1. 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 Some of the following slides are adapted from Matt Welsh, Harvard Univ.

2. 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.. Harvard Mark I, 1944 IBM 360, 1960's ENIAC, 1945

3. 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. 4 Punch Card

5. 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. 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. 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. 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. 9 Personal Computing 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... Bill Gates and Paul Allen, c.1975 Commodore VIC-20 IBM PC, 1981 Apple LISA, 1983 Apple I, 1976

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11. 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. 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. 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. 14 Operating System Overview User application Kernel Memory management Disk I/O Process management Device drivers Filesystem TCP/IP stack Accounting CPU support Protection boundary Hardware/software interface

15. 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. 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. 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. 18 Xerox PARC Alto

19. 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. 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. 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. 22 API – System Call – OS Relationship

23. 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. 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. 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. 26 Memory 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.)‏ Emacs Firefox xmms sshd

27. 27 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 OS Kernel (device drivers, file systems, virtual memory, etc.)‏ Memory Emacs Firefox xmms sshd System call (open network socket)‏ Interrupt (disk block read)‏

28. 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. 29 Interrupt handler example Interrupt handler table Interrupt handler for interrupt 4 Interrupt handler for interrupt 5 1) OS fills in interrupt handler table (usually at boot time) ‏ 2) Interrupt occurs – e.g., hardware signal !!! 3) CPU state saved to stack

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

31. 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. 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. 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. 34 Boundary Crossing Kernel mode User mode Mozilla calls read() system call Kernel trap handler read() system call Perform internal read()‏ Trap to kernel mode Save application registers and state Lookup read() in system call table Invoke internal read() function Return to trap handler Restore app registers Return CPU to user mode

35. 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. 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. 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. 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. 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 System call User application Kernel Memory management Disk I/O Process management Device drivers Filesystem TCP/IP stack Accounting CPU support Protection boundary

40. 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 System call User application Kernel Memory management Disk I/O Process management Device drivers Filesystem TCP/IP stack Accounting CPU support Protection boundary

41. 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. 42 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??? User application Protection boundary Microkernel Memory management Disk I/O Process management Filesystem TCP/IP stack Accounting CPU support Device drivers Inter-process procedure call

43. 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. 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. 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 Virtualbox.org Vmware.com parallels.com

46. 46 VMware Architecture

47. 47 Virtualization

48. 48 The Java Virtual Machine