Angl-kniha

ss main memory, but in the process it copies whatever it finds into the cache so that
it is there ready for the next time it is needed. The whole process is controlled by a group of logic circuits
called the cache controller.
One of the cache controller's main jobs is to look after 'cache coherency' which means ensuring that
any changes written to main memory are reflected within the cache and vice versa. There are several
techniques for achieving this, the most obvious being for the processor to write directly to both the
cache and main memory at the same time. This is known as a 'write-through' cache and is the safest
solution, but also the slowest.
The main alternative is the 'write-back' cache which allows the processor to write changes only to
the cache and not to main memory. Cache entries that have changed are flagged as 'dirty', telling the
cache controller to write their contents back to main memory before using the space to cache new data.
A write-back cache speeds up the write process, but does require a more intelligent cache controller.
Most-cache controllers move a 'line' of data rather than just a single item each time they need to
transfer data between man memory and the cache. This tends to improve the chance of a cache hit as
most programs spend their time stepping through instructions stored sequentially in memory, rather than
jumping about from one area to another. The amount of data transferred each time is known as the 'line
size'.
If there is a cache hit then the processor only needs to access the cache. If there is a miss then it
needs to both fetch data from main memory and update the cache, which takes longer. With a standard
write-through cache, data has to be written both to main memory and to the cache. With a write-back
cache the processor needs only write to the cache, leaving the cache controller to write data back to
main memory later on.
How a Disk Cache Works
Disk caching works in essentially the same way whether you have a cache on your disk controller or
you are using a software-based solution. The CPU requests specific data from the cache. In some
cases, the information will already be there and the request can be met without accessing the hard disk.
If the requested information isn't in the cache, the data is read from the disk along with a large chunk
of adjacent information. The cache then makes room for the new data by replacing old. Depending on
the algorithm that is being applied this may be the information that has been in the cache the longest, or
the information that is the least recently used. The CPU's request can then be met, and the cache
already has the adjacent data: loaded in anticipation of that information being requested next.
UNIT 3 Computer Applications
INTRODUCTION
As computer systems become more intelligent, they are used in a wider variety of work situations
where previously it was necessary to employ people. Hospitals can increasingly use computers where
highly trained people were required to deal with life-threatening situations. Computers can also be used
in airports where highly trained experts were previously required to ensure safety and the police can
make more use of computers to detect and investigate increasingly sophisticated crimes.
One of the uses considered in this unit is police speed traps used to catch drivers that are breaking
the official speed limit. In earlier systems, radar equipment was used to bounce radio waves off the
moving car. A small processor, known as a microprocessor, calculated the speed of the car from the
changes in the radio waves and triggered an ordinary camera with a flashgun to take a photograph of
the car if it was speeding. The details were stored on a smart card (a plastic card with a built-in
computer system that can store large amounts of data). When the smart card was taken back to the
police station, the driver's details were obtained from the DVLC (Driver and Vehicle Licensing Centre)
database i.e. the central computerised records of all licensed drivers and vehicles.
Newer systems prevent 'surfing' i.e. where the driver only slows down as they pass through the
speed trap, by using two computerised units with digital cameras placed at a fixed distance apart. Each
unit records the time that a vehicle passes it, as well as photographing and identifying the car licence
Page 6 from 33 VB035 English I
number using OCR software (optical character recognition software that changes picture images of
letters and numbers into digital form for use by a computer system). The computer then uses the
difference in recorded times to calculate the speed of the vehicle. The registration numbers of vehicles
exceeding the speed limit are immediately downloaded (copied from the computer to a server
computer) to the computer at police headquarters where each vehicle is matched with the DVLC
database. Standard letters are then printed off addressed to the vehicle owners using mailmerge (a
wordprocessing feature that produces a separate standard letter containing details obtained from each
record in a database).
There are many ways in which computer systems can be used in large supermarkets, particularly for
financial calculations and in stock control using EPOS tills (electronic point of sale cash tills). Each item
on a supermarket shelf has a barcode label with a barcode (a standard set of vertical bars of varying
thickness used to identify products) printed on it. The barcode number system giving standard price
and item code numbers used throughout Europe is known as EAN (European Article Number). The
barcodes are read by scanner devices called barcode readers that are attached to the EPOS tills.
