Category: operating System

Operating System – Virtual Memory ”; Previous Next A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard disk that”s set up to emulate the computer”s RAM. The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by using disk. Second, it allows us to have memory protection, because each virtual address is translated to a physical address. Following are the situations, when entire program is not required to be loaded fully in main memory. User written error handling routines are used only when an error occurred in the data or computation. Certain options and features of a program may be used rarely. Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used. The ability to execute a program that is only partially in memory would counter many benefits. Less number of I/O would be needed to load or swap each user program into memory. A program would no longer be constrained by the amount of physical memory that is available. Each user program could take less physical memory, more programs could be run the same time, with a corresponding increase in CPU utilization and throughput. Modern microprocessors intended for general-purpose use, a memory management unit, or MMU, is built into the hardware. The MMU”s job is to translate virtual addresses into physical addresses. A basic example is given below − Virtual memory is commonly implemented by demand paging. It can also be implemented in a segmentation system. Demand segmentation can also be used to provide virtual memory. Demand Paging A demand paging system is quite similar to a paging system with swapping where processes reside in secondary memory and pages are loaded only on demand, not in advance. When a context switch occurs, the operating system does not copy any of the old program’s pages out to the disk or any of the new program’s pages into the main memory Instead, it just begins executing the new program after loading the first page and fetches that program’s pages as they are referenced. While executing a program, if the program references a page which is not available in the main memory because it was swapped out a little ago, the processor treats this invalid memory reference as a page fault and transfers control from the program to the operating system to demand the page back into the memory. Advantages Following are the advantages of Demand Paging − Large virtual memory. More efficient use of memory. There is no limit on degree of multiprogramming. Disadvantages Number of tables and the amount of processor overhead for handling page interrupts are greater than in the case of the simple paged management techniques. Page Replacement Algorithm Page replacement algorithms are the techniques using which an Operating System decides which memory pages to swap out, write to disk when a page of memory needs to be allocated. Paging happens whenever a page fault occurs and a free page cannot be used for allocation purpose accounting to reason that pages are not available or the number of free pages is lower than required pages. When the page that was selected for replacement and was paged out, is referenced again, it has to read in from disk, and this requires for I/O completion. This process determines the quality of the page replacement algorithm: the lesser the time waiting for page-ins, the better is the algorithm. A page replacement algorithm looks at the limited information about accessing the pages provided by hardware, and tries to select which pages should be replaced to minimize the total number of page misses, while balancing it with the costs of primary storage and processor time of the algorithm itself. There are many different page replacement algorithms. We evaluate an algorithm by running it on a particular string of memory reference and computing the number of page faults, Reference String The string of memory references is called reference string. Reference strings are generated artificially or by tracing a given system and recording the address of each memory reference. The latter choice produces a large number of data, where we note two things. For a given page size, we need to consider only the page number, not the entire address. If we have a reference to a page p, then any immediately following references to page p will never cause a page fault. Page p will be in memory after the first reference; the immediately following references will not fault. For example, consider the following sequence of addresses − 123,215,600,1234,76,96 If page size is 100, then the reference string is 1,2,6,12,0,0 First In First Out (FIFO) algorithm Oldest page in main memory is the one which will be selected for replacement. Easy to implement, keep a list, replace pages from the tail and add new pages at the head. Optimal Page algorithm An optimal page-replacement algorithm has the lowest page-fault rate of all algorithms. An optimal page-replacement algorithm exists, and has been called OPT or MIN. Replace the page that will not be used for the longest period of time. Use the time when a page is to be used. Least Recently Used (LRU) algorithm Page which has not been used for the longest time in main memory is the one which will be selected for replacement. Easy to implement, keep a list, replace pages by looking back into time. Page Buffering algorithm To get a process start quickly, keep a pool of free frames. On page fault, select a page to be replaced. Write the new page in the frame of free pool, mark the page table and restart the process. Now write the dirty page out of disk and place the frame holding replaced

Discuss Operating System ”; Previous Next An operating system (OS) is a collection of software that manages computer hardware resources and provides common services for computer programs. The operating system is a vital component of the system software in a computer system. This tutorial will take you through step by step approach while learning Operating System concepts. Print Page Previous Next Advertisements ”;

