CENG334 Introduction to Operating Systems

1. Erol Sahin Dept of Computer Eng. Middle East Technical University Ankara, TURKEY URL: http://kovan.ceng.metu.edu.tr/~erol/Courses/CENG334 Operating System Overview Topics • Brief History • OS Services • System calls • Basic Operation • OS structures CENG334Introduction to Operating Systems Some of the following slides are adapted from Matt Welsh, Harvard Univ.

2. In the Beginning... • There was no OS – just libraries • Computer only ran one program at a time, so no need for an OS • Programming through wiring.. ENIAC, 1945 Harvard Mark I, 1944 IBM 360, 1960's

3. In the Beginning... • There was no OS – just libraries • Computer only ran one program at a time, so no need for an OS • And then there were batch systems • Programs printed on stacks of punchhole cards • OS was resident in a portion of machine memory • When previous program was finished, OS loaded next program to run

4. Punch Card

5. In the Beginning... • There was no OS – just libraries • Computer only ran one program at a time, so no need for an OS • And then there were batch systems • Programs printed on stacks of punchhole cards • OS was resident in a portion of machine memory • When previous program was finished, OS loaded next program to run

6. In the Beginning... • There was no OS – just libraries • Computer only ran one program at a time, so no need for an OS • And then there were batch systems • Programs printed on stacks of punchhole cards • OS was resident in a portion of machine memory • When previous program was finished, OS loaded next program to run • Disk spooling • Disks were much read stack onto disk while previous program is running • With multiple programs on disk, need to decide which to run next! • But, CPU still idle while program accesses a peripheral (e.g., tape or disk!)‏

7. Multiprogramming • To increase system utilization, multiprogramming OS’s were invented • keeps multiple runnable jobs loaded in memory at once • Overlaps I/O of a job with computing of another • While one job waits for I/O to compile, CPU runs instructions from another job • To benefit, need asynchronous I/O devices • need some way to know when devices are done performing I/O • Goal: optimize system throughput • perhaps at the cost of response time… Dennis Ritchie and Ken Thompson at a PDP11, 1971

8. Timesharing • To support interactive use, timesharing OS's were created • multiple terminals connected to one machine • each user has illusion of entire machine to him/herself • optimize response time, perhaps at the cost of throughput • Timeslicing • divide CPU fairly among the users • if job is truly interactive (e.g. editor), then can switch between programs and users faster than users can generate load • MIT Multics (mid-1960’s) was the first large timeshared system • nearly all modern OS concepts can be traced back to Multics

9. Personal Computing Apple I, 1976 • Totally changed the computing industry. • CP/M: First personal computer OS • IBM needed OS for their PCs, CP/M behind schedule • Bill Gates to the rescue: Bought 86-DOS and made MS-DOS • DOS is basically a subroutine library! • Many popular personal computers follow • Apple, Commodore, TRS-80, TI 99/4, Atari, etc... Apple LISA, 1983 IBM PC, 1981 Bill Gates and Paul Allen, c.1975 Commodore VIC-20

11. Parallel Computing and Clusters • High-end scientific apps want to use many CPUs at once • Parallel processing to crunch on enormous data sets • Need OS and language primitives for dividing program into parallel activities • Need OS primitives for fast communication between processors • degree of speedup dictated by communication/computation ratio • Many kinds of parallel machines: • SMPs: symmetric multiprocessors – several CPUs accessing the same memory • MPPs: massively parallel processors – each CPU may have its own memory • Clusters: connect a lot of commodity machines with a fast network

12. Distributed OS • Goal – Make use of geographically distributed resources • workstations on a LAN • servers across the Internet • Supports communication between applications • interprocess communication (on a single machine): • message passing and shared memory • networking procotols (across multiple machines): • TCP/IP, Java RMI, .NET SOAP • “The Grid”, .NET, and OGSA • Idea: Seamlessly connect vast computational resources across the Internet

