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Chapter 1: Introduction Chapter 1: Introduction What Operating Systems Do Computer-System Organization Computer-System Architecture Operating-System Structure Operating-System Operations Process Management Memory Management Storage Management Protection and Security Distributed Systems Special-Purpose Systems Computing Environments Operating System Concepts 1.2 Silberschatz, Galvin and Gagne ©2005 Four Components of a Computer System Operating System Concepts 1.3 Silberschatz, Galvin and Gagne ©2005 What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware. The job of the OS is to adapt to hardware. Examples: MS-DOS/Windows, MacOS, Unix, and many more 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. Operating System Concepts 1.4 Silberschatz, Galvin and Gagne ©2005 Computer System Structure Computer system can be divided into four components Hardware – provides basic computing resources 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 CPU, memory, I/O devices Word processors, compilers, web browsers, database systems, video games Users Operating System Concepts People, machines, other computers 1.5 Silberschatz, Galvin and Gagne ©2005 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 Operating System Concepts 1.6 Silberschatz, Galvin and Gagne ©2005 Operating System Definition (Cont.) No universally accepted definition “Everything a vendor ships when you order an operating system” is good approximation (but varies wildly) A Provider of Abstractions A Resource Coordinator A Magician: makes your system appear to be more than it is (more than one processor, more memory) “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 Operating System Concepts 1.7 Silberschatz, Galvin and Gagne ©2005 What is in the OS? What is included in the OS? Given that very few people write operating systems, why study them? OS concepts are relevant throughout computer science. An understanding of OS concepts gives you a firm foundation for work building large software systems. Understanding the OS is key in achieving a deep understanding of the operation of a computer system. This is useful even if you don’t want to write your own OS. Operating System Concepts 1.8 Silberschatz, Galvin and Gagne ©2005 운영체제의 기능 자원관리자 처리기 – 병행성 (프로세스) 저장장치 – 메모리 입출력장치 – 인터럽트 통신장치 – 통신 프로토콜, 분산 시스템 데이터 – 파일시스템 기타기능 사용자 인터페이스 하드웨어의 공유 데이터 공유 에러 회복 계정관리 Operating System Concepts 1.9 Silberschatz, Galvin and Gagne ©2005 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 Operating System Concepts 1.10 Silberschatz, Galvin and Gagne ©2005 OS and Architectures What an OS can do is dictated, at least in part, by the architecture Architecture support can greatly simplify (or complicate) OS tasks Example: PC operating systems have been primitive, in part because PCs lacked hardware support (e.g., for VM) Operating System Concepts 1.11 Silberschatz, Galvin and Gagne ©2005 Architectural Features for OS Features that directly support OS needs include: 1. Timer (clock) operation 2. Synchronization (atomic instructions) 3. Memory protection 4. I/O control and operation 5. Interrupts and exceptions 6. OS protection (kernel/user mode) 7. Protected instructions 8. System calls Operating System Concepts 1.12 Silberschatz, Galvin and Gagne ©2005 Computer-System Operation I/O devices and the CPU can execute concurrently. Each device controller is in charge of a particular device type. Each device controller has a local buffer. CPU moves data from/to main memory to/from local buffers I/O is from the device to local buffer of controller. Device controller informs CPU that it has finished its operation by causing an interrupt. Operating System Concepts 1.13 Silberschatz, Galvin and Gagne ©2005 Common Functions of Interrupts Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines. Interrupt architecture must save the address of the interrupted instruction. Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt. A trap is a software-generated interrupt caused either by an error or a user request. An operating system is interrupt driven. Operating System Concepts 1.14 Silberschatz, Galvin and Gagne ©2005 Interrupt Handling The operating system preserves the state of the CPU by storing registers and the program counter. Determines which type of interrupt has occurred: Separate segments of code determine what action should be taken for each type of interrupt Operating System Concepts 1.15 Silberschatz, Galvin and Gagne ©2005 Interrupt Timeline Operating System Concepts 1.16 Silberschatz, Galvin and Gagne ©2005 Exceptions Hardware must detect special conditions: page fault, write to a read-only page, overflow, trace trap, odd address trap, privileged