Operating System – Properties ”; Previous Next Following are the different properties of an Operating System. This tutorial will explain these properties in detail one by one: Batch processing Multitasking Multiprogramming Interactivity Real Time System Distributed Environment Spooling Batch processing Batch processing is a technique in which an Operating System collects the programs and data together in a batch before processing starts. An operating system does the following activities related to batch processing − The OS defines a job which has predefined sequence of commands, programs and data as a single unit. The OS keeps a number a jobs in memory and executes them without any manual information. Jobs are processed in the order of submission, i.e., first come first served fashion. When a job completes its execution, its memory is released and the output for the job gets copied into an output spool for later printing or processing. Advantages Batch processing takes much of the work of the operator to the computer. Increased performance as a new job get started as soon as the previous job is finished, without any manual intervention. Disadvantages Difficult to debug program. A job could enter an infinite loop. Due to lack of protection scheme, one batch job can affect pending jobs. Multitasking Multitasking is when multiple jobs are executed by the CPU simultaneously by switching between them. Switches occur so frequently that the users may interact with each program while it is running. An OS does the following activities related to multitasking − The user gives instructions to the operating system or to a program directly, and receives an immediate response. The OS handles multitasking in the way that it can handle multiple operations/executes multiple programs at a time. Multitasking Operating Systems are also known as Time-sharing systems. These Operating Systems were developed to provide interactive use of a computer system at a reasonable cost. A time-shared operating system uses the concept of CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared CPU. Each user has at least one separate program in memory. A program that is loaded into memory and is executing is commonly referred to as a process. When a process executes, it typically executes for only a very short time before it either finishes or needs to perform I/O. Since interactive I/O typically runs at slower speeds, it may take a long time to complete. During this time, a CPU can be utilized by another process. The operating system allows the users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user. As the system switches CPU rapidly from one user/program to the next, each user is given the impression that he/she has his/her own CPU, whereas actually one CPU is being shared among many users. Multiprogramming Sharing the processor, when two or more programs reside in memory at the same time, is referred as multiprogramming. Multiprogramming assumes a single shared processor. Multiprogramming increases CPU utilization by organizing jobs so that the CPU always has one to execute. The following figure shows the memory layout for a multiprogramming system. An OS does the following activities related to multiprogramming. The operating system keeps several jobs in memory at a time. This set of jobs is a subset of the jobs kept in the job pool. The operating system picks and begins to execute one of the jobs in the memory. Multiprogramming operating systems monitor the state of all active programs and system resources using memory management programs to ensures that the CPU is never idle, unless there are no jobs to process. Advantages High and efficient CPU utilization. User feels that many programs are allotted CPU almost simultaneously. Disadvantages CPU scheduling is required. To accommodate many jobs in memory, memory management is required. Interactivity Interactivity refers to the ability of users to interact with a computer system. An Operating system does the following activities related to interactivity − Provides the user an interface to interact with the system. Manages input devices to take inputs from the user. For example, keyboard. Manages output devices to show outputs to the user. For example, Monitor. The response time of the OS needs to be short, since the user submits and waits for the result. Real Time System Real-time systems are usually dedicated, embedded systems. An operating system does the following activities related to real-time system activity. In such systems, Operating Systems typically read from and react to sensor data. The Operating system must guarantee response to events within fixed periods of time to ensure correct performance. Distributed Environment A distributed environment refers to multiple independent CPUs or processors in a computer system. An operating system does the following activities related to distributed environment − The OS distributes computation logics among several physical processors. The processors do not share memory or a clock. Instead, each processor has its own local memory. The OS manages the communications between the processors. They communicate with each other through various communication lines. Spooling Spooling is an acronym for simultaneous peripheral operations on line. Spooling refers to putting data of various I/O jobs in a buffer. This buffer is a special area in memory or hard disk which is accessible to I/O devices. An operating system does the following activities related to distributed environment − Handles I/O device data spooling as devices have different data access rates. Maintains the spooling buffer which provides a waiting station where data can rest while the slower device catches up. Maintains parallel computation because of spooling process as a computer can perform I/O in parallel fashion. It becomes possible to have the computer read data from a tape, write data to disk and to write out to a tape printer while it is doing its computing task. Advantages The spooling operation uses a disk as a very large buffer. Spooling is capable of overlapping I/O operation

