Computer system Computer System 1 Book I: computer system fundamentals. Chapter 1: INTRODUCTION TO COMPUTER. Question 1. What is a computer? A computer may be defined as a machine which accepts data from an input device, processes it by performing arithmetical and logic operations in accordance with a program of instructions and returns the results through an output unit. A computer is basically an electronic machine operating on current. Question 2. Components of a Computer system? A computer system comprises of the following components: 1. Central Processing Unit (CPU). - CPU is the heart of the whole sys - CPU consists of the : • control unit (CU) • arithmetic logic unit (ALU) • accumulator (ACC) • program counter (PC) • instruction register (IR) • memory address register (MAR) • memory data register (MDR) • status register (SR) • general purpose register - The function of each components of CPU: • Control unit:  control and co_ordinate all hardware functions of the CS.  examine and decode all program instructions to the computer and initiate their execution by sending the appropriate signals. • ALU:  performs all arithmetic <addition, subtraction, multiplication, division & exponentiation> and logic comparision two values functions required by computer. • ACC:  holds the first operand of the temporary result of the ALU. • PC:  contains the add of the next instruction to be excuted. • IR:  contains the current instruction to be executed. Main memory • MAR:  holds the address location to or from which data is to be transferred • MDR:  contains the data to be written to or read out of the addressed location. • SR:  keeps track of the status of the accumalator. • General Purpose Register:  for general purpose procedures. Please refer to diagram for an illustratin of the basic components of the CPU. 2 I N T E R N A L CPU Control unit Arithmetic Logic Unit Accumulator Program Couter to main Instruction Register memory Memory Address Register Memory Data Register Status Register General Purpose Register Basic components of a CPU. Control Unit Input Unit ALU Output Unit Main Memory Backing Storage Control signals Data flow Components of a CS. 2. Input units - Used to enter data( raw unprocessed facts) and instructions to the computer. 3. Output units - Used for delevering the processed result from the computer in useful form. 4. Backing storage units - Backing storage units need for high capacity data storage devices that can store data in a more permanent form for later retrieral, updating and referencing. - Backing storage is also called secondary storage external storage and auxiliary storage. Chapter 2: MICOPROCESSOR. Question 1. Cache Memory? - Cache memory is a small amount of very fast store with faster access time than the main memory. 3 B U S - Cache memory is used to temporaryty store data instructions that are likely to be retrieved many times, thus speeds up the processing of data. - Sits between main storage and the processor acting as holding area through which all data and instructions pass. - Old data in the cache memory is over written by new then cache is full. Question 2. Virtual Memory? - Virtual memory makes use of both the main memory and backing store. - In a virtual memory sys, each user has the illusion that his program is in the main memory all the time. - The sys maintains this illusion by keeping some of the “unused” portion of the program’s code and data on a backing store device which is usually magnetic disk - The movement of the unused portion from the backing store to the mian memory is transparent to the users. - Please refer to diagram for virtual memory. Backing Store Main Memory Virtual Memory Chapter 3: BATCH/ ONLINE AND REAL TIME PROCESSING SYSTEM. Question 1. Batch Processing System? - Def: Computer processing does not begin until all the input data has been collected and grouped together called Batched Generally data is accumulated for a certain period of time or unitl a certain quantity. - Ads:  Response time is not critical.  Need to process large volumn of data.  Computer efficiency is more important than response time. - Dis:  Time between recording and processing of source document is long  Rereen normally required if errors are encountered.  Data is not current.  Error correction is more difficult. Question 2. Online Processing System? - Def: Inputs data enters the computer directly as soon as it is being transacted. There information will be processed immediately and updated into the master file. - Ads:  Enter availability of information for decision making.  More accurate data capture.  Schedules suits user. 4 A3 A1 A2 A2 A3 A1 - Dis:  CPU time is used less efficiently.  Random arrival of transactions, terminal operator process each transaction separately.  More expensive than batch processing. Question 3. Real Time Processing System? - Def: One which controls the environment by receiving data processing them and returning results sufficiently quickly to affect the functioning of the environment at that time. - Ads:  Response time is very critical and sufficient quick. - Dis:  Expensive hardware & software.  Very complex in terms of hardware & software. Chapter 4: PRINTERS AND TERMINALS. Question 1. Classification of printers? 