OPERATING SYSTEMS. Table of Contents. Republic of Cameroon Peace Work Fatherland School Year 2013/2014

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1 Ministry of Secondary Education Progressive Comprehensive High School & PCHS Mankon Bamenda Department of Computer Studies Republic of Cameroon Peace Work Fatherland School Year 2013/2014 OPERATING SYSTEMS Class: Comp. Sc A/L By: DZEUGANG PLACIDE CHAPTER OBJECTIVES This chapter aims: To describe the services an operating system provides to users, processes, and other systems. To explain some Operating system concept such as system call, kernel mode, deadlock, paging, segmentation To discuss the various ways of structuring an operating system. To describe in detail some functions of operating systems such as process management, memory management, device management, To describe some examples of today s operating systems and contrast Linux and Windows operating system Table of Contents I. INTRODUCTION TO OPERATING SYSTEM... 2 II. OPERATNG SYSTEM STRUCTURES... 4 III. TYPES OF OPERATING SYSTEMS... 8 IV. FUNCTIONS OF OPERATING SYSTEM... 9 V. PROCESS MANAGEMENT VI. DEADLOCK VII. MEMORY MANAGEMENT VIII. FILE MANAGEMENT IX. DEVICE MANAGEMENT X. SECURITY MANAGEMENT XI. EXAMPLES OF OS Visit for further learning tools for ICT & Comp. Sc Page 1

2 I. INTRODUCTION TO OPERATING SYSTEM I.1 Definition and objectives An operating system is a collection of system programs that together controls the operation of a computer system. Operating system along with hardware, application and other system software, and users constitute a computer system. It is the most important part of any computer system. The roles of an OS in a computer systems are many: Operating System is a Resource Manager. Handles multiple computer resources: CPU, Internal/External memory, Processes, Tasks, Applications, Users, etc Manages and allocates resources to multiple users or multiple jobs running at the same time (e.g., processor time, memory space, I/O devices) Arranges to use the computer hardware in an efficient manner (maximize throughput, minimize response time) and in a fair manner. It is a Control Program. Manages all the components of a complex computer system in an integrated manner. Controls the execution of user programs and I/O devices to prevent errors and improper use of the computer resources. Looks over and protects the computer. It is an extended/virtual machine It is an interface between the user and hardware, for it hides the details of the hardware from the user (e.g., I/O). Therefore, the end-users are not particularly concerned with the computer s architecture, and they view the computer system in terms of an application. To programmers, it provides some basic utilities to assist him in creating programs, the management of files, and the control of I/O devices. Visit for further learning tools for ICT & Comp. Sc Page 2

3 I.2 Evolution of OS Operating systems, like computer hardware, have undergone a series of revolutionary changes called generations. Ther exist five main generations of OS. I.2.1 First Generation ( ): Vacuum Tubes and Plugboards - No programming language nor OS - Machine language - The use of plugboards, punched cards I.2.2 Second Generation ( ): Transistor and Batch System - Introduction of transistor - Fortran, Assembler - Batch system - Offline system: - Operating System usually used: FMS (Fortran Monitor System), IBSYS (OS for IBM 7094) - Example of FMS job structure: I.2.3 Third Generation ( ): ICs and Multiprogramming - IBM 360 for scientific calculation (i.e numerical) and commercial (i.e character-oriented) - IC (Integrated Circuit) - Introduction of multiprogramming - Introduction of SPOOLING (Simultaneous Peripheral Operation On Line) - Introduction of time-sharing - Development of computer utility Þ machine that supports hundreds of timesharing users. Eg: MULTICS (Multiplex Information and Computing Service) - Development of minicomputer. Eg: DEC PDP-1 until PDP-11 - Development of UNIX (Uniplexed Information and Computing Service) I.2.4 Fourth Generation (1980-now): LSI and PC - LSI (Large Scale Integration) circuit and chips consisting of thousands of transistor Þ birth of PC (Personal Computer) - User-friendly software - Dominant Operating System: MS-DOS, Win ME, Win NT, Unix, Window - Network Operating System - Distributed Operating System Visit for further learning tools for ICT & Comp. Sc Page 3