instruction trap, syscall … Must transfer control to handler within the OS Hardware must save state on fault (PC, etc) so that the faulting process can be restarted afterwards Modern operating systems use VM traps for many functions: debugging, distributed VM, garbage collection, copy-on-write … Exceptions are a performance optimization, i.e., conditions could be detected by inserting extra instructions in the code (at high cost) Operating System Concepts 1.17 Silberschatz, Galvin and Gagne ©2005 I/O Control I/O issues: How to start an I/O (special instructions or memorymapped I/O I/O completion (interrupts) Interrupts are the basis for asynchronous I/O Device controller performs an operation asynch to CPU Device sends an interrupt signal on bus when done In memory is a vector table containing a list of addresses of kernel routines to handle various events CPU switches to address indicated by vector specified by the interrupt signal Operating System Concepts 1.18 Silberschatz, Galvin and Gagne ©2005 I/O Control Device interrupts CPU stops current operation, switches to kernel mode, and saves current PC and other state on kernel stack CPU fetches proper vector from vector table and branches to that address (to routine to handle interrupt) Interrupt routine examines device database and performs action required by interrupt Handler completes operation, restores saved (interrupt state) and returns to user mode (or calls scheduler to switch to another program) Operating System Concepts 1.19 Silberschatz, Galvin and Gagne ©2005 I/O Structure After I/O starts, control returns to user program only upon I/O completion. Wait instruction idles the CPU until the next interrupt Wait loop (contention for memory access). At most one I/O request is outstanding at a time, no simultaneous I/O processing. After I/O starts, control returns to user program without waiting for I/O completion. System call – request to the operating system to allow user to wait for I/O completion. Device-status table contains entry for each I/O device indicating its type, address, and state. Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt. Operating System Concepts 1.20 Silberschatz, Galvin and Gagne ©2005 Two I/O Methods Synchronous Operating System Concepts Asynchronous 1.21 Silberschatz, Galvin and Gagne ©2005 Device-Status Table Operating System Concepts 1.22 Silberschatz, Galvin and Gagne ©2005 Timer Operation How does the OS prevent against runaway user programs (infinite loops) ? A timer can be set to generate an interrupt in a given time Before it transfers to a user program, the OS loads the timer with a time to interrupt When the time arrives, the executing program is interrupted and the OS regains control This ensures that the OS can get the CPU back even if a user program erroneously or purposely continues to execute past some allotted time The timer is privileged: only the OS can load it Operating System Concepts 1.23 Silberschatz, Galvin and Gagne ©2005 Synchronization Interrupts cause potential problems because an interrupt can occur at any time – causing code to execute that interferes with code that was interrupted. OS must be able to synchronize concurrent processes. This involves guaranteeing that short instruction sequences (read-modify-write) execute atomically. One way to guarantee this is to turn off interrupts before the sequence, execute it, and re-enable interrupts; CPU must have a way to disable interrupts. Another is to have special instructions that can perform a read/modify/write in a single cycle, or can atomically test and conditionally set a bit, based on its previous value. Operating System Concepts 1.24 Silberschatz, Galvin and Gagne ©2005 Direct Memory Access Structure Used for high-speed I/O devices able to transmit information at close to memory speeds. Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention. Only on interrupt is generated per block, rather than the one interrupt per byte. Operating System Concepts 1.25 Silberschatz, Galvin and Gagne ©2005 Storage Structure Main memory – only large storage media that the CPU can access directly. Secondary storage – extension of main memory that provides large nonvolatile storage capacity. Magnetic 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. Operating System Concepts 1.26 Silberschatz, Galvin and Gagne ©2005 Storage Hierarchy Storage systems organized in hierarchy. Speed Cost Volatility Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage. Operating System Concepts 1.27 Silberschatz, Galvin and Gagne ©2005 Storage-Device Hierarchy Operating System Concepts 1.28 Silberschatz, Galvin and Gagne ©2005 Caching Use of high-speed memory to hold recently-accessed data. Requires a cache