drives – electronic memory, organized as disks Requirement Preserve data contents during power-off or disasters Directory / Folder Special kind of file that contains links pointing to other files Associates names with files Introduction to Operating Systems

48. Implementations of Files Usually on disks (or devices that mimic disks) Magnetic – hard drive or floppy Optical – CD, DVD Flash drives – electronic memory, organized as disks Requirement Preserve data contents during power-off or disasters Directory / Folder Special kind of file that contains links pointing to other files Associates names with files Older systems also used magnetic tape, paper tape, trays of punched cards, etc. Introduction to Operating Systems

49. File Access in C See Kernighan & Ritchie, Chapter 8 Raw file access Without simplifying stream functions – e.g., scanf, fscanf, printf, fprintf, fgetc, etc. read and write raw disk blocks Seek to a file position lseek, fseek — sets file pointer to specified location Subsequent read, write, etc., start there ftell – returns file pointer Introduction to Operating Systems

50. Organizations of Files Contiguous Blocks stored contiguously on storage medium E.g., CD, DVD, some large database systems Access time to any block is O(1) Linked Blocks linked together – File Allocation Table (FAT) Access time is O(n) Indexed Blocks accessed via tree of index blocks (i-nodes) Access time is O(log n) However, base of logarithm may be very large (>100) Introduction to Operating Systems