4 II. OPERATNG SYSTEM STRUCTURES OS structure deals with how operating systems are structured and organized. Different design issues and choices are examined and compared, and the basic structure of several popular OSes are presented. II.1 OS services An operating system provides an environment for the execution of programs. It provides certain services to programs and to the users of those programs. The specific services provided, of course, differ from one operating system to another, but we can identify common classes. One set of operating-system services provides functions that are helpful to the user. II.1.1 User Interface Almost all operating systems have a user interface (UI). This interface can take several forms. command-line interface(cli), which uses text commands and a method for entering them (say, a program to allow entering and editing of commands). Command Line Interface (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 Batch interface, in which commands and directives to control those commands are entered into files, and those files are executed. A graphical user interface (GUI) is used. Here, the interface is a window system with a pointing device to directi/o, choose from menus, and make selections and a keyboard to enter text. - A GUI is a User-friendly desktop interface - It usually uses mouse, keyboard, and monitor - It provides icons to 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) Some systems provide two or all three of these variations. Microsoft Windows is GUI with CLI command shell Apple Mac OS X as Aqua GUI interface with UNIX kernel underneath and shells available Solaris is CLI with optional GUI interfaces (Java Desktop, KDE) Visit for further learning tools for ICT & Comp. Sc Page 4

5 II.1.2 Program execution. The system must be able to load a program into memory and to run that program. The program must be able to end its execution, either normally or abnormally (indicating error). II.1.3 I/O operations. A running program may require I/O, which may involve a file or an I/O device. For specific devices, special functions may be desired (such as recording to a CD or DVD drive or blanking a CRT screen). For efficiency and protection, users usually cannot control I/O devices directly. Therefore, the operating system must provide a means to do I/O. II.1.4 File-system manipulation The file system is of particular interest. Obviously, programs need to read and write files and directories. They also need to create and delete them by name, search for a given file, and list file information. Finally, some programs include permissions management to allow or deny access to files or directories based on file ownership. II.1.5 Communication There are many circumstances in which one process needs to exchange information with another process. Such communication may occur between processes that are executing on the same computer or between processes that are executing on different computer systems tied together by a computer network. Communications may be implemented via shared memory or through message passing, in which packets of information are moved between processes by the operating system. II.1.6 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 II.1.7 Other services Another set of operating-system functions exists not for helping the user but rather for ensuring the efficient operation of the system itself. Systems with multiple users can gain efficiency by sharing the computer resources among the users. 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, main memory, and file storage) may have special allocation code, others (such as I/O devices) may have general request and release code Visit for further learning tools for ICT & Comp. Sc Page 5

6 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 II.2 Operating system concepts II.2.1 OS Kernel Fig: OS services The kernel is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components). It is the part of the OS which is loaded into memory during the booting process. Its job is to regulate disk files, memory management, program objectives and tasks, and program execute and process. It is considered the operating system score because it controls a computer s hardware, and is responsible for either directly activating computer hardware or for interfacing with software that drives the hardware II.2.2 System call In computing, a system call is how a program requests a service from an operating system's kernel that it does not normally have permission to run. System calls provide the interface between a process and the operating system. Types of System Calls Six major categories, as outlined in Figure 2.8 and the following six subsections: Visit for further learning tools for ICT & Comp. Sc Page 6

