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OS Services, System call, Virtual Machine
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- 2. Contents: Operating SystemServices User Operating System Interface System Calls Types of System Calls System Programs Operating System Design and Implementation Operating System Structure Virtual Machine
- 3. Objectives To describethe services an operating system provides to users, processes, and other systems To discuss the various ways of structuring an operating system To explain how operating systems are installed and customized and how they boot
- 4. 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
- 5. Simple Batch OS SimpleBatch OS
- 6. Batch Interface BatchInterface
- 7. Operating System Services(Cont.) One set of operating-system services provides functions that are helpful to the user (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
- 8. 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
- 9. A View ofOperating System Services
- 10. User Operating SystemInterface - 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
- 11. Bourne Shell CommandInterpreter
- 12. User Operating SystemInterface - 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, GNOME)
- 13. Touchscreen Interfaces Touchscreen devices requirenew interfaces ◦ Mouse not possible or not desired ◦ Actions and selection based on gestures ◦ Virtual keyboard for text entry Voice commands.
- 14. The Mac OSX GUI
- 15. System Calls Programminginterface 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) Note that the system-call names used throughout this text are generic
- 16. Example of SystemCalls System call sequence to copy the contents of one file to another file
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- 18. 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)
- 19. API – SystemCall – OS Relationship
- 20. System Call ParameterPassing 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
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- 22. Types of SystemCalls 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
- 23. Types of SystemCalls 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
- 24. Types of SystemCalls (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
- 25. Types of SystemCalls (Cont.) Protection ◦ Control access to resources ◦ Get and set permissions ◦ Allow and deny user access
- 26. Examples of Windowsand Unix System Calls
- 28. Standard C LibraryExample C program invoking printf() library call, which calls write() system call
- 29. System Programs Systemprograms 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 Most users ’ view of the operation system is defined by system programs, not the actual system calls
- 30. System Programs Providea 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
- 31. 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
- 32. 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
- 33. Operating System Designand 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
- 34. Operating System Designand 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
- 35. 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
- 36. Virtual Machines • Implementsan observation that dates to Turing – One computer can “emulate” another computer – One OS can implement abstraction of a cluster of computers, each running its own OS and applications • Incredibly useful! – System building – Protection • Cons – implementation • Examples – VMWare, JVM
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- 38. • The virtual-machineconcept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.