Unit I OS CS.ppt
- 1. Unit -I 1
- 2. Unit I Introduction: Operating Systems - Operating-System Structure - Operating System operations. Operating System Structures: Operating System Services - User and Operating System Interface - System Calls - System Programs - Operating System Design and Implementation - Operating System Debugging- Operating System Generation - Types of System Calls. 2
- 4. What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals: Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner 4
- 5. Computer System Structure Computer system can be divided into four components: Hardware – provides basic computing resources CPU, memory, I/O devices Operating system Controls and coordinates use of hardware among various applications and users Application programs – define the ways in which the system resources are used to solve the computing problems of the users Word processors, compilers, web browsers, database systems, video games Users People, machines, other computers 5
- 6. Four Components of a Computer System Figure 1. Four components of a Computer System 6
- 7. What Operating Systems Do? • Depends on the point of view • Users want convenience, ease of use and good performance ▫ Don’t care about resource utilization • But shared computer such as mainframe or minicomputer must keep all users happy • Users of dedicate systems such as workstations have dedicated resources but frequently use shared resources from servers • Handheld computers are resource poor, optimized for usability and battery life • Some computers have little or no user interface, such as embedded computers in devices and automobiles 7
- 8. Operating System Definition • OS is a resource allocator ▫ Manages all resources ▫ Decides between conflicting requests for efficient and fair resource use • OS is a control program ▫ Controls execution of programs to prevent errors and improper use of the computer 8
- 9. Operating System Definition (Cont.) • No universally accepted definition • “Everything a vendor ships when you order an operating system” is a good approximation ▫ But varies wildly • “The one program running at all times on the computer” is the kernel. • Everything else is either ▫ a system program (ships with the operating system) , or ▫ an application program. 9
- 10. Computer Startup • bootstrap program is loaded at power-up or reboot ▫ Typically stored in ROM or EPROM, generally known as firmware ▫ Initializes all aspects of system ▫ Loads operating system kernel and starts execution 10
- 11. Computer System Organization • Computer-system operation ▫ One or more CPUs, device controllers connect through common bus providing access to shared memory ▫ Concurrent execution of CPUs and devices competing for memory cycles Figure 2. Computer System Organization 11
- 12. Storage Structure • Main memory – only large storage media that the CPU can access directly ▫ Random access ▫ Typically volatile • Secondary storage – extension of main memory that provides large nonvolatile storage capacity • Hard disks – rigid metal or glass platters covered with magnetic recording material ▫ Disk surface is logically divided into tracks, which are subdivided into sectors ▫ The disk controller determines the logical interaction between the device and the computer • Solid-state disks – faster than hard disks, nonvolatile ▫ Various technologies ▫ Becoming more popular 12
- 13. Storage Hierarchy • Storage systems organized in hierarchy ▫ Speed ▫ Cost ▫ Volatility • Caching – copying information into faster storage system; main memory can be viewed as a cache for secondary storage • Device Driver for each device controller to manage I/O ▫ Provides uniform interface between controller and kernel 13
- 14. Storage-Device Hierarchy Figure 4. Storage-Device Hierarchy 14
- 15. Operating System Structure • Multiprogramming (Batch system) needed for efficiency ▫ Single user cannot keep CPU and I/O devices busy at all times ▫ Multiprogramming organizes jobs (code and data) so CPU always has one to execute ▫ A subset of total jobs in system is kept in memory ▫ One job selected and run via job scheduling ▫ When it has to wait (for I/O for example), OS switches to another job • Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing ▫ Response time should be < 1 second ▫ Each user has at least one program executing in memory process ▫ If several jobs ready to run at the same time CPU scheduling ▫ If processes don’t fit in memory, swapping moves them in and out to run ▫ Virtual memory allows execution of processes not completely in memory 15
- 16. Memory Layout for Multi programmed System Figure 9. Memory layout 16
- 17. Operating-System Operations • Interrupt driven (hardware and software) ▫ Hardware interrupt by one of the devices ▫ Software interrupt (exception or trap): Software error (e.g., division by zero) Request for operating system service Other process problems include infinite loop, processes modifying each other or the operating system 17
