unit 1.4 (3) (1) computer hardware as level

7/28/2019 UNIT 1.4 (3) (1) Computer Hardware AS level

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HARDWARE

UNIT 1.4

#013A039-Presented by: ANUSHKA K.C.


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1.4(a) PROCESSOR COMPONENTS

Control Unit,

Memory Unit, and

Arithmetic Logic Unit


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1.4.1(i)Control Unit

All computers follow instructions that are given to it in a program.These instructions are in a particular order in the program, andfollowing them, and carrying them out, will mean that the computerhas accomplished whatever task it was meant to do. Something, inthe computer, has to manage the instructions and make sure that allthe other parts of the processor do what they should be doing. Thisis the job of the control unit.

The control unit has three jobs : 1. It has to decide which instruction to carry out next and then go

and get it.

2. It has to decipher the instruction, in other words it has to workout what needs to be done to carry the instruction out. 3. It has to tell other parts of the processor what they should do so

that the instruction will be carried out.


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1.4.1(ii)Arithmetic Logic Unit(ALU)

The ALU includes the accumulator which is where things are actuallydone in the processor.

1. The ALU contains circuitry that allows it to add numbers together

(do arithmetic). 2. It allows for logic decisions to be made (If the value is negative then

, the part of the processor that makes the decision is the ALU).

3. The third task of the ALU is to act as the gateway between theprocessor and the other parts of the computer system. All input and

output to and from the processor goes into the ALU and waits therewhile the control unit decides what to do with it.


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1.4.1(iii)Memory Unit

The third part of the processor is where everything that the processoris going to use is stored.

This includes all the program instructions and all the data needed to

carry out those instructions. One of the jobs of the control unit is to be able to find the information

stored there when it is to be used.

The parts of the operating system, which the computer is using at thetime, also need to be stored in memory.


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1.4.(b) Types of Primary Memory

There are two types of primary memory in a computer:

a. RAM: Random Access Memory

b. ROM: Read Only Memory


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Differences between RAM & ROM

RAM ROM

Random Access Memory or RAM is aform of data storage that can beaccessed randomly at any time, in anyorder and from any physical location.,

allowing quick access andmanipulation.

Read-only memory or ROM is also aform of data storage that can not beeasily altered or reprogrammed.Stores instructions that are not

necessary for re-booting up to makethe computer operate when it isswitched off. They are hardwired.

RAM allows the computer to readdata quickly to run applications. Itallows reading and writing.

ROM stores the program required toinitially boot the computer. It onlyallows reading.

RAM is a type of volatile memory.Data in RAM is not permanentlywritten. When you power off yourcomputer the data stored in RAM isdeleted.

ROM is a type of non- volatilememory. Data in ROM is permanentlywritten and is not erased when youpower off your computer.


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1.4 (c) Secondary Storage Media

The memory that is available in the processor is limited in sizeand volatile. Thus a memory which is less temporary in natureand does not have the same restrictions as far as size isconcerned and is outside the processor is called secondarystorage.

There are a number of different types of secondary storagethat can be categorized according to Means by which the data is stored, optically or magnetically The technique used for storage of the data, sequential

storage or direct access storage

The capacity of the medium, how much can be stored on it Portability of the medium, can it be moved around easily Access times to the data stored.


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1. Magnetic tape

Magnetic tape is still widely used, particularly in the formof tape streamers.

The data in magnetic tape is stored in such a way that it canonly be accessed sequentially.

This makes it largely unsuitable for most data handlingapplications because of the time necessary to find aparticular item of data.

However, large volumes of data can be held and

The medium is freely transportable.

These qualities make it valuable for producing back upcopies of files stored on a computer system.


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2.Magnetic floppy disk

Floppy disks hold a relatively small amount of information.Most software is too large to be stored on a floppy disk, as arecommercial files of data.

However, a floppy disk is very portable, allowing for easycommunication of data from one stand-alone computer to

another. It is also readily available, almost all microcomputers having a

floppy disk drive. It is also convenient for storing those files that are particularly

confidential because the files cannot be broken into if they are

being carried by the owner, or locked away in a safe. Despite being direct, access times are slow because of the

limited speed of rotation possible and the relatively crudenature of the read heads.


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3.Magnetic hard disk

Data is stored in the same way as on a floppy disk. Thedifferences being that the hard disk is made of a rigid materialrather than a floppy plastic, and it is contained in a sealedunit.

These differences mean that the hard disk can rotate faster,

the heads can get closer to the surface meaning that thestorage density can be greater, Consequently that the amount that can be stored is greater

and that the access times are much faster. The hard drive is likely to be the main secondary storage for a

computer system, having very large storage capacities. Although they can be portable, it is more likely that they will

be fixed to the chassis of the machine because of thetolerances that they have to work to.


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4.CDROM

A CDROM is different from the storage devices so farmentioned because it is not magnetic.

A CDROM is an optical storage device, using the reflectionof a laser off a pitted surface to store information.

Large quantities of data can be stored on the surface and itis completely portable from one machine to another.

In addition, most computer systems can now be relied onto have a CD drive, and a CD is not alterable by the user.