management policy. Caching introduces another level in storage hierarchy. This requires data that is simultaneously stored in more than one level to be consistent. Operating System Concepts 1.29 Silberschatz, Galvin and Gagne ©2005 Caching Important principle, performed at many levels in a computer (in hardware, operating system, software) Information in use copied from slower to faster storage temporarily Faster storage (cache) checked first to determine if information is there If it is, information used directly from the cache (fast) If not, data copied to cache and used there Cache smaller than storage being cached Cache management important design problem Cache size and replacement policy Operating System Concepts 1.30 Silberschatz, Galvin and Gagne ©2005 Performance of Various Levels of Storage Movement between levels of storage hierarchy can be explicit or implicit Operating System Concepts 1.31 Silberschatz, Galvin and Gagne ©2005 Migration of Integer A from Disk to Register Multitasking environments must be careful to use most recent value, not matter where it is stored in the storage hierarchy Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent value in their cache Distributed environment situation even more complex Several copies of a datum can exist Various solutions covered in Chapter 17 Operating System Concepts 1.32 Silberschatz, Galvin and Gagne ©2005 Computer Startup bootstrap program is loaded at power-up or reboot Typically stored in ROM or EEPROM, generally known as firmware Initializates all aspects of system Loads operating system kernel and starts execution Operating System Concepts 1.33 Silberschatz, Galvin and Gagne ©2005 Operating System Structure Multiprogramming 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 Operating System Concepts 1.34 Silberschatz, Galvin and Gagne ©2005 Memory Layout for Multiprogrammed System Operating System Concepts 1.35 Silberschatz, Galvin and Gagne ©2005 OS Features Needed for Multiprogramming I/O routine supplied by the system. Memory management – the system must allocate the memory to several jobs. CPU scheduling – the system must choose among several jobs ready to run. Allocation of devices. Operating System Concepts 1.36 Silberschatz, Galvin and Gagne ©2005 Operating-System Operations Interrupt driven by hardware Software error or request creates exception or trap Division by zero, request for operating system service Other process problems include infinite loop, processes modifying each other or the operating system 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 Operating System Concepts 1.37 Silberschatz, Galvin and Gagne ©2005 Transition from User to Kernel Mode Timer to prevent infinite loop / process hogging resources Set interrupt after specific period Operating system decrements counter When counter zero generate an interrupt Set up before scheduling process to regain control or terminate program that exceeds allotted time Operating System Concepts 1.38 Silberschatz, Galvin and Gagne ©2005 Process Management A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task CPU, memory, I/O, files Initialization data Process termination requires reclaim of any reusable resources Single-threaded process has one program counter specifying location of next instruction to execute Process executes instructions sequentially, one at a time, until completion Multi-threaded process has one program counter per thread Typically system has many processes, some user, some operating system running concurrently on one or more CPUs Concurrency by multiplexing the CPUs among the processes / threads Operating System Concepts 1.39 Silberschatz, Galvin and Gagne ©2005 Process Management Activities The operating system is responsible for the following activities in connection with process management: Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Providing mechanisms for deadlock handling Operating System Concepts 1.40 Silberschatz, Galvin and Gagne ©2005 Memory Management All data in memory before and after processing All instructions in memory in order to execute Memory management determines what is in memory when Optimizing CPU utilization and computer response to users Memory management activities Keeping track of which parts of memory are currently being used and by whom Deciding which processes (or parts thereof) and data to move into and out of memory Allocating and deallocating memory space as needed Operating System Concepts 1.41 Silberschatz, Galvin and Gagne ©2005 Storage Management OS provides uniform, logical view of information storage Abstracts physical properties to logical storage unit - file Each medium is controlled by device (i.e., disk drive, tape drive) Varying