- 18. Operating-System Operations (cont.) • Dual-mode operation allows OS to protect itself and other system components ▫ User mode and kernel mode ▫ Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged, only executable in kernel mode System call changes mode to kernel, return from call resets it to user • Increasingly CPUs support multi-mode operations ▫ i.e. virtual machine manager (VMM) mode for guest VMs 18
- 19. Transition from User to Kernel Mode • Timer to prevent infinite loop / process hogging resources ▫ Timer is set to interrupt the computer after some time period ▫ Keep a counter that is decremented by the physical clock. ▫ Operating system set the counter (privileged instruction) ▫ When counter zero generate an interrupt ▫ Set up before scheduling process to regain control or terminate program that exceeds allotted time Figure 10. Transition from user to Kernel mode 19
- 20. Activity Case Study on Computer Operating System Hint :MS-Windows, Ubuntu, Mac OS, Fedora, Solaris, Free BSD, Chrome OS
- 22. Operating System Services • Operating systems provide an environment for execution of programs and services to programs and users • One set of operating-system services provides functions that are helpful to the user: ▫ User interface - Almost all operating systems have a user interface (UI). Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch ▫ Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error) ▫ I/O operations - A running program may require I/O, which may involve a file or an I/O device 22
- 23. Operating System Services (Cont.) ▫ File-system manipulation - The file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management. ▫ Communications – Processes may exchange information, on the same computer or between computers over a network Communications may be via shared memory or through message passing (packets moved by the OS) ▫ Error detection – OS needs to be constantly aware of possible errors May occur in the CPU and memory hardware, in I/O devices, in user program For each type of error, OS should take the appropriate action to ensure correct and consistent computing Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system 23
- 24. Operating System Services (Cont.) • Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing ▫ Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them Many types of resources - CPU cycles, main memory, file storage, I/O devices. ▫ Accounting - To keep track of which users use how much and what kinds of computer resources ▫ Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other Protection involves ensuring that all access to system resources is controlled Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts 24
- 25. A View of Operating System Services Figure 15. View of OS 25
- 26. User and Operating System Interface - CLI CLI or command interpreter allows direct command entry ▫ Sometimes implemented in kernel, sometimes by systems program ▫ 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 26
- 27. Bourne Shell Command Interpreter 27
- 28. User and 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 is “Aqua” GUI interface with UNIX kernel underneath and shells available ▫ Unix and Linux have CLI with optional GUI interfaces (CDE, KDE(Kool Desktop environment),GNOME(GNU Network Object Model Environment)) 28
- 29. Touch screen Interfaces n Touchscreen devices require new interfaces l Mouse not possible or not desired l Actions and selection based on gestures l Virtual keyboard for text entry l Voice commands. Figure 16. Touch screen Interfaces 29
- 30. The Mac OS X GUI Figure 17.The max OS X GUI 30
- 31. 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 Programming 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) 31
- 32. Example of System Calls • System call sequence to copy the contents of one file to another file Figure 18. Example of system calls 32
- 33. Example of Standard API Figure 19. Example of Standard API 33
- 34. 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 the 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) 34
- 35. API – System Call – OS Relationship Figure 20. API 35
- 36. System Call Parameter Passing • Often, more information is required than simply identity of desired system call ▫ Exact type and amount of information vary according to OS and call • Three general methods used to pass parameters to the OS ▫ Simplest: pass the parameters in registers In some cases, may be more parameters than registers ▫ Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register This approach taken by Linux and Solaris ▫ Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system ▫ Block and stack methods do not limit the number or length of parameters being passed 36