7 Type Process control File Management Device Management Information maintenance Communication System calls o End, abort o Load, execute o Create process, terminate process o Get process attributes, set process attributes o Wait for time o Wait event, signal event o Allocate and free memory o Create file, delete file o Open, close o Read, write, reposition o Get file attributes, set file attributes o Request device, release device o Read, write, reposition o Get device attributes, set device attributes o Locally attach or detach devices o Get time or date, set time or date o Get system date, set system date o Get process, file or devices attributes o Set process, file or devices attributes o Create, delete communication connection o Send, receive messages o Transfer status information o Attach or detach remote devices II.2.3 System program System programs provide a convenient environment for program development and execution. They can be divided into: File manipulation create, delete, copy, rename, print, list etc. Status information Display date, time, disk space, memory size, etc. File modification Create and modify files using text editors. Programming language support Compilers, assemblers, and interpreters. Program loading and execution Loaders, linkers. Communications remote login, send and receive messages. Most users view of the operation system is defined by system programs, not the actual system calls. II.2.4 Operating system modes Many CPU modes can be implemented by an Operating System: But en general a CPU can be either in kernel mode or User mode. In kernel mode, also called master mode, supervisor mode, privileged mode, supervisor state, etc, the CPU has instructions to manage memory and how it can be accessed, plus the ability to access peripheral devices like disks and network cards. The CPU can also switch itself from one running program to another. It is an unrestricted mode Visit for further learning tools for ICT & Comp. Sc Page 7

8 In user mode, access to memory is limited to only some memory locations, and access to peripheral devices is denied. The ability to keep or relinquish the CPU is removed, and the CPU can be taken away from a program at any time. Now, all programs will be run in user mode, and this prevents them from accessing the data in other programs, as well as preventing the disk etc. II.2.5 Interrupt Generally, the mechanisms for invoking the operating system are called interrupts. They are the gateways to the operating system. Modern computers will go into system mode when they handle an interrupt. System mode provides the foundation for operating system security. The three types of interrupts are software interrupts (syscall) - invoked by software external interrupts - invoked by external devices exceptions - invoked by the processor when errors occur Interrupt Handling The code that is installed at the target address for interrupts is called an interrupt handler. The first thing that it has to do is save the state of the currently executing process. Then it calls a subprogram to deal with the specific type of interrupt. When that subprogram returns, the interrupt handler restores the state of the process that was executing when the interrupt occurred. III. TYPES OF OPERATING SYSTEMS Within the broad family of operating systems, there are generally four types, categorized based on the types of computers they control and the sort of applications they support. The categories are: III.1 Real-time operating system (RTOS) Abbreviated as RTOS, a real-time operating system or embedded operating system is a computer operating system designed to handle events as they occur. Real-time operating systems are commonly found and used in robotics, cameras, complex multimedia animation systems, communications, and has various military and government uses. Many operating systems such as Windows and Linux have embedded versions and other examples included Chimera, Lynx, MTOS, QNX, RTMX, RTX, and VxWorks. III.2 Single-user, single task As the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system. Visit for further learning tools for ICT & Comp. Sc Page 8

9 III.3 Single-user, multi-tasking This is the type of operating system most people use on their desktop and laptop computers today. Microsoft's Windows and Apple's MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. III.4 Multi-user A multi-user operating system allows many different users to take advantage of the computer's resources simultaneously. Unix, VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems. III.5 Multiprocessing OS Multiprogramming OS have two or more processors for a single running process. Processing takes place in parallel and is also called parallel processing. Each processor works on different parts of the same task, or, on two or more different tasks. Linux, UNIX and Windows 7 are examples of multiprocessing OS. III.6 Time sharing Operating System: It allows execution of more than one tasks or processes concurrently. For this, the processor time is divided amongst different tasks. This division of time is also called time sharing. The processor switches rapidly between various processes. III.7 Distributed Operating System: They are also called Network Operating System(NOS). On a network data is stored and processed on multiple locations. The Distributed Operating System is used on networks as it allows shared data/files to be accessed from any machine on the network in a transparent manner. We can insert and remove the data and can even access all the input and output devices. The users feel as if all data is available on their workstation itself. Some example of network operating system are Windows NT, Novell s Netware ect. III.8 Multiprocessor Operating System: A multiprocessor operating system can incorporate more than one processor dedicated to the running processes. This technique of using more than one processor is often called parallel processing. III.9 Embedded Operating System: An embedded operating system refers to the operating system that is self-contained in the device and resident in the read-only memory (ROM). IV. FUNCTIONS OF OPERATING SYSTEM The main functions of a modern operating system are as follows: Visit for further learning tools for ICT & Comp. Sc Page 9