properties include access speed, capacity, data-transfer rate, access method (sequential or random) File-System management Files usually organized into directories Access control on most systems to determine who can access what OS activities include Operating System Concepts Creating and deleting files and directories Primitives to manipulate files and dirs Mapping files onto secondary storage Backup files onto stable (non-volatile) storage media 1.42 Silberschatz, Galvin and Gagne ©2005 Mass-Storage Management Usually disks used to store data that does not fit in main memory or data that must be kept for a “long” period of time. Proper management is of central importance Entire speed of computer operation hinges on disk subsystem and its algorithms OS activities Free-space management Storage allocation Disk scheduling Some storage need not be fast Tertiary storage includes optical storage, magnetic tape Still must be managed Varies between WORM (write-once, read-many-times) and RW (read-write) Operating System Concepts 1.43 Silberschatz, Galvin and Gagne ©2005 I/O Subsystem One purpose of OS is to hide peculiarities of hardware devices from the user I/O subsystem responsible for Memory management of I/O including buffering (storing data temporarily while it is being transferred), caching (storing parts of data in faster storage for performance), spooling (the overlapping of output of one job with input of other jobs) General device-driver interface Drivers for specific hardware devices Operating System Concepts 1.44 Silberschatz, Galvin and Gagne ©2005 Hardware Protection Dual-Mode Operation I/O Protection Memory Protection CPU Protection Operating System Concepts 1.45 Silberschatz, Galvin and Gagne ©2005 Use of A System Call to Perform I/O Operating System Concepts 1.46 Silberschatz, Galvin and Gagne ©2005 Memory Protection Must provide memory protection at least for the interrupt vector and the interrupt service routines. In order to have memory protection, add two registers that determine the range of legal addresses a program may access: Base register – holds the smallest legal physical memory address. Limit register – contains the size of the range Memory outside the defined range is protected. Operating System Concepts 1.47 Silberschatz, Galvin and Gagne ©2005 Use of A Base and Limit Register Operating System Concepts 1.48 Silberschatz, Galvin and Gagne ©2005 Hardware Address Protection Operating System Concepts 1.49 Silberschatz, Galvin and Gagne ©2005 Hardware Protection When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory. The load instructions for the base and limit registers are privileged instructions. Operating System Concepts 1.50 Silberschatz, Galvin and Gagne ©2005 CPU Protection Timer – interrupts computer after specified period to ensure operating system maintains control. Timer is decremented every clock tick. When timer reaches the value 0, an interrupt occurs. Timer commonly used to implement time sharing. Time also used to compute the current time. Load-timer is a privileged instruction. Operating System Concepts 1.51 Silberschatz, Galvin and Gagne ©2005 Protection and Security Protection – any mechanism for controlling access of processes or users to resources defined by the OS Security – defense of the system against internal and external attacks Huge range, including denial-of-service, worms, viruses, identity theft, theft of service Systems generally first distinguish among users, to determine who can do what User identities (user IDs, security IDs) include name and associated number, one per user User ID then associated with all files, processes of that user to determine access control Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file Privilege escalation allows user to change to effective ID with more rights Operating System Concepts 1.52 Silberschatz, Galvin and Gagne ©2005 운영체제의 발전 1940 - 1950 Batch processing 1960 Multi programming (System 360) Software 공학의 출현 1970 Multimode timesharing system 1980 Personal computer, Workstation Computer 통신망 1990 년대와 이후 Operating System Concepts Distributed Computing Open Systems 1.53 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 0: Computers are exotic experimental science: No operating system. Use plug-boards to direct computer. User sits at console. All activity is sequential: no overlap between computation, I/O, and user think or response time. People manually load card decks to run programs. Eventually, people developed libraries, used by all users. This was the precursor to the operating system. Problem: Too much waiting. Operating System Concepts The user waits for the machine. The machine waits for user. Everybody waits for card reader. 