13. Embedded OS • The rise of tiny computers everywhere – ubiquitous computing • Processor cost low enough to embed in many devices • PDAs, cell phones, pagers, ... • How many CPUs are in your car? On your body right now? • Gets more interesting with ubiquitous networking! • Wireless networks becoming pervasive • Sensor networks are an exciting new direction here • Little “motes” with less 4KB of RAM, some sensors, and a radio • Typically very constrained hardware resources • slow processors • very small amount of memory (e.g. 8 MB)‏ • no disk – but maybe quasi-permanent storage such as EEPROM

14. Operating System Overview User application User application User application Protection boundary Kernel Memory management Process management Filesystem TCP/IP stack Accounting CPU support Device drivers Disk I/O Hardware/software interface

15. Operating System Services(What things does the OS do?)‏ • Services that (more-or-less) map onto components • Program execution • How do you execute concurrent sequences of instructions? • I/O operations • Standardized interfaces to extremely diverse devices • File system manipulation • How do you read/write/preserve files? • Looming concern: How do you even find files??? • Communications • Networking protocols/Interface with CyberSpace? • User interface- Almost all operating systems have a user interface (UI)‏ • Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch • Cross-cutting capabilities • Error detection & recovery • Resource allocation • Accounting • Protection

16. User Operating System Interface - CLI • CLI allows direct command entry • Sometimes implemented in kernel, sometimes by systems programs • Sometimes multiple flavors implemented – shells • Primarily fetches a command from user and executes it • Sometimes commands built-in, sometimes just names of programs • If the latter, adding new features doesn’t require shell modification

17. User Operating System Interface - GUI • User-friendly desktop metaphor interface • Usually mouse, keyboard, and monitor • Icons represent files, programs, actions, etc • Various mouse buttons over objects in the interface cause various actions • provide information, options, • execute function, open directory (known as a folder) • Invented at Xerox PARC • Many systems now include both CLI and GUI interfaces • Microsoft Windows is GUI with CLI “command” shell • Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and shells available • Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)

18. Xerox PARC Alto

19. System Calls • Programming interface to the services provided by the OS • Typically written in a high-level language (C or C++) • Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use • Three most common APIs are • Win32 API for Windows, • POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and • Java API for the Java virtual machine (JVM) • Why use APIs rather than system calls?

20. Example of Standard API • Consider the ReadFile() function in the • Win32 API—a function for reading from a file • A description of the parameters passed to ReadFile()‏ • HANDLE file—the file to be read • LPVOID buffer—a buffer where the data will be read into and written from • DWORD bytesToRead—the number of bytes to be read into the buffer • LPDWORD bytesRead—the number of bytes read during the last read • LPOVERLAPPED ovl—indicates if overlapped I/O is being used

21. System Call Implementation • Typically, a number associated with each system call • System-call interface maintains a table indexed according to these numbers • The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values • The caller need know nothing about how the system call is implemented • Just needs to obey API and understand what OS will do as a result call • Most details of OS interface hidden from programmer by API Managed by run-time support library (set of functions built into libraries included with compiler)

22. API – System Call – OS Relationship

23. System Programs • System programs provide a convenient environment for program development and execution. They can be divided into: • File manipulation • Status information • File modification • Programming language support • Program loading and execution • Communications • Application programs • Most users’ view of the operation system is defined by system programs, not the actual system calls

24. System Programs • Provide a convenient environment for program development and execution • Some of them are simply user interfaces to system calls; others are considerably more complex • File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories • Status information • Some ask the system for info - date, time, amount of available memory, disk space, number of users • Others provide detailed performance, logging, and debugging information • Typically, these programs format and print the output to the terminal or other output devices • Some systems implement a registry - used to store and retrieve configuration information

25. System Programs (cont’d)‏ • File modification • Text editors to create and modify files • Special commands to search contents of files or perform transformations of the text • Programming-language support - Compilers, assemblers, debuggers and interpreters sometimes provided • Program loading and execution- Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language • Communications - Provide the mechanism for creating virtual connections among processes, users, and computer systems • Allow users to send messages to one another’s screens, browse web pages, send electronic-mail messages, log in remotely, transfer files from one machine to another