10 Process Management: As a process manager, the operating system handles the creation and deletion of processes, suspension and resumption of processes and scheduling and synchronization of processes. Memory Management: As a memory manager, the operating system handles the allocation and deallocation of memory space as required by various programs. File Management: The operating system is responsible for creation and deletion of files and directories. It also takes care of other file-related activities such as organizing, storing, retrieving, naming, and protecting the files. Device Management: Operating system provides input/output subsystem between process and device driver. It handles the device caches, buffers and interrupts. It also detects the device failures and notifies the same to the user. Security Management: The operating system protects system resources and information against destruction and unauthorized use. User Interface: Operating system provides the interface between the user and the hardware. The user interface is the layer that actually interacts with the computer operator. The interface consists of a set of commands or menus through which a user communicates with a program. V. PROCESS MANAGEMENT V. 1 What is a process? A process is a name given to a program instance that has been loaded into memory and managed by the operating system. Shortly, a process is a program in execution. (Program = static file (image). Process = executing program = program + execution state) V.2 Process States A Process in execution will have to pass through different stages : New: The process is being created.. Ready: The process is ready to be assigned to the processor. Running: The Process is currently using CPU to execute its instruction is in this stage. Waiting: The process is waiting for signal from some other process. Terminated: The Process that finishes its execution will be terminated. Steps followed in the Process execution: A Process when first created will be in New stage, then when it is waiting for CPU time in order to get executed will in ready stage. In Ready stage Process will be waiting for CPU time in Ready Queue (A queue where Process waits for the CPU). Once the CPU become free and process acquires it, Process will enter into Running stage where it will execute its instructions. If during this stage if any interrupt occurs then it will move back to ready stage and wait for the scheduler dispatcher. If it has Visit for further learning tools for ICT & Comp. Sc Page 10

11 I/O operation then it will move to Waiting stage where it will perform I/O operation and then move back to ready stage and waits for the scheduler dispatcher. After the process get executed it will terminate. The change of the state of the process from one form to another is called context change and this course of action is known as context switching. V.3 Threads Fig: Process states A thread is a task that runs concurrently with other tasks within the same process. This is also known as lightweight process. A thread is the simplest unit of a process. The single thread of control allows the process to perform only one task at a time. An example of a single thread in a process is a text editor where a user can either edit the text or perform any other task like printing the document. In a multitasking operating system, a process may contain several threads, all running at the same time inside the same process. Writing a program in which a process creates multiple threads is called multithread programming. V.4 Process Scheduling In a multiprogrammed system, at any given time, several processes will be competing for the CPU time. Thus, a choice has to be made which process to allocate the CPU next. This procedure of determining the next process to be executed on the CPU is called process scheduling and the module of operating system that makes this decision is called a scheduler. V.4.1 Preemptive and Non-preemptive Scheduling CPU-scheduling decisions may take place under the following four circumstances: 1. When a process switches from the running state to the waiting state (for example, as the result of an I/O request, ). 2. When a process switches from the running state to the ready state (for example, when an interrupt occurs). Visit for further learning tools for ICT & Comp. Sc Page 11