1.54 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 1: Computers are expensive: People are cheap. Make more efficient use of the computer by decoupling the people’s activity from the computer’s activity. The OS functions as a batch monitor, continually loading a job, running it, and moving on to the next job. If program failed, the OS saves a copy of the memory contents for debugging (circa 1955 – 1965) Better use of hardware, but harder to debug! Data channels and interrupts => overlap I/O and computation. Operating System Concepts – Buffering and interrupts handling is done by OS. – Spool jobs onto drum. 1.55 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 1 (Cont’d) Problems: Hardware to the rescue (again): Add memory protection and relocation Multiprogramming: many users can share the system. Small jobs can complete quickly. OS must manage the interaction between concurrent jobs. IBM OS/360: The first OS designed for a family of computers. Use the same operating system on all machines, smallest to largest. OS becomes a field of study in computer science. People began to study operating systems because they didn’t work. OS/360 is an example. It was announced in 1963 but didn’t really work until 1968. New Problems: Operating System Concepts The OS was enormously complicated. It was all in assembly code. People are still waiting for computers. This motivates Phase 2. 1.56 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 2: Computers are fast; People are slow; both are expensive. Need to help people be more productive. Operating System Concepts Interactive timesharing: let many users use the same machine at once. Give everybody a terminal. Keep data on line: use a structured file systems. Provide reasonable response time (or not: thrashing). 1.57 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 3: Computers are cheap; People are expensive. Give everybody a computer. The personal workstation – The PERQ. – The Sun workstation (Stanford University Network) – The Xerox Alto The Apple II The IBM PC The Macintosh Operating System Concepts 1.58 Silberschatz, Galvin and Gagne ©2005 A Social History of Operating Systems Phase 4: Personal Computers take over the planet. Software vendors produce high quality personal software with limited OS support. Protection and multiprogramming are of secondary importance. Networks (an offshoot of OS research) make electronic mail, bboard, WWW important applications. Client/server architecture. Server machines motivate more advanced OS technology. Problems People are still waiting for computers (see Phase 1). Viruses. Hackers. Operating System Concepts 1.59 Silberschatz, Galvin and Gagne ©2005 A Technical Perspective Timesharing Systems CTS (1962) MIT One of the first timesharing systems Very successful: Motivated MULTICS MULTICS (1965) Joint project between MIT, Bell Labs, and General Electric. Envisioned a computing “utility”. People would buy computing services like electricity. Many fundamental ideas: (protection rings, treestructured file system) Building it was more difficult than expected. Operating System Concepts 1.60 Silberschatz, Galvin and Gagne ©2005 A Technical Perspective Timesharing Systems (Cont’d) UNIX (1970) Operating System Concepts Ken Thompson (from Multics project) found on old PDP-7 lying around Bell Labs. He and Dennis Ritchie built a system designed by programmers for programmers. First in assembly language, then in C Source code made available to universities. Berkeley added virtual memory support. UNIX becomes a commercial operating system. Important ideas popularized by UNIX: – High level language – Portable operating system – Tree structured file system – Many others 1.61 Silberschatz, Galvin and Gagne ©2005 Personal Computers and Workstations Computers are cheap, so don’t bother with terminal. CP/M first PC operating system. IBM needed software for their new PC. CP/M was behind schedule. Asked Microsoft (Bill Gates, BASIC, Unix) to build one. Gates bought 86-DOS from a memory board manufacturer. Primary goal: finish quickly and be compatible with CP/M. OS becomes a subroutine library and command executive. Today: Networks Computing system are made up of personal as well as shared resources. People don’t want to share their processor or memory, but they do want to share electronic mail, news, data bases, programs. Operating System Concepts 1.62 Silberschatz, Galvin and Gagne ©2005 Operating Systems Today Today’s operating systems are: Enormous: if your operating system kernel is under 1 megabyte, it’s considered small. Typically hundreds of thousands of lines of code 100-1000 man years of development effort Complex: Asynchronous. Hardware idiosyncracies (must run on every platform). Different classes of users have different needs. Performance is crucial! Poorly understood: Operating System Concepts The system outlives its creators. Too large for a single person to comprehend. Never fully debugged (OS/360 releases with 1000 bugs). Behavior is hard to predict. Performance tuning often based on hunch. Intuition is often wrong. 