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Only if the data isnot in the cache does it need to access mainmemory, but in the process it copies whatever itfinds into the cache so that it is there ready forthe next time it is needed. The whole process iscontrolled by a group of logic circuits called thecache controller.30One of the cache controller's main jobs is to lookafter 'cache coherency' which means ensuring thatany changes written to main memory are reflectedwithin the cache and vice versa.
There are severaltechniques for achieving this, the most obviousProcessorCacheControllerMainMemory354045being for the processor to write directly to boththe cache and main memory at the same time.This is known as a 'write-through' cache and is thesafest solution, but also the slowest.The main alternative is the 'write-back' cachewhich allows the processor to write changes onlyto the cache and not to main memory. Cacheentries that have changed are flagged as 'dirty',telling the cache controller to write their contentsback to main memory before using the space tocache new data.
A write-back cache speeds up thewrite process, but does require a more intelligentcache controller.Most cache controllers move a 'line' of data ratherthan just a single item each time they need totransfer data between main memory and thecache. This tends to improve the chance of acache hit as most programs spend their timestepping through instructions stored sequentiallyin memory, rather than jumping about from onearea to another.
The amount of data transferredeach time is known as the 'line size'.ProcessorProcessorCacheCacheControllerMainMemoryIf there is a cache hit then the processor only needs to accessthe cache. If there is a miss then it needs to both fetch datafrom main memory and update the cache, which takes longer.With a standard write-through cache, data has to be writtenCacheControllerMainMemoryProcessorCacheControllerMainMemoryboth to main memory and to the cache. With a write-backcache the processor needs only write to the cache, leaving thecache controller to write data back to main memory later on.[Adapted from Cache Memory, PC Plus, February 1994, Future Publishing Ltd.]UNIT 2 Computer Architecture17How a Disk Cache Works51015Disk caching works in essentially the same waywhether you have a cache on your diskcontroller or you are using a software-basedsolution.
The CPU requests specific data fromthe cache. In some cases, the information willalready be there and the request can be metwithout accessing the hard disk.If the requested information isn't in the cache,the data is read from the disk along with a largechunk of adjacent information.
The cache thenmakes room for the new data by replacing old.Depending on the algorithm that is beingapplied, this may be the information that hasbeen in the cache the longest, or theinformation that is the least recently used. TheCPU's request can then be met, and the cachealready has the adjacent data loaded inanticipation of that information beingrequested next.B1 Match the terms in Table A with thestatements in Table B.Table ACache hitCache controllerCache coherencydWrite-through cacheeWrite-back cachefLine sizeTable BThe process of writing changes only to thecache and not to main memory unless thespace is used to cache new data5Cache suppliesCPU withrequested data1CPU requestsnew dataCache(before)Cache(after)4Cache replacesold data withnew dataRe-read the texts to find the answers tothese questions.ii The amount of data transferred to thecache at any one timeiii The process of writing directly to both thecache and main memory at the same timeiv The processor is successful in finding thedata in the cachevEnsuring that any changes written to mainmemory are reflected within the cacheand vice versavi The logic circuits used to control thecache process2Data is notfound in cacheCache request:data fromhard disk3Cache reads datafrom multiplesectors on disk,including dataadjacent to thatrequested[Adapted from 'How a Disk Cache Works', PC Magazine,September 1990]2 Mark the following as True or False:a Cache memory is faster than RAM.b The processor looks for data in the mainmemory first.c Write-through cache is faster than write-backcache.d Write-back cache requires a more intelligentcache controller.e Most programs use instructions that arestored in sequence in memory.f Most cache controllers transfer one item ofdata at a time.g Hardware and software disk caches work inmuch the same way.UNIT3Computer ApplicationsSTARTERWork in groups.
List as many uses as you can for computers inone of these areas.1234READINGsupermarketshospitalsairportspolice headquartersStudy this diagram. Using only the diagram, try to list eachstage in the operation of this computerised speed trap to make anexplanation of how it operates. For example:1Camera 1 records the time each vehicle passes.Details of speeding carssent to Police HQ computerDigital camera + processor 2Digital camera + processor 1data linkTime and number plate recorded.Speed recordedTime and number platerecorded using OCRFig lNew speed camera sysmeasured distancePart 1 of the text describes the system which predates the oneshown in Fig 1.
Does it contain any information that may helpcomplete your explanation? Read it quickly to find out. Ignore anyinformation which is not helpful to you.In the last ten years, police have installed speedtrap units on many busy roads. These contain aradar set, a microprocessor and a camera equippedwith a flash.
The radar sends out a beam of radiowaves at a frequency of 24 gigahertz. This isequivalent to a wavelength of 1.25 cms. If a car ismoving towards the radar, the reflected signal willbounce back with a slightly smaller wavelength. Ifaway from the radar, the waves will reflect with aslightly longer wavelength. The microprocessorwithin the unit measures the difference inwavelength between outgoing and returningsignals and calculates the speed of each vehicle.