26. Memory Emacs Firefox xmms sshd Operating System operation • The OS kernel is just a bunch of code that sits around in memory, waiting to be executed OS Kernel (device drivers, file systems, virtual memory, etc.)‏

27. Interrupt (disk block read)‏ System call (open network socket)‏ Operating System operation • The OS kernel is just a bunch of code that sits around in memory, waiting to be executed • OS is triggered in two ways: system calls and hardware interrupts • System call: Direct “call” from a user program • For example, open() to open a file, or exec() to run a new program • Hardware interrupt: Trigger from some hardware device • For example, when a disk block has been read or written Memory Emacs OS Kernel (device drivers, file systems, virtual memory, etc.)‏ Firefox xmms sshd

28. Interrupts – a primer • An interrupt is a signal that causes the CPU to jump to a pre-defined instruction – called the interrupt handler • Interrupt can be caused by hardware or software • Hardware interrupt examples • Timer interrupt (periodic “tick” from a programmable timer) • Device interrupts • e.g., Disk will interrupt the CPU when an I/O operation has completed • Software interrupt examples (also called exceptions) • Division by zero error • Access to a bad memory address • Intentional software interrupt – e.g., x86 “INT” instruction • Can be used to trap from user program into the OS kernel! • Why might this be useful?

29. Interrupt handler for interrupt 4 Interrupt handler for interrupt 5 !!! Interrupt handler example 1) OS fills in interrupt handlertable (usually at boot time)‏ Interrupt handler table 2) Interrupt occurs – e.g., hardwaresignal 3) CPU state saved to stack

30. Interrupt handler for interrupt 4 Interrupt handler for interrupt 4 Interrupt handler for interrupt 5 Interrupt handler example 1) OS fills in interrupt handlertable (usually at boot time)‏ Interrupt handler table 2) Interrupt occurs – e.g., hardwaresignal !!! 3) CPU state saved to stack 4) CPU consults interrupt table and invokes appropriate handler

31. Protection • A major job of the OS is to enforce protection • Prevent malicious or buggy programs from: • Allocating too many resources (denial of service)‏ • Corrupting or overwriting shared resources (files, shared memory, etc.)‏ • Prevent different users, groups, etc. from: • Accessing or modifying private state (files, shared memory, etc.)‏ • Killing each other's processes • How does the OS enforce protection boundaries?

32. Enforcing Resource Limits • The OS limits what resources user programs can access • For example, Emacs can't modify memory in use by Mozilla. • xmms can't hog the CPU and prevent other programs from running. • One user cannot read/write another user's files • (Unless permissions are set appropriately)‏ • How does the OS enforce these limits? • This implies that regular user programs cannot “break out” of these limits! • We'll see how on the next slide. • A lot of viruses, worms, etc. exploit security holes in the OS • Overrunning a memory buffer in the kernel can give a non-root process root privileges • Kernel code needs to be rock solid in order to be secure!!!

33. User mode vs. kernel mode • What makes the kernel different from user programs? • Kernel can execute special privileged instructions • Examples of privileged instructions: • Access I/O devices • Poll for IO, perform DMA, catch hardware interrupt • Manipulate memory management • Set up page tables, load/flush the TLB and CPU caches, etc. • Configure various “mode bits” • Interrupt priority level, software trap vectors, etc. • Call halt instruction • Put CPU into low-power or idle state until next interrupt • These are enforced by the CPU hardware itself. • CPU has at least two protection levels: Kernel mode and user mode • CPU checks current protection level on each instruction • What happens if user program tries to execute a privileged instruction?

34. Restore app registers Return CPU to user mode Trap to kernel mode Save application registers and state Return to trap handler Lookup read() in system call table Invoke internal read() function Boundary Crossing Mozilla calls read() system call User mode Kernel mode Kernel trap handler read() system call Perform internal read()‏

35. Web surfing homework for Wednesday! • Learn • More about XEROX PARC • What else had they invented • More about Ken Thomson and Dennis Ritchie • What are they known for • More about Microsoft • How did MS-DOS become so successful? • More about Apple • What’s the relation between XEROX PARC GUI and Apple GUI? • Use Wikipedia, and google the web..