- 37. Parameter Passing via Table Figure 21. Parameter passing via table 37
- 38. Types of System Calls • Process control ▫ create process, terminate process ▫ end, abort ▫ load, execute ▫ get process attributes, set process attributes ▫ wait for time ▫ wait event, signal event ▫ allocate and free memory ▫ Dump memory if error ▫ Debugger for determining bugs, single step execution ▫ Locks for managing access to shared data between processes 38
- 39. Types of System Calls • File management ▫ create file, delete file ▫ open, close file ▫ read, write, reposition ▫ get and set file attributes • Device management ▫ request device, release device ▫ read, write, reposition ▫ get device attributes, set device attributes ▫ logically attach or detach devices 39
- 40. Types of System Calls (Cont.) • Information maintenance ▫ get time or date, set time or date ▫ get system data, set system data ▫ get and set process, file, or device attributes • Communications ▫ create, delete communication connection ▫ send, receive messages if message passing model to host name or process name From client to server ▫ Shared-memory model create and gain access to memory regions ▫ transfer status information ▫ attach and detach remote devices 40
- 41. Types of System Calls (Cont.) • Protection ▫ Control access to resources ▫ Get and set permissions ▫ Allow and deny user access 41
- 42. Examples of Windows and Unix System Calls 42
- 43. Standard C Library Example • C program invoking printf() library call, which calls write() system call Figure 22. Standard C library Example 43
- 44. Example: MS-DOS • Single-tasking • Shell invoked when system booted • Simple method to run program ▫ No process created • Single memory space • Loads program into memory, overwriting all but the kernel • Program exit -> shell reloaded At system startup running a program 44
- 45. Example: FreeBSD • Unix variant • Multitasking • User login -> invoke user’s choice of shell • Shell executes fork() system call to create process ▫ Executes exec() to load program into process ▫ Shell waits for process to terminate or continues with user commands • Process exits with: ▫ code = 0 – no error ▫ code > 0 – error code Figure 23. Free BSD 45
- 46. System Programs • System programs provide a convenient environment for program development and execution. They can be divided into: ▫ File manipulation ▫ Status information sometimes stored in a File modification ▫ Programming language support ▫ Program loading and execution ▫ Communications ▫ Background services ▫ Application programs • Most users’ view of the operation system is defined by system programs, not the actual system calls 46
- 47. 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 47
- 48. System Programs (Cont.) • 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 48
- 49. System Programs (Cont.) • Background Services ▫ Launch at boot time Some for system startup, then terminate Some from system boot to shutdown ▫ Provide facilities like disk checking, process scheduling, error logging, printing ▫ Run in user context not kernel context ▫ Known as services, subsystems, daemons • Application programs ▫ Don’t pertain to system ▫ Run by users ▫ Not typically considered part of OS ▫ Launched by command line, mouse click, finger poke 49
- 50. Operating System Design and Implementation • Design and Implementation of OS not “solvable”, but some approaches have proven successful • Internal structure of different Operating Systems can vary widely • Start the design by defining goals and specifications • Affected by choice of hardware, type of system • User goals and System goals ▫ User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast ▫ System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient 50
- 51. Operating System Design and Implementation (Cont.) • 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 (example – timer) • Specifying and designing an OS is highly creative task of software engineering 51
- 52. Implementation • Much variation ▫ Early OSes in assembly language ▫ Then system programming languages like Algol, PL/1 ▫ Now C, C++ • Actually usually a mix of languages ▫ Lowest levels in assembly ▫ Main body in C ▫ Systems programs in C, C++, scripting languages like PERL, Python, shell scripts • More high-level language easier to port to other hardware ▫ But slower • Emulation can allow an OS to run on non-native hardware 52
- 53. Operating System Structure • General-purpose OS is very large program • Various ways to structure ones ▫ Simple structure – MS-DOS ▫ More complex -- UNIX ▫ Layered – an abstraction ▫ Microkernel -Mac 53
- 54. Simple Structure -- MS-DOS • MS-DOS – written to provide the most functionality in the least space ▫ Not divided into modules ▫ Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated Figure 24. Simple structure – MS DOS 54