OS Exams Questions with Answers ”; Previous Next These selected questions and answers are prepared from Operating Systems Exam point of view and will also help in quick revision to get good marks in Operating Systems Examination. These questions has been prepared for the computer science graduates (B.C.A, M.C.A, B.Tech, B.E. and so…), to help them understand and revise the basic to advanced concepts related to Operating System. Following is the selected list of questions and their answers and will help in quick revision to get good marks in Operating Systems Examination. Operating Systems Overview What is the relationship between operating systems and computer hardware? How Buffering can improve the performance of a Computer system? What are the primary differences between Network Operating System and Distributed Operating System? What inconveniences that a user can face while interacting with a computer system, which is without an operating system? Operating Systems Process What is the Difference between a Job and a Process? What are the advantages of multiprogramming? What are the advantages of Multiprocessing or Parallel System? Operating Systems Types What are the differences between Batch processing system and Real Time Processing System? What are the differences between Real Time System and Timesharing System? What are the differences etween multiprocessing and multiprogramming? Operating Systems Process Scheduling What is a process scheduler? State the characteristicsof a good process scheduler?ORWhat is scheduling? What criteria affects the schedulers performance? Explain time slicing. How its duration affects the overall working of the system. What is Shortest Remaining Time, SRT scheduling? What is Highest Response Ratio Next (HRN) Scheduling? What are the different principles which must be considered while selection of a scheduling algorithm? Find out which algorithm among FCFS, SJF And Round Robin with quantum 10, would give the minimum average time for a given workload. Explain pseudo parallelism? Describe the process model that makes parallelism easier to deal with. Operating Systems Memory Allocation What are the differences between paging and segmentation? Explain various allocation algorithms. When does a page fault occur? Explain various page replacement strategies/algorithms. Operating Systems Semaphores Explain semophores and write a short note on it. Print Page Previous Next Advertisements ”;

Operating System Scheduling algorithms ”; Previous Next A Process Scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms. There are six popular process scheduling algorithms which we are going to discuss in this chapter − First-Come, First-Served (FCFS) Scheduling Shortest-Job-Next (SJN) Scheduling Priority Scheduling Shortest Remaining Time Round Robin(RR) Scheduling Multiple-Level Queues Scheduling These algorithms are either non-preemptive or preemptive. Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state. First Come First Serve (FCFS) Jobs are executed on first come, first serve basis. It is a non-preemptive, pre-emptive scheduling algorithm. Easy to understand and implement. Its implementation is based on FIFO queue. Poor in performance as average wait time is high. Wait time of each process is as follows − Process Wait Time : Service Time – Arrival Time P0 0 – 0 = 0 P1 5 – 1 = 4 P2 8 – 2 = 6 P3 16 – 3 = 13 Average Wait Time: (0+4+6+13) / 4 = 5.75 Shortest Job Next (SJN) This is also known as shortest job first, or SJF This is a non-preemptive, pre-emptive scheduling algorithm. Best approach to minimize waiting time. Easy to implement in Batch systems where required CPU time is known in advance. Impossible to implement in interactive systems where required CPU time is not known. The processer should know in advance how much time process will take. Given: Table of processes, and their Arrival time, Execution time Process Arrival Time Execution Time Service Time P0 0 5 0 P1 1 3 5 P2 2 8 14 P3 3 6 8 Waiting time of each process is as follows − Process Waiting Time P0 0 – 0 = 0 P1 5 – 1 = 4 P2 14 – 2 = 12 P3 8 – 3 = 5 Average Wait Time: (0 + 4 + 12 + 5)/4 = 21 / 4 = 5.25 Priority Based Scheduling Priority scheduling is a non-preemptive algorithm and one of the most common scheduling algorithms in batch systems. Each process is assigned a priority. Process with highest priority is to be executed first and so on. Processes with same priority are executed on first come first served basis. Priority can be decided based on memory requirements, time requirements or any other resource requirement. Given: Table of processes, and their Arrival time, Execution time, and priority. Here we are considering 1 is the lowest priority. Process Arrival Time Execution Time Priority Service Time P0 0 5 1 0 P1 1 3 2 11 P2 2 8 1 14 P3 3 6 3 5 Waiting time of each process is as follows − Process Waiting Time P0 0 – 0 = 0 P1 11 – 1 = 10 P2 14 – 2 = 12 P3 5 – 3 = 2 Average Wait Time: (0 + 10 + 12 + 2)/4 = 24 / 4 = 6 Shortest Remaining Time Shortest remaining time (SRT) is the preemptive version of the SJN algorithm. The processor is allocated to the job closest to completion but it can be preempted by a newer ready job with shorter time to completion. Impossible to implement in interactive systems where required CPU time is not known. It is often used in batch environments where short jobs need to give preference. Round Robin Scheduling