12 3. When a process switches from the waiting state to the ready state (for example, at completion of I/O, on a semaphore, or for some other reason). 4. When a process terminates. If no process is ready, a system-supplied idle process is normally run. For situations 1 and 4, there is no choice in terms of scheduling. A new process (if one exists in the ready queue) must be selected for execution. There is a choice, however, for situations 2 and 3. When scheduling takes place only under circumstances 1 and 4, we say that the scheduling scheme is nonpreemptive or cooperative; otherwise, it is pre-emptive. o o Under nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state. Unfortunately, pre-emptive scheduling incurs a cost associated with access to shared data. Consider the case of two processes that share data. While one is updating the data, it is preempted so that the second process can run. The second process then tries to read the data, which are in an inconsistent state. In such situations, we need new mechanisms to coordinate access to shared data. V.4.2 Algorithm to select process to execute The scheduler uses some scheduling procedure to carry out the selection of a process for execution. 1) First-Come-First-Served: As the name suggests, in FCFS scheduling, the processes are executed in the order of their arrival in the ready queue, which means the process that enters the ready queue first, gets the CPU first. To implement FCFS scheduling procedure, the ready queue is managed as a firstin first-out (FIFO) queue. FCFS falls under nonpreemptive scheduling and its main drawback is that a process may take a very long time to complete, and thus holds up other waiting processes in the queue Process Arrival time Execute Time Service Time Wait Time : Service Time - Arrival Time P = 0 P = 4 P = 6 P = 13 Average Wait Time: ( ) / 4 = 5.55 Visit for further learning tools for ICT & Comp. Sc Page 12

13 2) Round Robin: Round robin scheduling was designed, keeping in mind the limitations of the FCFS scheduling procedure. This procedure falls under preemptive scheduling in which a process is selected for execution from the ready queue in FIFO sequence. However, the process is executed only for a fixed period known as time slicing or quantum period after which it will be interrupted and returned to the end of the ready queue. In round robin procedure, processes are allocated the CPU time on a turn basis Quantum = 3 Process Arrival time Execute Time Wait Time : Service Time - Arrival Time P0 0 5 (0-0) + (12-3) = 9 P1 1 3 (3-1) = 2 P ) + (15-9) = 10 P4 3 6 (9-3) + (18-12) = 12 Average Wait Time: ( ) / 4 = ) Shortest-Job-First Scheduling, SJF The idea behind the SJF algorithm is to pick the quickest fastest little job that needs to be done, get it out of the way first, and then pick the next smallest fastest job to do next. ( Technically this algorithm picks a process based on the next shortest CPU burst, not the overall process time. ). For example, the Gantt chart below is based upon the following CPU burst times, ( and the assumption that all jobs arrive at the same time. ) Process Arrival time Execute Time Service Time Wait Time : Service Time - Arrival Time P = 3 P = 0 P = 14 P = 5 Visit for further learning tools for ICT & Comp. Sc Page 13

14 Average Wait Time: ( ) / 4 = ) Priority Scheduling Priority scheduling is a more general case of SJF, in which each job is assigned a priority and the job with the highest priority gets scheduled first. ( SJF uses the inverse of the next expected burst time as its priority - The smaller the expected burst, the higher the priority. ) Note that in practice, priorities are implemented using integers within a fixed range, but there is no agreed-upon convention as to whether "high" priorities use large numbers or small numbers. This book uses low number for high priorities, with 0 being the highest possible priority. Processes with same priority are executed on first come first serve basis. Priority can be decided based on memory requirements, time requirements or any other resource requirement. Wait time of each process is following Process Wait Time : Service Time - Arrival Time P0 0-0 = 0 P1 3-1 = 2 P2 8-2 = 6 P = 13 Average Wait Time: ( ) / 4 = ) Shortest-Remaining-Time (SRT) Scheduling Shortest remaining time, also known as shortest remaining time first (SRTF), is the preemtive counterpart of SJF and useful in time-sharing environment. The process with the smallest estimated run-time to completion is run next, including new arrivals. Visit for further learning tools for ICT & Comp. Sc Page 14