For these reasons, manufacturers have tended to use CDs toproduce software and large data files like encyclopedias.


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5. CD Rewriteables.

It is now possible to write to CDs by using a specialperipheral device called a CD writer and a special CD,

But it is important to remember that these are notCDROMs, which cannot be changed.

They tend to be used for things like making a back up copyof files, or archiving for long term storage.


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6. Solid state storage.

Solid-state storage is a nonvolatile, removable storagemedium that employs ICs or integrated circuits (allconfined entirely within the solid materi.al) rather thanmagnetic or optical media.

It is the equivalent of large-capacity, nonvolatile memory.

Examples include:

flash memory Universal Serial Bus (USB) devices andvarious proprietary removable packages intended to replace

external hard drives.


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7.Others

There are many other types of secondary storage,two of which are worth a mention:

Zip drives are a more robust version of a floppy disk, having afaster access speed and a greater capacity. They have become

fairly popular for backing up work on microcomputers.

DVD is a relatively new technology that will probably replaceCDROM as it can store much larger quantities of data andallow access at high speed.


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STORAGE DEVICES


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Speed of access to data.

Notice that there are no figures quoted for access times. Thereis little point in doing this because access times vary accordingto the manufacturer and the point of development so farreached. If the author were to quote a number, not only wouldit be wrong as far as this work is concerned because there is

such a wide range, but it would certainly be out of date by thetime you read it. In general terms the tape streamer is the slowest access

because of the way that the data is stored sequentially, thencomes the floppy disk which suffers because of the nature ofthe medium.

Access times from CDROM and hard drives are faster than theother two, but because there is such a range of both types ofdrive available, it is impossible to say that one is faster thanthe other.


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Capacity.

Again, much depends on the type of drive or size ofmedium being used.

Tape streamers come in different sizes, but tend to

be comparable in capacity with smaller hard drives. Of the different types of disk, a reasonable analogy is

that a floppy would store the text from a book, aCDROM would store a multi volume encyclopedia,

while a hard drive could store a library.


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Uses.

Obviously, no list of uses will be complete. However,there are some obvious uses for each storage type

which take into account the advantages anddisadvantages of each.

A Tape Streamer is used for making backups of files held oncomputer systems. This makes use of the fact that it can store alarge amount of data, but the disadvantage of the access beingslow does not matter because it is rare that a backup file would

be used anyway.


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A floppy disk has the advantage that it can be written onto and takenaway from the computer. Because of this it can be used for storingconfidential files. Add to this the fact that all computers can be reliedon to have a floppy disk drive and it becomes a sensible way oftransferring information from one machine to another.

A hard drive has the advantages of being fast to access and also storesmassive amounts of data. These advantages mean that it is going to beused for storing software and user files. The disadvantage of beingattached to one machine is not important if the same users always usethe same machines, some other method of sharing will have to bedevised if this is not true.

A CDROM cannot be altered. This disadvantage can be turned into anadvantage if the owner does not want the contents of the storage to bealtered. Examples of files stored on CDROM are software for importto a system and large reference files like encyclopedias.


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1.4 (e) and (f) Common Peripheral Devices

Any hardware device that is part of the computersystem but is not part of the processor itself is calleda peripheral device. Peripheral devices can becategorized under four headings.

1. Communication devices. These devices allow for

communication between machines.


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INPUT DEVICES

a) Keyboard. A standard keyboard uses keys that stand for thedifferent characters that the computer recognizes in its characterset.

Most keyboards contain the letters of the alphabet, but not all do,for instance most calculator keyboards are very different, as are

the keyboards for use at ATM machines.

The characters needed for specialist use machines are determinedby the use to which the machine is to be put.

Keyboards are the most common form of input device to a systembecause they are universally available and understood.


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b) Mouse. A mouse is a device designed to be used with apointer on the screen.

It is particularly useful because it mimics the natural humanreaction of being able to point at something. A mouse is reallytwo input devices in one. One is the movement around thescreen created by actually moving the mouse in a physical way,and the other is the ability to select, which is done by using aswitch (the mouse buttons).

Variations have been developed which use the same basicprinciples but are designed for particular applications. Anexample is the tracker ball used in many laptop computers.This is like an upside down mouse where the user moves theball directly rather than moving an object around a flatsurface. This is necessary because when using a laptop theremay not be a flat surface available.


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c) Barcode readers. A barcode reader is a laser scanner thatreads the reflected laser light from a series of dark and lightcolored lines of varying thickness.

The different widths of pairs of lines make up a code that can beconverted into a number. This number can then be used as the

key field relating to a file of items that have been bar-coded. Barcodes are particularly useful because they do not rely on

human beings to input the data, although, if the barcode isdamaged so that the laser scanner cannot read it properly, thedigits represented by the code are printed underneath so that

they can be input by a user at a keyboard. Barcodes are usedwhere the data does not change, and so can be printed onoriginal packaging.


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d) MICR (magnetic ink character reader). This is a device thatreads characters that are printed on an original document at thetime of it being created.

The characters are printed using magnetic ink. The value is thatthe characters are readable by humans and by machines.