Round Robin is the preemptive process scheduling algorithm. Each process is provided a fix time to execute, it is called a quantum. Once a process is executed for a given time period, it is preempted and other process executes for a given time period. Context switching is used to save states of preempted processes. Wait time of each process is as follows − Process Wait Time : Service Time – Arrival Time P0 (0 – 0) + (12 – 3) = 9 P1 (3 – 1) = 2 P2 (6 – 2) + (14 – 9) + (20 – 17) = 12 P3 (9 – 3) + (17 – 12) = 11 Average Wait Time: (9+2+12+11) / 4 = 8.5 Multiple-Level Queues Scheduling Multiple-level queues are not an independent scheduling algorithm. They make use of other existing algorithms to group and schedule jobs with common characteristics. Multiple queues are maintained for processes with common characteristics. Each queue can have its own scheduling algorithms. Priorities are assigned to each queue. For example, CPU-bound jobs can be scheduled in one queue and all I/O-bound jobs in another queue. The Process Scheduler then alternately selects jobs from each queue and assigns them to the CPU based on the algorithm assigned to the queue. Print Page Previous Next Advertisements ”;

Operating System – Memory Management ”; Previous Next Memory management is the functionality of an operating system which handles or manages primary memory and moves processes back and forth between main memory and disk during execution. Memory management keeps track of each and every memory location, regardless of either it is allocated to some process or it is free. It checks how much memory is to be allocated to processes. It decides which process will get memory at what time. It tracks whenever some memory gets freed or unallocated and correspondingly it updates the status. This tutorial will teach you basic concepts related to Memory Management. Process Address Space The process address space is the set of logical addresses that a process references in its code. For example, when 32-bit addressing is in use, addresses can range from 0 to 0x7fffffff; that is, 2^31 possible numbers, for a total theoretical size of 2 gigabytes. The operating system takes care of mapping the logical addresses to physical addresses at the time of memory allocation to the program. There are three types of addresses used in a program before and after memory is allocated − S.N. Memory Addresses & Description 1 Symbolic addresses The addresses used in a source code. The variable names, constants, and instruction labels are the basic elements of the symbolic address space. 2 Relative addresses At the time of compilation, a compiler converts symbolic addresses into relative addresses. 3 Physical addresses The loader generates these addresses at the time when a program is loaded into main memory. Virtual and physical addresses are the same in compile-time and load-time address-binding schemes. Virtual and physical addresses differ in execution-time address-binding scheme. The set of all logical addresses generated by a program is referred to as a logical address space. The set of all physical addresses corresponding to these logical addresses is referred to as a physical address space. The runtime mapping from virtual to physical address is done by the memory management unit (MMU) which is a hardware device. MMU uses following mechanism to convert virtual address to physical address. The value in the base register is added to every address generated by a user process, which is treated as offset at the time it is sent to memory. For example, if the base register value is 10000, then an attempt by the user to use address location 100 will be dynamically reallocated to location 10100. The user program deals with virtual addresses; it never sees the real physical addresses. Static vs Dynamic Loading The choice between Static or Dynamic Loading is to be made at the time of computer program being developed. If you have to load your program statically, then at the time of compilation, the complete programs will be compiled and linked without leaving any external program or module dependency. The linker combines the object program with other necessary object modules into an absolute program, which also includes logical addresses. If you are writing a Dynamically loaded program, then your compiler will compile the program and for all the modules which you want to include dynamically, only references will be provided and rest of the work will be done at the time of execution. At the time of loading, with static loading, the absolute program (and data) is loaded into memory in order for execution to start. If you are using dynamic loading, dynamic routines of the library are stored on a disk in relocatable form and are loaded into memory only when they are needed by the program. Static vs Dynamic Linking As explained above, when static linking is used, the linker combines all other modules needed by a program into a single executable program to avoid any runtime dependency. When dynamic linking is used, it is not required to link the actual module or library with the program, rather a reference to the dynamic module is provided at the time of compilation and linking. Dynamic Link Libraries (DLL) in Windows and Shared Objects in Unix are good examples of dynamic libraries. Swapping Swapping is a mechanism in which a process can be swapped temporarily out of main memory (or move) to secondary storage (disk) and make that memory available to other processes. At some later time, the system swaps back the process from the secondary storage to main memory. Though performance is usually affected by swapping process but it helps in running multiple and big processes in parallel and that”s the reason Swapping is also known as a technique for memory compaction. The total time taken by swapping process includes