15 A nonpreemptive scheduling algorithm picks a process to run and then just lets it run until it blocks (either on I/O or waiting for another process) or until it voluntarily releases the CPU. First-Come-First-Served (FCFS), Shortest Job first (SJF). In contrast, a pre-emptive scheduling algorithm picks a process and lets it run for a maximum of some fixed time. If it is still running at the end of the time interval, it is suspended and the scheduler picks another process to run. Round-Robin (RR), Priority Scheduling. VI. DEADLOCK VI.1 What is Deadlock? In a multiprogramming environment, several processes may compete for a limited number of resources. A process requests for the required resource and if it is not available, then the process enters in the waiting state and remains in that state until it acquires the resource. There might be a situation when the process has to wait endlessly because the requested resource may be held by other waiting process. This type of situation is known as deadlock. VI.2 Real-life Examples of deadlock 1) Bridge traffic can only be in one direction. Each entrance of the bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (resource preemption). Starvation is possible (Processes wait indefinitely). 2) An utility program copy a file from tape to disk and print the file to printer Resources: Tape, Disk, Printer A deadlock - A holds tape and disk, then requests for a printer - Bholds printer, then requests for tape and disk Visit for further learning tools for ICT & Comp. Sc Page 15

16 VI.3 Necessary Conditions A deadlock situation arises if the following four conditions hold simultaneously on the system: Mutual Exclusion: At least one resource must be held in a non-sharable mode; If any other process requests this resource, then that process must wait for the resource to be released. Hold and Wait: In this situation, a process might be holding some resource while waiting for additional resources, which are currently being held by other processes. No Preemption: Resources cannot be preemptive, that is, resources cannot be forcibly removed from a process. A resource can only be released voluntarily by the holding process, after that process has completed its task. Circular Wait: This situation may arise when a set of processes waiting for allocation of resources held by other processes forms a circular chain in which each process is waiting for the resource held by its successor process in the chain. VI.4 Methods for Handling Deadlocks Generally speaking there are three ways of handling deadlocks: 1. Deadlock prevention or avoidance - Do not allow the system to get into a deadlocked state. 2. Deadlock detection and recovery - Abort a process or preempt some resources when deadlocks are detected. 3. Ignore the problem all together - If deadlocks only occur once a year or so, it may be better to simply let them happen and reboot as necessary than to incur the constant overhead and system performance penalties associated with deadlock prevention or detection. This is the approach that both Windows and UNIX take. VI.5 The Banker's algorithm: The Banker's algorithm is run by the operating system whenever a process requests resources. The algorithm prevents deadlock by denying or postponing the request if it determines that accepting the request could put the system in an unsafe state (one where deadlock could occur). When a new process enters a system, it must declare the maximum number of instances of each resource type that may not exceed the total number of resources in the system. Also, when a process gets all its requested resources it must return them in a finite amount of time. - This algorithm handles multiple instances of the same resource. - Force threads to provide advance information about what resources they may need for the duration of the execution. - The resources requested may not exceed the total available in the system. - The algorithm allocates resources to a requesting thread if the allocation leaves the system in a safe state. - Otherwise, the thread must wait. Safe state A state is safe if a sequence of processes exist such that there are enough resources for the first to finish, and as each finishes and releases its resources there are Visit for further learning tools for ICT & Comp. Sc Page 16

17 enough for the next to finish. The rule is simple: If a request allocation would cause an unsafe state, do not honor that request. NOTE: All deadlocks are unsafe, but all unsafe are NOT deadlocks. Example VII. MEMORY MANAGEMENT In addition to managing processes, the operating system also manages the primary memory of the computer. The part of the operating system that handles this job is called the memory manager. The major tasks accomplished by the memory manager so that all the processes function in harmony are as follows: Relocation: Each process must have enough memory to execute. Protection and Sharing: A process should not run into the memory space of another process. VII.1 Relocation Visit for further learning tools for ICT & Comp. Sc Page 17