When a checkout operator moves the barcode label across the scanner, the label is scanned and the
barcode number for that item is read. The scanner signals are converted to a digital form (where the
changing signal is either off or on) and sent to the supermarket branch computer. The branch
computer checks the digital EAN code against a computer database (a type of applications program
used for storing information so that it can be easily searched and sorted) that holds a record of each
type of item. In this way the item and the price of the item can be identified and the sale of the product
can be recorded by the computer. The item and the price are shown on the EPOS till display and printed
on a paper receipt.
Computers are also used to provide cash to users and to process bank cards such as Visa cards
using an ATM (automatic teller machine - the type of machine used by banks for enabling customers to
withdraw money from their bank accounts).
READING
In the last ten years, police have installed speed trap units on many busy roads. These contain a
radar set, a microprocessor and a camera equipped with a flash. The radar sends out a beam of radio
waves at a frequency of 24 gigahertz. This is equivalent to a wavelength of 1.25 cms. If a car is moving
towards the radar, the reflected signal will bounce back with a slightly smaller wavelength. If away from
the radar, the waves will reflect with a slightly longer wavelength. The microprocessor within the unit
measures tile difference in wavelength between outgoing and returning signals and calculates the
speed of each vehicle. If it is above the speed pre-set by the police, the camera takes a picture of the
vehicle. The information is stored on a smart card for transfer to the police computer. The owner of the
vehicle can then be traced using the Driver and Vehicle Licensing Centre database.
Some drivers have now got used to these traps. They slow down when they approach one to ensure
that the camera is not triggered. They speed up again as soon as they have passed. This is known as
'surfing'. One way of outwitting such motorists is a new computerised system. This consists of two
units equipped with digital cameras positioned at a measured distance apart. The first unit records the
time each vehicle passes it and identifies each vehicle by its number plates using optical character
recognition software. This information is relayed to the second unit which repeats the exercise. The
microprocessor within the second unit then calculates the time taken by each vehicle to travel between
the units. The registration numbers of those vehicles exceeding the speed limit are relayed to police
headquarters where a computer matches each vehicle with the DVLC database. Using mailmerge a
standard letter is then printed off addressed to the vehicle owner.
SPECIALIZED READING
Data Mining
Data mining is simply filtering through large amounts of raw data for useful information that gives
businesses a competitive edge. This information is made up of meaningful patterns and trends that
are already in the data but were previously unseen.
The most popular tool used when mining is artificial intelligence (AI). AI technologies try to work the
way the human brain works, by making intelligent guesses, learning by example, and using deductive
reasoning. Some of the more popular AI methods used in data mining include neural networks,
clustering, and decision trees.
Page 7 from 33 VB035 English I
Neural networks look at the rules of using data, which are based on the connections found on a
sample set of data. As a result, the software continually analyzes value and compares it to the other
factors, and it compares these factors repeatedly until it finds patterns emerging. These patterns are
known as rules. The software then looks for other patterns based on these rules or sends out an alarm
when a trigger value is hit.
Clustering divides data into groups based on similar features or limited data ranges. Clusters are
used when data, isn't labelled in a way that is favourable to mining. For instance, an insurance
company that wants to find instances of fraud wouldn't have its records labelled as fraudulent or not
fraudulent. But after analyzing patterns within clusters, the mining software can start to figure out the
rules that point to which claims are likely to be false.
Decision trees, like clusters, separate the data into subsets and then analyze the subsets to divide
them into further subsets, and so on (for a few more levels). The final subsets, are then small enough
that the mining process can find interesting patterns and relationships within data.
Once the data to be mined is identified, it should be cleansed. Cleansing data frees it from duplicate
information and erroneous data. Next, the data should be stored in a uniform format within relevant
categories or fields. Mining tools can work with all types of data storage, from large data warehouses to
smaller desktop databases to flat files. Data warehouses and data marts are storage methods that
involve archiving large amounts of data in a way that makes it easy to access when necessary.
When the process is complete, the mining software generates a report. An analyst goes over the
report to see if further work needs to be done, such as refining parameters, using other data analysis
tools to examine the data, or even scrapping the data if it's unusable. If no further work is required, the
report proceeds to the decision makers for appropriate action.