the time it takes to move the entire process to a secondary disk and then to copy the process back to memory, as well as the time the process takes to regain main memory. Let us assume that the user process is of size 2048KB and on a standard hard disk where swapping will take place has a data transfer rate around 1 MB per second. The actual transfer of the 1000K process to or from memory will take 2048KB / 1024KB per second = 2 seconds = 2000 milliseconds Now considering in and out time, it will take complete 4000 milliseconds plus other overhead where the process competes to regain main memory. Memory Allocation Main memory usually has two partitions − Low Memory − Operating system resides in this memory. High Memory − User processes are held in high memory. Operating system uses the following memory allocation mechanism. S.N. Memory Allocation & Description 1 Single-partition allocation In this type of allocation, relocation-register scheme is used to protect user processes from each other, and from changing operating-system code and data. Relocation register contains value of smallest physical address whereas limit register contains range of logical addresses. Each logical address must be less than the limit register. 2 Multiple-partition allocation In this type of allocation, main memory is divided into a number of fixed-sized partitions where each partition should contain only one

Operating System – I/O Softwares ”; Previous Next I/O software is often organized in the following layers − User Level Libraries − This provides simple interface to the user program to perform input and output. For example, stdio is a library provided by C and C++ programming languages. Kernel Level Modules − This provides device driver to interact with the device controller and device independent I/O modules used by the device drivers. Hardware − This layer includes actual hardware and hardware controller which interact with the device drivers and makes hardware alive. A key concept in the design of I/O software is that it should be device independent where it should be possible to write programs that can access any I/O device without having to specify the device in advance. For example, a program that reads a file as input should be able to read a file on a floppy disk, on a hard disk, or on a CD-ROM, without having to modify the program for each different device. Device Drivers Device drivers are software modules that can be plugged into an OS to handle a particular device. Operating System takes help from device drivers to handle all I/O devices. Device drivers encapsulate device-dependent code and implement a standard interface in such a way that code contains device-specific register reads/writes. Device driver, is generally written by the device”s manufacturer and delivered along with the device on a CD-ROM. A device driver performs the following jobs − To accept request from the device independent software above to it. Interact with the device controller to take and give I/O and perform required error handling Making sure that the request is executed successfully How a device driver handles a request is as follows: Suppose a request comes to read a block N. If the driver is idle at the time a request arrives, it starts carrying out the request immediately. Otherwise, if the driver is already busy with some other request, it places the new request in the queue of pending requests. Interrupt handlers An interrupt handler, also known as an interrupt service routine or ISR, is a piece of software or more specifically a callback function in an operating system or more specifically in a device driver, whose execution is triggered by the reception of an interrupt. When the interrupt happens, the interrupt procedure does whatever it has to in order to handle the interrupt, updates data structures and wakes up process that was waiting for an interrupt to happen. The interrupt mechanism accepts an address ─ a number that selects a specific interrupt handling routine/function from a small set. In most architectures, this address is an offset stored in a table called the interrupt vector table. This vector contains the memory addresses of specialized interrupt handlers. Device-Independent I/O Software The basic function of the device-independent software is to perform the I/O functions that are common to all devices and to provide a uniform interface to the user-level software. Though it is difficult to write completely device independent software but we can write some modules which are common among all the devices. Following is a list of functions of device-independent I/O Software − Uniform interfacing for device drivers Device naming – Mnemonic names mapped to Major and Minor device numbers Device protection Providing a device-independent block size Buffering because data coming off a device cannot be stored in final destination. Storage allocation on block devices Allocation and releasing dedicated devices Error Reporting User-Space I/O Software These are the libraries which provide richer and simplified interface to access the functionality of the kernel or ultimately interactive with the device drivers. Most of the user-level I/O software consists of library procedures with some exception like spooling system which is a way of dealing with dedicated I/O devices in a multiprogramming system. I/O Libraries (e.g., stdio) are in user-space to provide an interface to the OS resident device-independent I/O SW. For example putchar(), getchar(), printf() and scanf() are example of user level I/O library stdio available in C programming. Kernel I/O Subsystem Kernel I/O Subsystem is responsible to provide many services related to I/O. Following are some of the services provided. Scheduling − Kernel schedules a set of I/O requests to determine a good order in which to execute