- 55. Non Simple Structure -- UNIX UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts ▫ Systems programs ▫ The kernel Consists of everything below the system-call interface and above the physical hardware Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level 55
- 56. Traditional UNIX System Structure Beyond simple but not fully layered Figure 25. Traditional UNIX System Structure 56
- 57. Layered Approach • The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. • With modularity, layers are selected such that each uses functions (operations) and services of only lower- level layers Figure 26 . Layered Approach 57
- 58. Microkernel System Structure • Moves as much from the kernel into user space • Mach example of microkernel ▫ Mac OS X kernel (Darwin) partly based on Mach • Communication takes place between user modules using message passing • Benefits: ▫ Easier to extend a microkernel ▫ Easier to port the operating system to new architectures ▫ More reliable (less code is running in kernel mode) ▫ More secure • Detriments: ▫ Performance overhead of user space to kernel space communication 58
- 59. Microkernel System Structure Figure 27. Microkernel System Structure 59
- 60. Modules • Many modern operating systems implement loadable kernel 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 with more flexible ▫ Linux, Solaris, etc 60
- 61. Solaris Modular Approach Figure 28. Solaris Modular Approach 61
- 62. Hybrid Systems • Most modern operating systems are actually not one pure model ▫ Hybrid combines multiple approaches to address performance, security, usability needs ▫ Linux and Solaris kernels in kernel address space, so monolithic, plus modular for dynamic loading of functionality ▫ Windows mostly monolithic, plus microkernel for different subsystem personalities • Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment ▫ Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions) 62
- 63. Mac OS X Structure graphical user interface Aqua application environments and services kernel environment Java Cocoa Quicktime BSD Mach I/O kit kernel extensions BSD Figure 29. Mac OXS X Structure 63
- 64. iOS • Apple mobile OS for iPhone, iPad ▫ Structured on Mac OS X, added functionality ▫ Does not run OS X applications natively Also runs on different CPU architecture (ARM vs. Intel) ▫ Cocoa Touch Objective-C API for developing apps ▫ Media services layer for graphics, audio, video ▫ Core services provides cloud computing, databases ▫ Core operating system, based on Mac OS X kernel 64
- 65. Android • Developed by Open Handset Alliance (mostly Google) ▫ Open Source • Similar stack to IOS • Based on Linux kernel but modified ▫ Provides process, memory, device-driver management ▫ Adds power management • Runtime environment includes core set of libraries and Dalvik virtual machine ▫ Apps developed in Java plus Android API Java class files compiled to Java bytecode then translated to executable than runs in Dalvik VM • Libraries include frameworks for web browser (webkit), database (SQLite), multimedia, smaller libc 65
- 66. Android Architecture Applications Application Framework Android runtime Core Libraries Dalvik virtual machine Libraries Linux kernel SQLite openGL surface manager webkit libc media framework Figure 30. Android Architecture 66
- 67. Operating-System Debugging • Debugging is finding and fixing errors, or bugs • OS generate log files containing error information • Failure of an application can generate core dump file capturing memory of the process • Operating system failure can generate crash dump file containing kernel memory • Beyond crashes, performance tuning can optimize system performance ▫ Sometimes using trace listings of activities, recorded for analysis ▫ Profiling is periodic sampling of instruction pointer to look for statistical trends Kernighan’s Law: “Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.” 67
- 68. Performance Tuning • Improve performance by removing bottlenecks • OS must provide means of computing and displaying measures of system behavior • For example, “top” program or Windows Task Manager Figure 31. Performance Tuning 68
- 69. DTrace DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation on production systems Probes fire when code is executed within a provider, capturing state data and sending it to consumers of those probes Example of following XEventsQueued system call move from libc library to kernel and back Figure 32. DTrace 69
- 70. Dtrace (Cont.) DTrace code to record amount of time each process with UserID 101 is in running mode (on CPU) in nanoseconds 70
- 71. Operating System Generation Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site SYSGEN program obtains information concerning the specific configuration of the hardware system Used to build system-specific compiled kernel or system-tuned Can general more efficient code than one general kernel 71