The only common use for such characters is the data printed onthe bottom of cheques containing account identification.


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e) OCR (optical character reader). This is a device that readscharacters and can distinguish between the different charactersin a given character set.

It works by comparing the shape of a scanned character with alibrary of shapes that it is intended that it should recognize.

OCR tends to be an unreliable form of input and works moreeffectively when it is restricted to having to recognize a standardcharacter set produced by printing rather than by using handwriting.

OCR is used for reading post codes on printed documents and

also for reading documents for blind people, the contents ofwhich can be output using a voice synthesizer.


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f) OMR (optical mark reader). This device can recognize thepresence of a mark on a sheet of paper. The position of themark conveys information to the machine.

For example a school register may consist of a list of namesof pupils in a class together with two columns of small

rectangles, one for present and one for absent. The sameaction (shading in a rectangle) stands for both being presentand being absent. The difference is the position that themark occupies on the paper.

Printing in the sensitive areas of the sheet is done using aspecial type of ink which the optical scanner does not see,

that is why OMR documents tend to be printed in a light blueor pink color. The other standard use for OMR documents isas multi choice examination answer sheets.

The big advantage of both OCR and OMR is that data can be input to acomputer system without having to be transcribed first, therebycutting down the number of errors on data input.


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g) Scanners. A scanner is a device that converts a document intoa series of pixels (picture elements these are small squaresthat, when put together, form a picture).

The larger the number of pixels, or conversely the smaller eachindividual pixel, the better the definition of the final picture.

There are different types of scanner, but all use the sameprinciple to create the image. A typical use for a scanner wouldbe to input a picture of a house so that it could be included withthe details of a house that is for sale in an estate agentspublication.

A scanner is an input device, not to be confused with a plotterwhich is an output device.


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h) Graphics Tablet. A graphics tablet is a flat surface on which apiece of paper is placed. The user can then draw on the paperand the tablet will sense where the pencil is pointing andtransfer the line to the screen.

i) Microphones. Used to input sound to a computer system.


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INPUT DEVICES


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4. Output Devices

There are too many output devices to be able to writenotes on all of them. Again, the same thing is trueabout output as is true about input, that it isimportant to know about those devices stated in thesyllabus and also a range of devices that will allowfor sensible decisions about peripheral devices to bemade for a given scenario in a question.


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a) Screens. Monitor screens are categorized according to the obvious

color/monochrome, also according to the number of pixels that thereare on the screen. The more pixels there are, the better the picturewill be, known as the screen resolution.

The more pixels that there are on the screen, the higher theresolution is said to be.

A particular type of screen, called a touch screen, acts as both aninput device and an output device.


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Information is output by the system onto the screen and the user isinvited to answer questions or make choices by pointing at a particulararea of the screen. The device can sense where the user is pointing andcan report the position to the processor. The processor can then deduce

what the users reply was according to the position that was pointed to. Touch screens are particularly useful in areas where keyboards are not

appropriate, e.g. where the device may suffer from vandalism.

They are also useful for users who would find difficulty using otherinput devices, e.g. very young children who want to be able to draw on a

screen.


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b) Printers. A printer is a device which provides the user with anoutput from the system which is permanent. This output is known ashard copy, so a printer is a device which produces hard copy.

There are many different types of printer:

The first type is a dot matrix printer. These tend to be slow, andthe output is particularly poor quality. The big advantage is thatthe output is produced by using pins to strike at the surface of thepaper. Because of the physical nature of the way that the printoutis produced, it is possible to obtain multiple copies by usingcarbon paper or self carbonating paper. A good example of this is

the receipt that a shopper is presented with if buying somethingusing a credit card, there are two copies produced, back to back,one for the shop to keep and one for the buyer to take away withthem.


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Ink jet printers, which produce output by spraying ink on to thepaper could not produce the two copies that the dot matrix can,but it can produce much better quality and in color, at low cost.This makes ink jet printers ideal for home use.

Laser printers can produce very high quality work at high speed.The cost is more than with the other types but used where it isnecessary to give a good impression, for instance sending lettersfrom a solicitors office to clients.

Plotters are a type of printer designed for drawing lines andgeometric designs rather than for producing characters. The imageis created by pens being moved across a piece of paper, under thecommand of the processor. Plotters tend to be used for drawingblueprints, perhaps in an architects office to produce detaileddrawings of buildings for builders to follow.


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c) Speakers: Used to output sound from a computer system. Speakersare one of the most common output devices used with computersystems. Some speakers are designed to work specifically withcomputers, while others can be hooked up to any type of sound system.

Regardless of their design, the purpose of speakers is to produceaudio output that can be heard by the listener.

Speakers typically come in pairs, which allows them to produce stereosound. This means the left and right speakers transmit audio on twocompletely separate channels. By using two speakers, music sounds

much more natural since our ears are used to hearing sounds from theleft and right at the same time. Surround systems may include four toseven speakers (plus a subwoofer), which creates an even more realisticexperience.


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OUTPUT DEVICES

unit 1.4 (3) (1) computer hardware as level

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