Ifit is above the speed pre-set by the police, thecamera takes a picture of the vehicle. Theinformation is stored on a smart card for transferto the police computer. The owner of the vehiclecan then be traced using the Driver and VehicleLicensing Centre database.Part 2 describes the new system. Read it to complete thestages in your explanation.Some drivers have now got used to these traps.They slow down when they approach one toensure that the camera is not triggered. Theyspeed up again as soon as they have passed. This isknown as 'surfing'.
One way of outwitting suchmotorists is a new computerised system. Thisconsists of two units equipped with digitalcameras positioned at a measured distance apart.The first unit records the time each vehicle passesit and identifies each vehicle by its number platesLANGUAGE WORKusing optical character recognition software. Thisinformation is relayed to the second unit whichrepeats the exercise. The microprocessor withinthe second unit then calculates the time taken byeach vehicle to travel between the units. Theregistration numbers of those vehicles exceedingthe speed limit are relayed to police headquarterswhere a computer matches each vehicle with theDVLC database. Using mailmerge a standard letteris then printed off addressed to the vehicle owner.Present passiveStudy these sentences.1 The radar sends out a beam of radio waves.2 The information is stored on a smart card.In 1 the verb is active and in 2 it is passive,the Present passive.
Why is this so? Whatdifference does it make? In 1 the agentresponsible for the action is included - theradar. In 2 the agent is not included althoughwe know what it is - the microprocessor. Thepassive is often used to describe the steps ina process where the action is moreimportant than the agent and where theagent is already known to the reader. If weneed to add the agent, we can do so like this:3 The information is stored on a smart cardby the microprocessor.Describe the operation of the new speed trap by convertingeach of these statements to the Present passive. Add information onthe agent where you think it is necessary.1 The first unit records the time each vehicle passes.2 It identifies each vehicle by its number plates using OCRsoftware.345It relays the information to the second unit.The second unit also records the time each vehicle passes.The microprocessor calculates the time taken to travel betweenthe units.6It relays the registration numbers of speeding vehicles to policeheadquarters.78A computer matches each vehicle with the DVLC database.It prints off a letter to the vehicle owners using mailmerge.20UNIT 3 Computer ApplicationsWith the help of this diagram, sequence these steps in theoperation of an EPOS till.
Then write a description of its operation inthe Present passive.abcdefghiThe scanner converts the barcode into electrical pulses.The branch computer sends the price and description of theproduct to the EPOS till.The scanner reads the barcode.The branch computer records the sale of the product.The till shows the item and price.The checkout operator scans the item.The scanner sends the pulses to the branch computer.The till prints the item and price on the paper receipt.The branch computer searches the stock file for a productmatching the barcode EAN.Bar code to branch computer.Item and price shownon digital display andprinted on receipt.ItemPrice and descriptionto EPOS till.Branch computer records thatone of these products has been sold.Fig 2Operation of EPOS tillBranch computer searchesstock file for product.UNIT 3 Computer ApplicationsPROBLEM-SOLVINGSPEAKINGAssuming cost is not a problem, what computer applicationswould make today's cars safer, more comfortable, more secure andmore efficient? List your ideas; then compare ideas with others inyour group.Work in pairs, A and B.
Be prepared to describe the processshown in your diagram to your partner. Take notes on the processdescribed to you. Ask your partner to repeat or explain further if youdo not understand any of the steps in his/her description. If youprefer, you may describe another computing process you are familiarwith.Student A Your process is on page 184.Student B Your process is on page 190.WRITING21Write a description of the process you described in Task 8.22UNIT 3 Computer ApplicationsDATA M I N I N GQFind the answers to these questions in thefollowing text.1 What tool is often used in data mining?2 What Al method is used for the followingprocesses?a Separate data into subsets and thenanalyse the subsets to divide them intofurther subsets for a number of levels.b Continually analyse and compare datauntil patterns emerge.c Divide data into groups based on similarfeatures or limited data ranges.3 What term is used for the patterns found byneural networks?4 When are clusters used in data mining?5 What types of data storage can be used indata mining?6 What can an analyst do to improve the datamining results?7 Name some of the ways in which data miningis currently used.Data mining is simply filtering through largeamounts of raw data for useful information thatgives businesses a competitive edge.
Thisinformation is made up of meaningful patternsand trends that are already in the data but werepreviously unseen.The most popular tool used when mining isartificial intelligence (AI). AI technologies try towork the way the human brain works, by makingintelligent guesses, learning by example, andusing deductive reasoning. Some of the morepopular AI methods used in data mining includeneural networks, clustering, and decision trees.Neural networks look at the rules of using data,which are based on the connections found or ona sample set of data. As a result, the softwarecontinually analyses value and compares it to theother factors, and it compares these factorsrepeatedly until it finds patterns emerging. Thesepatterns are known as rules.
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