them. When an application issues a blocking I/O system call, the request is placed on the queue for that device. The Kernel I/O scheduler rearranges the order of the queue to improve the overall system efficiency and the average response time experienced by the applications. Buffering − Kernel I/O Subsystem maintains a memory area known as buffer that stores data while they are transferred between two devices or between a device with an application operation. Buffering is done to cope with a speed mismatch between the producer and consumer of a data stream or to adapt between devices that have different data transfer sizes. Caching − Kernel maintains cache memory which is region of fast memory that holds copies of data. Access to the cached copy is more efficient than access to the original. Spooling and Device Reservation − A spool is a buffer that holds output for a device, such as a printer, that cannot accept interleaved data streams. The spooling system copies the queued spool files to the printer one at a time. In some operating systems, spooling is managed by a system daemon process. In other operating systems, it is handled by an in kernel thread. Error Handling − An operating system that uses protected memory can guard against many kinds of hardware and application errors. Print Page Previous Next Advertisements ”;

Types of Operating System ”; Previous Next Operating systems are there from the very first computer generation and they keep evolving with time. In this chapter, we will discuss some of the important types of operating systems which are most commonly used. Batch operating system The users of a batch operating system do not interact with the computer directly. Each user prepares his job on an off-line device like punch cards and submits it to the computer operator. To speed up processing, jobs with similar needs are batched together and run as a group. The programmers leave their programs with the operator and the operator then sorts the programs with similar requirements into batches. The problems with Batch Systems are as follows − Lack of interaction between the user and the job. CPU is often idle, because the speed of the mechanical I/O devices is slower than the CPU. Difficult to provide the desired priority. Time-sharing operating systems Time-sharing is a technique which enables many people, located at various terminals, to use a particular computer system at the same time. Time-sharing or multitasking is a logical extension of multiprogramming. Processor”s time which is shared among multiple users simultaneously is termed as time-sharing. The main difference between Multiprogrammed Batch Systems and Time-Sharing Systems is that in case of Multiprogrammed batch systems, the objective is to maximize processor use, whereas in Time-Sharing Systems, the objective is to minimize response time. Multiple jobs are executed by the CPU by switching between them, but the switches occur so frequently. Thus, the user can receive an immediate response. For example, in a transaction processing, the processor executes each user program in a short burst or quantum of computation. That is, if n users are present, then each user can get a time quantum. When the user submits the command, the response time is in few seconds at most. The operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time. Computer systems that were designed primarily as batch systems have been modified to time-sharing systems. Advantages of Timesharing operating systems are as follows − Provides the advantage of quick response. Avoids duplication of software. Reduces CPU idle time. Disadvantages of Time-sharing operating systems are as follows − Problem of reliability. Question of security and integrity of user programs and data. Problem of data communication. Distributed operating System Distributed systems use multiple central processors to serve multiple real-time applications and multiple users. Data processing jobs are distributed among the processors accordingly. The processors communicate with one another through various communication lines (such as high-speed buses or telephone lines). These are referred as loosely coupled systems or distributed systems. Processors in a distributed system may vary in size and function. These processors are referred as sites, nodes, computers, and so on. The advantages of distributed systems are as follows − With resource sharing facility, a user at one site may be able to use the resources available at another. Speedup the exchange of data with one another via electronic mail. If one site fails in a distributed system, the remaining sites can potentially continue operating. Better service to the customers. Reduction of the load on the host computer. Reduction of delays in data processing. Network operating System A Network Operating System runs on a server and provides the server the capability to manage data, users, groups, security, applications, and other networking functions. The primary purpose of the network operating system is to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks. Examples of network operating systems include Microsoft Windows Server 2003, Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD. The advantages of network operating systems are as follows − Centralized servers are highly stable. Security is server managed. Upgrades to new technologies and hardware can be easily integrated into the system. Remote access to servers is possible from different locations and types of systems. The disadvantages of network operating systems are as follows − High cost of buying and running a server. Dependency on a central location for most operations. Regular maintenance and updates are required. Real Time operating System A real-time system is defined as a data processing system in which the time interval required