18 When a process is to be executed, it has to be loaded from the secondary storage (like hard disk) to the main memory (RAM). This is called process loading. Since, main memory is limited and other processes also need it for their execution, an operating system swaps the two processes, which is called swapping. Once the process is 'swapped out', it is uncertain to predict when it will be 'swapped in' because of the number of processes running concurrently VII.2 Protection and Sharing In multiprogrammed systems, as a number of processes may reside in the main memory at the same time, there is a possibility that a user program during execution may access the memory location allocated either to other user processes or to the operating system. It is the responsibility of the memory manager to protect the operating system from being accessed by other processes and the processes from one another. VII.3 Memory Allocation The main challenge of efficiently managing memory comes when a system has multiple processes running at the same time. In such a case, the memory manager can allocate a portion of primary memory to each process for its own use. Different strategies are used to allocate space to processes competing for memory. Three of the most popular are: best fit, first fit and worst fit. Fig: Strategies for Memory Allocation Best Fit: In this case, the memory manager places a process in the smallest block of unallocated memory in which it will fit. For example, a process requests 12 KB of memory and the memory manager currently has a list of unallocated blocks of 6 KB, Visit for further learning tools for ICT & Comp. Sc Page 18

19 14 KB, 19 KB, 11 KB and 13 KB blocks. The best fit strategy will allocate 12 KB of the 13 KB block to the process. First Fit: The memory manager places the process in the first unallocated block that is large enough to accommodate the process. Using the same example to fulfil 12 KB request, first fit will allocate 12 KB of the 14 KB block to the process. Worst Fit: The memory manager places the process in the largest block of unallocated memory available. To furnish the 12 KB request again, worst fit will allocate 12 KB of the 19 KB block to the process, leaving a 7 KB block for future use. VII.4 Paging and segmentation 1) Paging Paging is a memory management scheme that allows the processes to be stored noncontiguously in the memory. The memory is divided into fixed size chunks called page frames. The operating system breaks the address space of the program (the collection of addresses used by the program) into fixed size chunks called pages, which are of same size as that of page frames. Generally, page size is of 4 KB. However, some systems support even larger page sizes such as 8 KB and 4 MB. When a process is to be executed, its pages are loaded into unallocated page frames (not necessarily contiguous). The main advantage of paging is that it minimizes the problem of fragmentation since memory allocated is always in fixed units and any free frame can be allocated to a process. 2) Segmentation Segmentation is a Memory Management technique in which memory is divided into variable sized chunks which can be allocated to processes. Each chunk is called a segment. A table stores the information about all such segments and is called Global Descriptor Table (GDT). A GDT entry is called Global Descriptor. 3) Difference between paging and segmentation:- Paging and segmentation are closed relative to each other but they have following difference. Paging divides the virtual memory in physical memory areas while the segmentation divides it logically. Paging divides the memory in fixed length memory areas while segmentation divides into variable length memory areas. In segmentation a full logical portion of the process in loaded, but in paging information about page is loaded. The segments are large length as compare to page. In segmentation memory is divided into segments through software and in paging the memory in divided through hardware. Visit for further learning tools for ICT & Comp. Sc Page 19

20 VII.5 Concept of Virtual Memory VII.5.1 What is virtual memory The basic idea of virtual memory is to hide the details of the real physical memory available in a given system from the user. In particular, virtual memory conceals the fact that physical memory is not allocated to a program as a single contiguous region and also conceals the actual size of available physical memory. Virtual memory is a method of using the computer hard drive to provide extra memory for the computer. Segments of memory are stored on the hard drive known as pages. When a segment of memory is requested that is not in memory it is moved from the virtual memory to an actual memory address. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit or MMU, automatically translates virtual addresses to physical addresses. With virtual memory, the system can run programs that are actually larger than the primary memory of the system. Virtual memory allows for very effective multiprogramming and relieves the user from the unnecessarily tight constraints of main memory VII.5.2 Page Faults Fig: illustration of the concept of virtual memory A page fault is an interruption that occurs when a software program attempts to access an invalid page in memory. Page faults are caused by a software issue or confliction, driver issue, or a hardware related issue. A page fault occurs when a program tries to access a page that is mapped in address space, but not loaded in the physical memory (the RAM). In other words, a page fault occurs when a program can not find a page that it s looking for in the Visit for further learning tools for ICT & Comp. Sc Page 20