The fixed head are positioned permanently over certain of the outer tracks, there being one head per track, so climimating the need for head movement. - The heads are very close disk surface. - Curshion of air carried by the rotating disk. Question 3. Winchester disks( hard disks )? - Comprises a number of platters(disks) permanently into an airtight enclosure. - All dust is excluded thus perimiting the read/write heads to be positioned even closer to the surfaces and so enabling greater recording densities to be employed. - The disks have greater storage capacity and a higher rate of data transger. - It has the lubricated surfaces allowing the heads “land” when the platters cease to rotate, so eliminating head crashes. - Winchester platters are either 14 in, 8 in, 5¼ in or 3½ in diameter. Question 4. Floppy disks? - Diskettes, generally called floppy disks, are single disks made of flexible plastic and permanently housed is an envelope. - The data on floppy disks is in concentric tracks on the outer part of the surfaces and access to it is via slot in the envelope. - The most common size are 3½ in, 5¼in, and 8 in diameter disks, the 3½ in disks have the advantages of a shutter. - Floppy disks may be either single or double sided and of course the drive needs to be correspondingly equipped. - Both the drives and the floppy disks themselves are inexpensive with the result that they have come into extensive used by small business and home computer buffs. - The range of capacities is from 1/4 to 2 megabytes and transfer rates around 125 to 250 kilobytes per seconds. Question 5. Optical disks? - Optical disk are comparatively new development for data storage. - Optical disks consist of a single removable glass, plastic or metal disk coated on one side with tellurium and protected by a 1 mm layer or transpacent plastic. - The disk diameters are mostly between 8 in and 14 in they rotate on a spindle in a similar fashion to magnetic disks. - The data is recorded in the form of minute pits burned into the telliurium coating by a finely-focused lazer beam. - Optical disks hold between 0.7 and GBs, this is about 20 times greater than magnetic dis cartridges. - The data is read by a low power laser beam which moved across the surface and is reflected into a photo cell. - Optical disks rotate mostly at 1500 r.p.m which, allowing for the movement of the laser unti, given access time of between 16 & 500 ms and data transfer rates of 0.6 to 3 MVs per second. - The draw back of optical disks is that the data cannot be erased so making them non-rewriteable. Question 6. Mass storage media? - Mass storage media is a high capacity disk system as when necessary by transferring data from a number of “data cartridges” house in cells. - Each cartridge consists of a 3 in wide magnetic modium inside a protective cover - In order to load the disk system, the data cartridges are moved automatically from the cells. - A typical system consists of 9440 cartridges giving a storage capacity of 472000 million bytes. Question 7. Magnetic drums? - A magnetic drum consists of a cylinder upon the surface of which data is stored in magnetic form in tracks running around its circumference, each track has its own read/write head. - A typical magnetic drum has 800 tracks each capable of holding 5000 bytes. Question 8. Charge_coupled Device Memory (CCD)? - CCD consists of thousands tiny metal squares each capable of holding an electric charge, thus representing a bit. - The squares are in the form of an array 64 x 64 holding 4096 bits. - It is very impact. - CCD is volate lity storage. Question 9. Magnetic Bubble Memory? - A thin wayer of magnetic garnet is capable of containing tiny domains or cylinders of magnetism, called bubbles. - By erasing unwanted bubbles, the resultant presence of a bubbles represent a 1 or a 0 bit. - The main ads are low power consumption, compactness, robustness reliability and non-volitility. Question 10. Megnetic tape? - The magnetic tape usage is now more as a backup medium rather than a primary method of backing storage. - It is often used as a depositony for disk dumped from fixed data storage. - It is in reells of up 3600 feet and is made of Mylar plastic tape, 1/2 in wide and coated with a magnetic material on one side. - The data is read from one read and written to another. - A reel of tape is loaded on a magnetic tape drive, and so as many drives are needed as reels during a processing run. - It is used as a backing medium than a primary method of backing storage. - The seconds usually have to be sequence where store in magnetic tape. Chapter 7: COMPUTER FILES. Question 1. File Processes? 1. Sorting a. The records in logical file are brought into some sequence as determined by key in the records. b. A computer is capable of sorting record into a “nested” sequence. c. Sorting is done by a “sorting generator”. This is part of the computer’s software and comprises several sophisticated sorting techniques that are called into use according to the file and the sort requirements. d. The need of sorting has dimished in line with the demise of magnetic tape as backing storage. 