to process and respond to inputs is so small that it controls the environment. The time taken by the system to respond to an input and display of required updated information is termed as the response time. So in this method, the response time is very less as compared to online processing. Real-time systems are used when there are rigid time requirements on the operation of a processor or the flow of data and real-time systems can be used as a control device in a dedicated application. A real-time operating system must have well-defined, fixed time constraints, otherwise the system will fail. For example, Scientific experiments, medical imaging systems, industrial control systems, weapon systems, robots, air traffic control systems, etc. There are two types of real-time operating systems. Hard real-time systems Hard real-time systems guarantee that critical tasks complete on time. In hard real-time systems, secondary storage is limited or missing and the data is stored in ROM. In these systems, virtual memory is almost never found. Soft real-time systems Soft real-time systems are less restrictive. A critical real-time task gets priority over other tasks and retains the priority until it completes. Soft real-time systems have limited utility than hard real-time systems. For example, multimedia, virtual reality, Advanced Scientific Projects like undersea exploration and planetary rovers, etc. Print Page Previous Next Advertisements ”;

Operating System – File System ”; Previous Next File A file is a named collection of related information that is recorded on secondary storage such as magnetic disks, magnetic tapes and optical disks. In general, a file is a sequence of bits, bytes, lines or records whose meaning is defined by the files creator and user. File Structure A File Structure should be according to a required format that the operating system can understand. A file has a certain defined structure according to its type. A text file is a sequence of characters organized into lines. A source file is a sequence of procedures and functions. An object file is a sequence of bytes organized into blocks that are understandable by the machine. When operating system defines different file structures, it also contains the code to support these file structure. Unix, MS-DOS support minimum number of file structure. File Type File type refers to the ability of the operating system to distinguish different types of file such as text files source files and binary files etc. Many operating systems support many types of files. Operating system like MS-DOS and UNIX have the following types of files − Ordinary files These are the files that contain user information. These may have text, databases or executable program. The user can apply various operations on such files like add, modify, delete or even remove the entire file. Directory files These files contain list of file names and other information related to these files. Special files These files are also known as device files. These files represent physical device like disks, terminals, printers, networks, tape drive etc. These files are of two types − Character special files − data is handled character by character as in case of terminals or printers. Block special files − data is handled in blocks as in the case of disks and tapes. File Access Mechanisms File access mechanism refers to the manner in which the records of a file may be accessed. There are several ways to access files − Sequential access Direct/Random access Indexed sequential access Sequential access A sequential access is that in which the records are accessed in some sequence, i.e., the information in the file is processed in order, one record after the other. This access method is the most primitive one. Example: Compilers usually access files in this fashion. Direct/Random access Random access file organization provides, accessing the records directly. Each record has its own address on the file with by the help of which it can be directly accessed for reading or writing. The records need not be in any sequence within the file and they need not be in adjacent locations on the storage medium. Indexed sequential access This mechanism is built up on base of sequential access. An index is created for each file which contains pointers to various blocks. Index is searched sequentially and its pointer is used to access the file directly. Space Allocation Files are allocated disk spaces by operating system. Operating systems deploy following three main ways to allocate disk space to files. Contiguous Allocation Linked Allocation Indexed Allocation Contiguous Allocation Each file occupies a contiguous address space on disk. Assigned disk address is in linear order. Easy to implement. External fragmentation is a major issue with this type of allocation technique. Linked Allocation Each file carries a list of links to disk blocks. Directory contains link / pointer to first block of a file. No external fragmentation Effectively used in sequential access file. Inefficient in case of direct access file. Indexed Allocation Provides solutions to problems of contiguous and linked allocation. A index block is created having all pointers to files. Each file has its own index block which stores the addresses of disk space occupied by the file. Directory contains the addresses of index blocks of files. Print Page Previous Next Advertisements ”;