1. Classifying printers according to speed. a. Serial printers Slow printers that print one character at a time. Eg: Dot matrix printers Daisywheel printers b. Line printers Medium to high speed printers that can print in excess of 2000 lines per minute. Eg: Chain Printers Band Printers Drum Printers 2. Calssifying printers according to method of printing a. Impact printers Use hammers or prints to strike a print rebbon in order to form the character on the paper. b. Non impact printers Use more silent methods of printing. Eg: Thermal printers Ink Jet printers Lazers printers 3. Classifying printers according to print quality Kinds of quality printers Draft quality Near letter quality(NLQ) Letter quality Graphic quality Question 2. Describe some types of printer? 1. According to speed: a. Dot matrix printer - Serial impact printers that can print draft, near letter quality and a limited amount of graphics. - The print resolution is generally lower than lazer printers. b. Daisywheel printers - Are serial impact printers, the speed of a daisywheel printer is slow(20-55 characters per second), noisy in operation. - The print head has the letters arranged at the end of spokes round a central hub. 5 c. Chain printers - The chains printers has its characters set rapidly rotating on a print chain. d. Band printers - The band printer has rotating scalloped steel band. e. Drum printers - Are line printers, the print character are raised in bands around a heavy metal drum which rotates at very high speed. - The print hammers strike the paper and a print ribbon against an apropriate character on the line. An entire line of the same character is printed on one rotation of the drum. f. Thermal printers - Uses special heat sensitive paper and a matrix of print wires that become hot when exposed to an electric current. The heated wires come into close contact with the paper, burning the image of the character onto it. - The more advanced thermal printers are using thermal transfer printing. - They have a special heat sensitive ribbon and a print head with wires that become hot when a currents is applied. - The heat from the print wires causes the ink from the ribbon to fuse to a piece of regular paper. g. Inl Jet Printers - The ink jet prints by using a small droplet generator to break special inks into tiny drops, which are then forced towards a paper supply. h. Lazer printers - Using a photoconductive drum. - A lazer is then used to write the image of the character onto the drum. - After exposure to the lazer, the drum rotates through a developing station, picks up toner and transfers it to the paper. - The character is fused onto the paper by heat. i. Ion deposition printers - Ions are created in a cavity, and directed electrically through an orifice onto the dielectric surface of a rotating cylinder. - The required characters are formed as an electric charge image on the cylinders surface. - Toner is the applied to the charged image and transferred to the paper on which it is transfixed by pressure(cold fusion). j. Electrostatic printers - Letterheads and logos are created electrostatically from a changeable metal cylinder. k. Magnetic printers - A drum in the printer has a surface that can be coated with sows of tiny spots of magnetion by means of thousands of minute recording heads. - As the drum rotates it becomes covered with these magnetic spots so as to from a latent image of the page to be printed. - Dry ink particles are brought into contact with the drum’s surface and these adthere to the magnetised spots. The ink was then pressunal on to the surface and subsequently transferred onto the paper. Question 3. Characteristics of a page printers? - Speed - Characters sets - Copies 6 - Intelligence - Output Chapter 5: DATA STORAGE MEDIA. Question 1. Data storage Requirements Characteristics? - Low access time: fast speed - Storage capacity: much enough - Interchangeability: can be change easily - Security: safe enough - Transfer rate: fast enough - Cost: economic Question 2. Magnetic disks? - This comprises a drive unit onto which one or perhaps two magnetic disk cartridges are loaded. - The drive consists of a control unit and a spindle housing that rotates continuously when switch on. - The cartridge are loaded by the operator so as to provide the data currently needed for the job in hand. - Bach tracks is devided up into sectors(often 4 or 8), sectors are read or written or more at a time as blocks by means of a read. - There are usually one head for each surface, all the heads are moved. - Sunchronously across the tracks. - Once in position all the data on the equiradial tracks can be read or written without further movement of the heads. - Cylinder is a set of equiradial tracks. - A cartridge comprises several flat disks mounted on a central sprindle. When mounted it rotates at a high speed enabling data to be read from or written to it. The data is recorded magnetically on both surfaces of each disk in the form of concertric tracks. •  Certain models of disk units also have a number of fixed read/write heads in addition to the movable heads.  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. 7 - 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. 8 - 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 9 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 10 . Generally data is accumulated for a certain period of time or unitl a certain quantity. - Ads:  Response time is not critical.  Need to process large volumn