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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. TUTORIALS POINT Simply Easy Learning Page 77
Space Allocation Files are allocated disk spaces by operating system. <strong>Operating</strong> 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. TUTORIALS POINT Simply Easy Learning Page 78
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Operating System Tutorial
Page 3 and 4:
ABOUT THE TUTORIAL Operating System
Page 5 and 6:
Multiprogramming ..................
Page 7 and 8:
Clocks ............................
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Following are some of important fun
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CHAPTER 2 Types of Operating System
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Distributed operating System Distri
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Real Time operating System Real tim
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Program execution Operating system
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Error handling Error can occur anyt
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Increased performance as a new job
Page 23 and 24:
Multiprogramming When two or more p
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Spooling Spooling is an acronym for
Page 27 and 28:
Program A program by itself is not
Page 29 and 30:
Process Control Block, PCB Each pro
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CHAPTER 6 Operating System Process
Page 33 and 34: Two State Process Model Two state p
Page 35 and 36: Running process may become suspende
Page 37 and 38: Context Switch A context switch is
Page 39 and 40: First Come First Serve (FCFS) Jo
Page 41 and 42: Priority Based Scheduling Each p
Page 43 and 44: Multi Queue Scheduling Multiple
Page 45 and 46: Difference between Process and Thre
Page 47 and 48: Types of Thread Threads are impleme
Page 49 and 50: Multithreading Models Some operatin
Page 51 and 52: Difference between User Level & Ker
Page 53 and 54: Instructions and data to memory add
Page 55 and 56: Logical versus Physical Address Spa
Page 57 and 58: Memory Allocation Main memory usual
Page 59 and 60: Paging External fragmentation is av
Page 61 and 62: Address generated by CPU is divided
Page 63 and 64: Virtual memory is commonly implemen
Page 65 and 66: Step Description Step 1 Check an in
Page 67 and 68: Page Replacement Algorithm Page rep
Page 69 and 70: Least Recently Used (LRU) algorithm
Page 71 and 72: I/O port An I/O port typically cons
Page 73 and 74: I/O devices I/O Devices can be cate
Page 75 and 76: 4 Disk controller sends each byte t
Page 77 and 78: CHAPTER 12 I/O Software This sectio
Page 79 and 80: Clocks Clocks are also called timer
Page 81 and 82: Device driver Device driver is a pr
Page 83: File Type File type refers to the a
Page 87 and 88: Authentication Authentication refer
Page 89 and 90: Program Threats Operating system's
Page 91 and 92: Computer Security Classifications A
Page 93 and 94: Components of Linux System Linux Op
Page 95 and 96: Basic Features Following are some o
Page 97: APPENDIX A About tutorialspoint.com
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