Operating System – Multi-Threading ”; Previous Next What is Thread? A thread is a flow of execution through the process code, with its own program counter that keeps track of which instruction to execute next, system registers which hold its current working variables, and a stack which contains the execution history. A thread shares with its peer threads few information like code segment, data segment and open files. When one thread alters a code segment memory item, all other threads see that. A thread is also called a lightweight process. Threads provide a way to improve application performance through parallelism. Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process. Each thread belongs to exactly one process and no thread can exist outside a process. Each thread represents a separate flow of control. Threads have been successfully used in implementing network servers and web server. They also provide a suitable foundation for parallel execution of applications on shared memory multiprocessors. The following figure shows the working of a single-threaded and a multithreaded process. Difference between Process and Thread S.N. Process Thread 1 Process is heavy weight or resource intensive. Thread is light weight, taking lesser resources than a process. 2 Process switching needs interaction with operating system. Thread switching does not need to interact with operating system. 3 In multiple processing environments, each process executes the same code but has its own memory and file resources. All threads can share same set of open files, child processes. 4 If one process is blocked, then no other process can execute until the first process is unblocked. While one thread is blocked and waiting, a second thread in the same task can run. 5 Multiple processes without using threads use more resources. Multiple threaded processes use fewer resources. 6 In multiple processes each process operates independently of the others. One thread can read, write or change another thread”s data. Advantages of Thread Threads minimize the context switching time. Use of threads provides concurrency within a process. Efficient communication. It is more economical to create and context switch threads. Threads allow utilization of multiprocessor architectures to a greater scale and efficiency. Types of Thread Threads are implemented in following two ways − User Level Threads − User managed threads. Kernel Level Threads − Operating System managed threads acting on kernel, an operating system core. User Level Threads In this case, the thread management kernel is not aware of the existence of threads. The thread library contains code for creating and destroying threads, for passing message and data between threads, for scheduling thread execution and for saving and restoring thread contexts. The application starts with a single thread. Advantages Thread switching does not require Kernel mode privileges. User level thread can run on any operating system. Scheduling can be application specific in the user level thread. User level threads are fast to create and manage. Disadvantages In a typical operating system, most system calls are blocking. Multithreaded application cannot take advantage of multiprocessing. Kernel Level Threads In this case, thread management is done by the Kernel. There is no thread management code in the application area. Kernel threads are supported directly by the operating system. Any application can be programmed to be multithreaded. All of the threads within an application are supported within a single process. The Kernel maintains context information for the process as a whole and for individuals threads within the process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread creation, scheduling and management in Kernel space. Kernel threads are generally slower to create and manage than the user threads. Advantages Kernel can simultaneously schedule multiple threads from the same process on multiple processes. If one thread in a process is blocked, the Kernel can schedule another thread of the same process. Kernel routines themselves can be multithreaded. Disadvantages Kernel threads are generally slower to create and manage than the user threads. Transfer of control from one thread to another within the same process requires a mode switch to the Kernel. Multithreading Models Some operating system provide a combined user level thread and Kernel level thread facility. Solaris is a good example of this combined approach. In a combined system, multiple threads within the same application can run in parallel on multiple processors and a blocking system call need not block the entire process. Multithreading models are three types Many to many relationship. Many to one relationship. One to one relationship. Many to Many Model The many-to-many model multiplexes any number of user threads onto an equal or smaller number of kernel threads. The following diagram shows the many-to-many threading model where 6 user level threads are multiplexing with 6 kernel level threads. In this model, developers can create as many user threads as necessary and the corresponding Kernel threads can run in parallel on a multiprocessor machine. This model provides the best accuracy on concurrency and when a thread performs a blocking system call, the kernel can schedule another thread for execution. Many to One Model Many-to-one model maps many user level threads to one Kernel-level thread. Thread management is done in user space by the thread library. When thread makes a blocking system call, the entire process will be blocked. Only one thread can access the Kernel at a time, so multiple threads are unable to run in parallel on multiprocessors. If the user-level thread libraries are implemented in the operating system in such a way that the system does not support them, then the Kernel threads use the many-to-one relationship modes. One to One Model There is one-to-one relationship of user-level thread to the kernel-level thread. This model provides more concurrency than the many-to-one model. It also allows another thread to run when a thread makes a blocking system call. It supports multiple threads to execute in parallel on microprocessors. Disadvantage of this model is that creating