The power of data mining is being used for many purposes, such as analyzing Supreme Court
decisions, discovering patterns in health care, pulling stories about competitors from newswires,
resolving bottlenecks in production processes, and analyzing sequences in the human genetic makeup.
There really is no limit to the type of business or area of study where data mining can be beneficial.
Data Stores
You must first have data to mine. Data stores include one or several databases or data warehouses.
Cleanse data
Data must be stored in a consistent format and free from errors and redundancies.
Search data
Actual mining occurs when data is combed for patterns and trends. Rules for patterns are noted.
Analyze reports
Someone must analyze mining results for validity and relevance.
Report findings
The mining results can then be reviewed and interpreted, and a plan of action determined.
UNIT 4 Peripherals
INTRODUCTION
EPOS (electronic point of sale) tills used in supermarkets form part of a computer system with
various input and output peripheral devices attached to the till, including: electronic scales for
weighing produce, barcode reader for looking up prices using barcodes, swipe card reader for reading
bank cards, numeric keypad for inputting prices manually, LCD (liquid crystal display) screen for
outputting purchase details.
Digital cameras are gradually being developed that are as good as conventional cameras. They
have various electronic devices inside, including:
a LCD (Liquid Crystal Display) screen used as a view-finder and for viewing the pictures after they
have been taken.
Page 8 from 33 VB035 English I
b CCD (Charge-Coupled Device) consisting of thousands of photo-transistors (light-sensitive
transistors - a transistor is an electronic switch). It creates the pictures as a set of dots or pixels
(picture elements).
c Memory cards e.g. flash cards - solid state memory (electronic integrated circuits, i.e. chips, used
for storing the pictures).
There is no delay in getting pictures from digital cameras because there is no film requiring chemical
processing. They can be attached to a computer to directly transfer pictures for editing using special
software and unwanted pictures can be deleted. However, they cost more than conventional cameras
and the quality is not quite as good. You also need to buy rechargeable batteries and a photo-quality
colour printer with high printing costs for paper, ink, etc. Two important features when buying a digital
camera are:
a picture quality or resolution. The resolution of a camera is measured in pixels and given as two
numbers, indicating how many pixels there are across the image and how many going down the
image e.g. 1280 by 960 (or 1280x960).
b the number of pictures the camera can store. The higher the resolution, i.e. the more pixels, the more
memory is required to store the pictures. Data can be compressed to allow more pictures to be
stored.
Storage devices are used to store data and programs that are not being used by the processor. They
usually consist of:
a storage media in the form of a circular disk or a tape where the data is stored
b a disk or tape drive that moves the media past a read/write head that reads the data from and
writes data to the storage media.
Types of storage devices include:
magnetic devices (that use
magnetism)
floppy disks (diskettes) and magnetic tape made of a
magnetic coated flexible plastic; hard disks made of
magnetic coated aluminium disks.
optical devices (that use laser
light)
CD-ROM
CD-R
CD-RW
DVD-ROM
DVD-RAM
– compact disk read only memory
– recordable compact disk
– re-writable compact disk
– digital versatile disk read only memory
– digital versatile disk random access
memory
magneto-optical devices (that
use a combination of magnetism
and laser light)
CD-MO – magneto optical compact disk
Read and write media enable the user to both read data from and write data to the media. Read
only media can only be used for reading data i.e. the stored data cannot be changed in any way.
Removable storage enables the user to change the media and transfer it to another computer.
Fixed storage does not allow the media to be changed or transferred to another computer.
Other factors that vary between storage devices include:
a the speed at which the drive moves the media past the read/write head and reads or writes data to
the storage media
b the capacity of the media i.e. how much data can be stored on each disk or tape
c the cost of the drive and the media.
There are various types of printers for, out-putting text and graphics to paper.
Some types of printers are mono (print in black and white only) and others can print in colour. The
speed, quality and cost of printing varies between different types of printer. Some are designed for
printing text and are not really suited to printing graphics.
Data can take many forms and there is a wide variety of input, output, storage and communication
peripherals.