36. OS design and implementation • There is no ultimate OS that would satisfy all requirements: • Trade-offs have to made at each level and for all aspects. • Important principle to separate • Policy: What will be done? • Mechanism: How to do it? • Mechanisms determine how to do something, policies decide what will be done • The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later

37. Operating Systems Structure(What is the organizational Principle?)‏ • Simple • Only one or two levels of code • Layered • Lower levels independent of upper levels • Microkernel • OS built from many user-level processes • Modular • Core kernel with Dynamically loadable modules

38. Simple Structure • MS-DOS – written to provide the most functionality in the least space • Not divided into modules • Interfaces and levels of functionality not well separated

39. Monolithic Kernels • Most common OS kernel design (used in UNIX and Linux)‏ • Kernel code is privileged and lives in its own address space • User applications are unprivileged and live in their own separate address spaces • All kernel functions loaded into memory as one large, messy program • Pros and cons??? User application User application User application System call Protection boundary Kernel Memory management Process management Filesystem TCP/IP stack Accounting CPU support Device drivers Disk I/O

40. Monolithic Kernels • Most common OS kernel design • Kernel code is privileged and lives in its own address space • User applications are unprivileged and live in their own separate address spaces • All kernel functions loaded into memory as one large, messy program • Pros and cons • Pro: Overhead of module interactions within the kernel is low (function call)‏ • Pro: Kernel modules can directly share memory • Con: Very complicated and difficult to organize • Con: A bug in any part of the kernel can crash the whole system! User application User application User application System call Protection boundary Kernel Memory management Process management Filesystem TCP/IP stack Accounting CPU support Device drivers Disk I/O

41. Layered kernels • Operating system is divided many layers (levels) • Each built on top of lower layers • Bottom layer (layer 0) is hardware • Highest layer (layer N) is the user interface • Each layer uses functions (operations) and services of only lower-level layers • Advantage: modularity  Easier debugging/Maintenance • Not always possible: Does process scheduler lie above or below virtual memory layer? • Need to reschedule processor while waiting for paging • May need to page in information about tasks • Important: Machine-dependent vs independent layers • Easier migration between platforms • Easier evolution of hardware platform

42. User application User application User application Memory management Process management Filesystem TCP/IP stack Accounting CPU support Device drivers Disk I/O Microkernel Microkernels • Use a very small, minimal kernel, and implement all other functionality as user level “servers” • Kernel only knows how to perform lowest-level hardware operations • Device drivers, filesystems, virtual memory, etc. all implemented on top • Use inter-process procedure call (IPC) to communicate between applications and servers • Pros and Cons??? Inter-process procedure call Protection boundary

43. Microkernels - 2 • Pros and cons • Pro: Kernel is small and simple, servers are protected from each other • Con: Overhead of invoking servers may be very high • e.g., A user process accessing a file may require inter-process communication through 2 or 3 servers! • Microkernels today • Lots of research in late 80's and early 90's • Windows NT uses “modified microkernel”: • Monolithic kernel for most things, OS APIs (DOS, Win3.1, Win32, POSIX) implemented as user-level services • Mac OS X has reincarnated the microkernel architecture as well: • Gnarly hybrid of Mach (microkernel) and FreeBSD (monolithic)

44. Modules-based Structure • Most modern operating systems implement modules • Uses object-oriented approach • Each core component is separate • Each talks to the others over known interfaces • Each is loadable as needed within the kernel • Overall, similar to layers but more flexible

45. Virtual Machines • A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware • A virtual machine provides an interface identical to the underlying bare hardware • The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory parallels.com Virtualbox.org Vmware.com

46. VMware Architecture

47. Virtualization

48. The Java Virtual Machine