12.1Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCSMC 412CSMC 412Operating SystemsProf. Ashok K Agrawala© 2005 Ashok AgrawalaSet 22.2Silberschatz, Galvin and Gagne ©2005Operating System ConceptsComputerComputer--System StructuresSystem Structures Operating System Services User Operating System Interface System Calls Types of System Calls System Programs Operating System Design and Implementation Operating System Structure Virtual Machines Operating System Generation System Boot22.3Silberschatz, Galvin and Gagne ©2005Operating System ConceptsObjectivesObjectives To describe the 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 boot2.4Silberschatz, Galvin and Gagne ©2005Operating System ConceptsOperating System ServicesOperating System Services One set of operating-system services provides functions that are helpful to the user:z User interface - Almost all operating systems have a user interface (UI) Varies between Command-Line (CLI), Graphics User Interface (GUI), Batchz 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)z I/O operations - A running program may require I/O, which may involve a file or an I/O device. z File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories, create and delete them, search them, list file Information, permission managementz 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)z 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 system32.5Silberschatz, Galvin and Gagne ©2005Operating System ConceptsOperating System Services (Cont.)Operating System Services (Cont.) Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharingz Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them Many types of resources - Some (such as CPU cycles,mainmemory, and file storage) may have special allocation code, others (such as I/O devices) may have general request and release code. z Accounting - To keep track of which users use how much and what kinds of computer resourcesz Protection and security - The owners of information stored in a multiuseror 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 If a system is to be protected and secure, precautions must be instituted throughout it. A chain is only as strong as its weakest link.2.6Silberschatz, Galvin and Gagne ©2005Operating System ConceptsUser Operating System Interface User Operating System Interface --CLICLI Two principle forms:z CLI 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 modification42.7Silberschatz, Galvin and Gagne ©2005Operating System ConceptsUser Operating System Interface User Operating System Interface --GUIGUI User-friendly desktop metaphor interfacez Usually mouse, keyboard, and monitorz Icons represent files, programs, actions, etcz Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function,open directory (known as a folder)z Invented at Xerox PARC Many systems now include both CLI and GUI interfacesz Microsoft Windows is GUI with CLI “command” shellz Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and shells availablez Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)2.8Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSystem CallsSystem 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?(Note that the system-call names used throughout this text are generic)52.9Silberschatz, Galvin and Gagne ©2005Operating System ConceptsExample of System CallsExample of System Calls System call sequence to copy the contents of one file to another file2.10Silberschatz, Galvin and Gagne ©2005Operating System ConceptsExample of Standard APIExample 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()z HANDLE file—the file to be readz LPVOID buffer—a buffer where the data will be read into and written fromz DWORD bytesToRead—the number of bytes to be read into the bufferz LPDWORD bytesRead—the number of bytes read during the last readz LPOVERLAPPED ovl—indicates if overlapped I/O is being used62.11Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSystem Call ImplementationSystem Call Implementation Typically, a number associated with each system callz 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
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