1.63 Silberschatz, Galvin and Gagne ©2005 UNIX 의 발전 1965 년 : Bell Lab + GE + MIT (project MAC) MULTICS 개발 착수. 1969 년 : Bell Lab team quit. Thompson 과 Ritchie 가 PDP-7 에 초기 시스템 구현. (어셈블리어) 1971 년 : PDP-11으로 porting. 1973 년 : C 로 다시 작성. (크기가 1/3 증가) 대학에 보급 시작 1974 년 : CACM 에 발표 1977 년 : Interdata 8/32 에 이식 1 BSD 1980 년 : XENIX (Microsoft) 1983 년 : UNIX 시스템 V 지원 개시 4.2 BSD, 4.2 on SUN Operating System Concepts 1.64 Silberschatz, Galvin and Gagne ©2005 UNIX 의 발전 1984 년 : System V Release 2 1989 년 : SVR 4.0 1991 년 : SVR 4.0 MP, LINUX v0.01 1992 년 : SVR 4.0 ES/MP 1993 년 : Novell UNIXWARE 1994 년 : LINUX v1.0 1995 년 : SCO UNIX 1996 년 : LINUX v2.0 1999 년 : LINUX v2.2 Operating System Concepts 1.65 Silberschatz, Galvin and Gagne ©2005 DOS 의 발전 1981 년 : DOS 1.0 CP/M, MP/M $ 40 96 % 차지 1982 (DOS 2.0), 1984 (DOS 3.0), 1986 (DOS 3.2), 1988 (DOS 4.0) 1987 년 : OS/2 80286 용 다중 태스킹 OS DOS 또는 OS/2 dual mode 로 동작 1989 년 : OS/386 (= OS/3) Concurrent DOS session 지원 DOS 는 다양한 S/W 가 존재, OS/2 는 S/W 부족 Operating System Concepts 1.66 Silberschatz, Galvin and Gagne ©2005 DOS 의 발전 1990 년 : Windows 3.0 1991 년 : DOS 5.0 Windows 3.1 1993 년 : DOS 6.0 Windows NT 3.1 1994 년 : Windows NT 3.5 (Daytona) 1995 년 : DOS 7.0 Windows 95 (Chicago) Windows NT (Cairo) 1998 년 : Windows 98 2000 년 : Windows 2000 2001 년 : Windows XP Operating System Concepts 1.67 Silberschatz, Galvin and Gagne ©2005 기종별 운영체제 IBM-PC Windows 98/2000 매킨토시 맥 OS SUN Solaris HP HP-UX IBM MVS, VM, AIX Operating System Concepts 1.68 Silberschatz, Galvin and Gagne ©2005 Computing Environments Traditional computer Blurring over time Office environment PCs connected to a network, terminals attached to mainframe or minicomputers providing batch and timesharing Now portals allowing networked and remote systems access to same resources Home networks Used Now Operating System Concepts to be single system, then modems firewalled, networked 1.69 Silberschatz, Galvin and Gagne ©2005 Computing Environments (Cont.) Client-Server Computing Dumb terminals supplanted by smart PCs Many systems now servers, responding to requests generated by clients Compute-server provides an interface to client to request services (i.e. database) File-server provides interface for clients to store and retrieve files Operating System Concepts 1.70 Silberschatz, Galvin and Gagne ©2005 Peer-to-Peer Computing Another model of distributed system P2P does not distinguish clients and servers Instead all nodes are considered peers May each act as client, server or both Node must join P2P network Registers its service with central lookup service on network, or Broadcast request for service and respond to requests for service via discovery protocol Examples include Napster and Gnutella Operating System Concepts 1.71 Silberschatz, Galvin and Gagne ©2005 Web-Based Computing Web has become ubiquitous PCs most prevalent devices More devices becoming networked to allow web access New category of devices to manage web traffic among similar servers: load balancers Use of operating systems like Windows 95, client-side, have evolved into Linux and Windows XP, which can be clients and servers Operating System Concepts 1.72 Silberschatz, Galvin and Gagne ©2005 Network Structure Local Area Networks (LAN) Wide Area Networks (WAN) Operating System Concepts 1.73 Silberschatz, Galvin and Gagne ©2005 Local Area Network Structure Operating System Concepts 1.74 Silberschatz, Galvin and Gagne ©2005 Wide Area Network Structure Operating System Concepts 1.75 Silberschatz, Galvin and Gagne ©2005 Real-Time Systems Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems. Well-defined fixed-time constraints. Real-Time systems may be either hard or soft real-time. Operating System Concepts 1.76 Silberschatz, Galvin and Gagne ©2005 Real-Time Systems (Cont.) Hard real-time: Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM) Conflicts with time-sharing systems, not supported by general-purpose operating systems. Soft real-time Limited utility in industrial control of robotics Useful in applications (multimedia, virtual reality) requiring advanced operating-system features. Operating System Concepts 1.77 Silberschatz, Galvin and Gagne ©2005 Handheld Systems Personal Digital Assistants (PDAs) Cellular telephones Issues: Limited memory Slow processors Small display screens. Operating System Concepts 1.78 Silberschatz, Galvin and Gagne ©2005 End of Chapter 1