1. Introduction to Operating Systems CS-2301 B-term 20081 Introduction to Operating Systems (continued) CS-2301, System Programming for Non-majors (Slides include materials from The C Programming Language, 2 nd ed., by Kernighan and Ritchie and from C: How to Program, 5 th ed., by Deitel and Deitel)

2. Introduction to Operating Systems CS-2301 B-term 20082 Review – 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

3. Introduction to Operating Systems CS-2301 B-term 20083 Review – 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)

4. Introduction to Operating Systems CS-2301 B-term 20084 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 look like each process has own CPU Can execute at same time! Uses interrupts to manage and enforce multiplexing of CPU

5. Introduction to Operating Systems CS-2301 B-term 20085 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

6. Introduction to Operating Systems CS-2301 B-term 20086 Review – Why Processes? Independent applications can share a computing system Enables applications to overlap computation with external activities Exploit parallelism in modern processors Vehicle for managing resources, security, etc.

7. Introduction to Operating Systems CS-2301 B-term 20087 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.

8. Introduction to Operating Systems CS-2301 B-term 20088 Note Every command on Linux command line creates a separate process May run in foreground or background Double-clicking on icon in Windows often creates a separate process May run in same window or another window Processes in embedded systems are usually created at system startup

9. Introduction to Operating Systems CS-2301 B-term 20089 Questions about Processes?

10. Introduction to Operating Systems CS-2301 B-term 200810 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

11. Introduction to Operating Systems CS-2301 B-term 200811 Definition – Virtual Memory The illusion that each process has its own memory –Separate from virtual memories of other processes –(Often) more memory than machine has installed Implemented by translating “logical” addresses of program into “physical” address of RAM (random access memory) –Every memory reference, both program and data, on-the-fly E.g., when the program utters 0x00105C, … –… the machine accesses 0x61605C OS creates illusion by swapping blocks of memory from RAM to disk and back as needed –Called pages

12. Introduction to Operating Systems CS-2301 B-term 200812 Virtual Memory for Process (Windows & Linux) 0x00000000 0xFFFFFFFF address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP Every process has one of these, separate from every other process.

13. Introduction to Operating Systems CS-2301 B-term 200813 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

14. Introduction to Operating Systems CS-2301 B-term 200814 Virtual Memory in Embedded Systems Implemented by partitioning RAM No swapping in or out May not even have a disk!

15. Introduction to Operating Systems CS-2301 B-term 200815 Virtual Memories in Embedded System OS Kernel stack Process 1 stack Process 2 0x00000000 0x0000FFFF Physical memory stack Process 3

16. Introduction to Operating Systems CS-2301 B-term 200816 Questions about Processes or Virtual Memory? Not in Kernighan & Ritchie

17. Introduction to Operating Systems CS-2301 B-term 200817 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

18. Introduction to Operating Systems CS-2301 B-term 200818 Thread Interface – POSIX standard #include 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

19. Introduction to Operating Systems CS-2301 B-term 200819 Thread Interface – POSIX standard #include 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

20. Introduction to Operating Systems CS-2301 B-term 200820 Thread Interface – POSIX standard #include 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

21. Introduction to Operating Systems CS-2301 B-term 200821 Thread Example pthread_create(tp, &f, &args) pthread_join(tp) main function f g f h f

22. Introduction to Operating Systems CS-2301 B-term 200822 Thread Example pthread_create(tp, &f, &args) pthread_join(tp) main function f g f h f Two threads run at the same time

23. Introduction to Operating Systems CS-2301 B-term 200823 Virtual Memory for Multiple Threads 0x00000000 0xFFFFFFFF Virtual address space code (text) static data heap thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3)

24. Introduction to Operating Systems CS-2301 B-term 200824 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 …

25. Introduction to Operating Systems CS-2301 B-term 200825 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

26. Introduction to Operating Systems CS-2301 B-term 200826 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

27. Introduction to Operating Systems CS-2301 B-term 200827 Questions about Threads? Not in Kernighan & Ritchie

28. Introduction to Operating Systems CS-2301 B-term 200828 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

29. Introduction to Operating Systems CS-2301 B-term 200829 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!

30. Introduction to Operating Systems CS-2301 B-term 200830 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 points to other files Associates names with files

31. Introduction to Operating Systems CS-2301 B-term 200831 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 points to other files Associates names with files Older systems also used magnetic tape, paper tape, trays of punched cards, etc.

32. Introduction to Operating Systems CS-2301 B-term 200832 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

33. Introduction to Operating Systems CS-2301 B-term 200833 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)

34. Introduction to Operating Systems CS-2301 B-term 200834 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) NTFS and Linux file systems on hard drives

35. Introduction to Operating Systems CS-2301 B-term 200835 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) Typical camera chips, flash drives, floppies

36. Introduction to Operating Systems CS-2301 B-term 200836 Questions about Files?

37. Introduction to Operating Systems CS-2301 B-term 200837 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

38. Introduction to Operating Systems CS-2301 B-term 200838 Sockets and Connections Connection: –a serial conversation over a network between two end points e.g., processes, threads, tasks on different computers –organized as a sequence of messages or datagrams –distinct from all other connections Socket: –An end point of a connection –An abstraction that allows a process to send or receive only the information of that connection –Multiplexed on network with all other connections

39. Introduction to Operating Systems CS-2301 B-term 200839 Sockets and Connections Defer to another course

40. Introduction to Operating Systems CS-2301 B-term 200840 General Questions?