2. Merging - Merging implies that two or more files in the same sequence are combined into one file. a. File merging Two or more separate files of similar seconds and in the same sequence are marged together so as to form one file. b. Record merging The records from two or more “input” files, usually in the same sequence, are combined one record in the output file. 3. Matching a. Two or more input files (generally in the same sequence) are compared records against record in order to ensure that there is a complete set of records for each key. b. Masmatched records are highlighted for subsequent action 4. Summanizing a. Records with the same key in one file are accumulated together to form one record in the output file. b. Summanizing usually applies to a file presorted into a certain sequence and the resultant file is in the same sequence. c. Records to be summarized are generally of a similar type. 5. Searching a. Searching is looking for records with certain keys or holding certain data and in some way making a note of these. b. An instance is a search for and count of all records with a debt balance of above a certain amount. 6. Information retrieval a. Information retrieval is the process that involves the bringing together of data from several files. b. Data may also be extracted from several files and combined before being presented as information. Chapter 8: DIRECT ACCESS FILE ORGANIZATION AND STRUCTURES. Question 1. Storage and Access Modes? There are 3 principal modes for storing and accessing accords on a disk or drum: 1. Serial mode: - The record are stored contigously regardless of their keys - The sole way of accessing serial seconds is to search through the complete file starting with the first record. - It is sometimes possible to partition a serial files thus reducing the search time by starting the search at the beginning of a known partition. - A serial file is normally of a temporary nature awaiting sorting into a useful sequence. 2. Sequential mode: - direct access sequential mode normally involves accessing sequential a file that is stored sequentially. - sequential mode is often associated with a master file held in a certain sequence and updated by a transaction file sorted into the same sequence. 3. Indexed_sequential/ selective_sequential mode - Indexed_sequential is a mode of storage where by records are held sequentially and accessed selectively. - Groups of unrequired records are skipped past. - Indexed sequential files may also be accessed haphazandly. 4. Random modes: - Each record is stored in a location determind from the second’s key by means of an add generation algorithm. - The only erricient way to find a record is to use the algorithm - Random mode is applicable to master files Ads of random modes No index is required thus saving storage space It is a fast access method because little or no searching is involved Transaction do not need storing, thus saving time New records are easily insertly into the random file provided they are not excessive in number Dis The main problem with the random mode is in achieving a uniform spread of records over the storage are allocated to the file Question 2. Direct Access Addressing? - The key of record is used to identify by record - The key of record also is used to decide its storage location(or address) 1. Self addressing: - Self addressing is a straight forwards method because a record’s address is equal to its key’s value - The file is inevitably stored in key sequence Ads of self addressing It leads directly to the wanted record No indexing or searching is required The key itself need not necessarily be held within the stored record- although it generally is Dis The storage space per second has to be the same When records one missing, storage locations related to its must be left empty 2. Self addressing with key conversion - This method a basically similar to self addressing except that the key required a little processing to turn it into the record’s address - This leads to either a pricise address 3. Matrix addressing - In somes case, it is necessary to find the add of a record held within a multi dimensional matrix of record it’s called matrix addressing. Question 3. Direct Access Searching? . representing a bit. - The squares are in the form of an array 64 x 64 holding 4096 bits. - It is very impact. - CCD is volate lity storage. Question 9. Magnetic Bubble Memory? - A thin wayer. - By erasing unwanted bubbles, the resultant presence of a bubbles represent a 1 or a 0 bit. - The main ads are low power consumption, compactness, robustness reliability and non-volitility - The magnetic tape usage is now more as a backup medium rather than a primary method of backing storage. - It is often used as a depositony for disk dumped from fixed data storage. - It