Units of measurement used in data storage include:
bit a binary digit i.e. a 1 or a 0
Page 9 from 33 VB035 English I
byte 8 bits = 1 character i.e. a letter, numerical digit or a punctuation mark
megabyte (MB) 1,048,576 bytes (approximately one million bytes)
gigabyte (GB) 1,073,741,824 bytes (approximately one thousand million bytes)
terabit 1,099,511,627,776 bits (approximately one thousand gigabits)
micron one millionth of a metre
angstrom the approximate radius of an atom
LISTENING
How a digital camera works
Digital cameras store images on memory cards so pictures can be transferred easily to a computer.
A lens focuses the image on to a CCD unit or Charge-Coupled Device where the film would normally
be.
So you can aim the camera accurately, there is an optical viewfinder.
So you can play back the images and decide which to keep and which to re-shoot, the image is
passed to a small LCD screen on the back of the camera.
SPECIALIST READING
Ready for the Bazillion Byte Drive?
Thinking about writing your memoirs - putting your life story down on paper for all eternity? Why not
skip the repetitive strain injury and just capture your whole life on full-motion video, putting it all in a
device the size of a sugar cube? It might not be as far of as you think.
Currie Munce, director of IBM's; Advanced HDD Technology Storage Systems Division, has one
avowed goal: Build bigger storage. Recently Munce and his fellow Ph.Ds restored Big Blue's lead in the
disk space race with a new world record for areal (bit) density: 35.3 gigabits per square inch - roughly
three times as dense as any drive shipping at press time.
During the 1990s, areal density doubled every 13 months, keeping pace with the transistor density
gains predicted by Moore's Law. But increasingly daunting technical challenges face those who would
push the storage envelope further. 'I think magnetic recording technology has another good 5 to 10
years,'says Munce. 'After that, we'll see substantial difficulties with further advances at the pace people
are accustomed to.'
From here on, a phenomenon called superpararmagnetism threatens to make densely-packed bits
unstable. Provided that new developments continue to thwart superparamagnetic corruption, scientists
speculate that the theoretical limit for discrete bit recording is 10 terabits per square inch (1 terabit
=1,000 gigabits).
Approaching this limit will require new technologies. Two possible contenders are atomic force
microscopy (AFM) and holographic storage. AFM would use a spinning plastic disk, perhaps inside a
wristwatch, and a tiny, 10-micron cantilever with a 40-angstrom tip (an angstrom represents the
approximate radius of an atom) to write data. In theory, AFM will allow densities of 300 to 400 gigabits
per square inch.
While AFM is still in the lab, holographic storage is closer to reality. According to Rusty Rosenberger,
optical program manager for Imation, 'We are targeting a 51/4 -inch disk with 125GB of storage and a
40MB-per-second transfer rate. Future iterations of holographic systems should improve substantially.
The three-dimensional nature of holography makes it an appealing storage medium because 'pages'
of data can be superimposed on a single volume - imagine transferring a whole page of text at once as
opposed to reading each letter in sequence. Hans Coufal, manager of IBM's New Directions in Science
and Technology, Research division, predicts that the fast access rates and transfer times of holographic
storage will lead to improved network searches, video on demand, high-end servers, enterprise
computing, and supercomputing.
Meanwhile, also-ran technologies are thriving, Tape, first used for data storage in 1951 with the
Univac I, has been revitalized by the corporate hunger for affordable archiving solutions. In the
consumer arena, says Dataquest analyst Mary Craig, recordable CD-ROMs and DVDs will remain the
dominant high-capacity removable storage media for the next decade. Despite their failure to match the
areal density gains of hard disks, optical disks are cheap to produce, making them ideal for software
Page 10 from 33 VB035 English I
distribution (until a mature digital rights management system facilitates online delivery). Finally,
solidstate options such as flash cards can't yet match the pricing of hard disks at high capacities.
Further out, scientists salivate over the prospect of data manipulation and storage on an atomic level.
Because consumer demand for capacity is lagging behind what technology can deliver, bringing new
storage options to the masses will depend on seeding the need for more space.
UNIT6 Operating Systems
INTRODUCTION
The OS (operating system) is the set of computer programs that allow the user to perform basic
tasks tike copying, moving, saving and printing files. It also provides an interface between (i.e. provides
communication between) applications programs (e.g. wordprocessors or spreadsheets) and the
computer hardware. As a user interacts with an applications program on the screen, the applications
program communicates with the operating system and the operating system communicates with the
computer hardware. The work of the operating system takes place in the background and is not always
obvious to the user