Chapter 1.5 Hardware

Chapter 1.5 Hardware.

1.5 (a)         Control Unit, Memory Unit, and Arithmetic Logic Unit

Control Unit.

All computers follow instructions that are given to it in a program. These instructions are in a particular order in the program, and following them, and carrying them out, will mean that the computer has accomplished whatever task it was meant to do. Something, in the computer, has to manage the instructions and make sure that all the other parts of the processor do what they should be doing. This is 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 work out 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.

Arithmetic Logic Unit (ALU)

The ALU includes the accumulator which is where things are actually done 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 the processor and the other parts of the computer system. All input and output to and from the processor goes into the ALU and waits there while the control unit decides what to do with it.

Memory Unit

The third part of the processor is where everything that the processor is 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 the time, also need to be stored in memory.

1.5 (b)         Types of Primary Memory

There are a number of different types of memory in the processor.

1. ROM (Read Only Memory)

ROM is memory that cannot be altered. This means that even switching the computer off will not affect the contents of the ROM. There is very little that needs to be stored in ROM so it tends to be very small. In the past the whole operating system was stored on ROM, but that means that if you want to use your computer in a different way or you want to install the latest edition of an operating system to replace the one in use, you can’t because the computer can’t delete the old one. When the computer is turned on it would be there. For this reason the operating system is stored on the hard drive of a computer along with all the other programs that may be used. A problem arises because when the computer is switched on it needs the operating system to be able to do anything useful, so a small program is stored on ROM whose job it is to go and get the operating system from the hard drive so that the computer can work. This little program is called the bootstrap, and the process of retrieving the operating system is known as booting.

2. RAM (Random Access Memory)

RAM stores the programs that are being used by the computer (including the operating system) and the data that is being used with those programs. When the computer is switched off all this data is lost. RAM is said to be volatile memory because it is so easily changed, whereas ROM is non-volatile because it cannot change.

1.5 (c)         Secondary Storage Media

Primary memory is memory within the processor. It is here that the computer stores data that are in current use because the control unit does not have direct access to data that is stored anywhere outside the processor. However, the memory that is available in the processor is limited in size and volatile. What is needed is something that is less temporary in nature and that does not have the same restrictions as far as size is concerned. This will be storage outside the processor. It is called secondary storage.

There are a number of different types of secondary storage that can be categorised 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.

1. Magnetic tape

Magnetic tape is still widely used, particularly in the form of tape streamers. The big disadvantage of tape is that the access to the data stored is, by necessity, sequential which makes it largely unsuitable for most data handling applications because of the time necessary to find a particular 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 up copies of files stored on a computer system.

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 are commercial files of data. However, a floppy disk is very portable, allowing for easy communication 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 crude nature of the read heads.

3.Magnetic hard disk

Data is stored in the same way as on a floppy disk. The differences being that the hard disk is made of a rigid material rather than a floppy plastic, and it is contained in a sealed unit. These differences mean that the hard disk can rotate faster, the heads can get closer to the surface meaning that the storage 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 the tolerances that they have to work to.

4.CDROM

A CDROM is different from the storage devices so far mentioned because it is not magnetic. A CDROM is an optical storage device, using the reflection of a laser off a pitted surface to store information. Large quantities of data can be stored on the surface and it is completely portable from one machine to another. In addition, most computer systems can now be relied on to have a CD drive, and a CD is not alterable by the user. For these reasons, manufacturers have tended to use CDs to produce software and large data files like encyclopaedias.

5. CDRewriteables.

It is now possible to write to CDs by using a special peripheral device called a CD writer and a special CD, but it is important to remember that these are not CDROMs, which cannot be changed. They tend to be used for things like making a back up copy of files, or archiving for long term storage.

6.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 a faster 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 replace CDROM as it can store much larger quantities of data and allow access at high speed. For the purposes of the syllabus, the original 4 types of secondary storage are quite enough at this level.

Speed of access to data.

In general terms the tape streamer is the slowest access because of the way that the data is stored sequentially, then comes the floppy disk which suffers because of the nature of the medium. Access times from CDROM and hard drives are faster than the other two, but because there is such a range of both types of drive available, it is impossible to say that one is faster than the other.

Capacity.

Again, much depends on the type of drive or size of medium 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, a CDROM would store a multi volume encyclopaedia, while a hard drive could store a library.

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 and disadvantages of each.

A Tape Streamer is used for making backups of files held on computer systems. This makes use of the fact that it can store a large amount of data, but the disadvantage of the access being slow does not matter because it is rare that a backup file would be used anyway.

A floppy disk has the advantage that it can be written onto and taken away from the computer. Because of this it can be used for storing confidential files. Add to this the fact that all computers can be relied on to have a floppy disk drive and it becomes a sensible way of transferring information from one machine to another.

A hard drive has the advantages of being fast to access and also stores massive amounts of data. These advantages mean that it is going to be used for storing software and user files. The disadvantage of being attached to one machine is not important if the same users always use the same machines, some other method of sharing will have to be devised if this is not true.

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

1.5 (d)         Transfer of Data, including Buffers and Interrupts

There are two places where data can be stored in a computer system, the primary memory and secondary storage. Sometimes it is necessary to transfer data from the primary memory to secondary storage, perhaps because the computer is going to be switched off in which case the data in primary memory would be lost. Sometimes it is necessary to transfer from secondary storage to primary memory, perhaps because the processor wants to use some data held in the secondary storage and, consequently, needs to move the data into the primary memory first. Whichever direction the data is to be transferred the method of transfer must be planned.

Normally the data would be transferred through a wire connecting the two storage areas together. The type of wire and the different rules for data transfer are explained in section 1.6.d, sufficient for the moment is to have a picture of data traveling freely in both directions.

Unfortunately, we know that things can’t be that simple. We know that primary memory is part of the processor, and that anything that goes in or out of the processor must go through the ALU, so the diagram should look like this

This causes a problem. The primary memory operates at great speed because it is part of the processor, while the secondary storage is probably some sort of disk or tape which (while it seems incredibly quick to us) is very slow at reading or writing the data, in comparison. This means that the processor should be able to get on with something else because the secondary storage is so slow. Unfortunately, it can’t because it needs the ALU that is being used for the transfer of the data. This problem is overcome by the use of a buffer.

Buffers.

The problem is caused because the secondary storage device is so slow compared to the processor, the solution is to put a small amount of fast memory in to the system between the ALU and the secondary storage device.

 This means that the processor can send data very quickly (or receive it), and then get on with something else while the storage device takes its time in reading (or sending) the data. This small amount of memory between the two parts of the system is called a buffer.

Stage One: Filling the buffer from the processor

Interrupts.

This system is fine if the buffer can hold all the data that needs to be sent to the storage device. However, if there is more data than will fit in the buffer, the storage device has to be able to tell the processor (actually the control unit) that it has used up all the data in the buffer and that it is ready for the next lot. This forces the processor to stop what it is doing and fill the buffer up with more data. Because this message from the secondary device makes the processor stop what it is doing it is called an interrupt. When the processor receives the interrupt, the whole procedure is repeated.

1.5 (e) and (f)       Common Peripheral Devices

Any hardware device that is part of the computer system but is not part of the processor itself, is called a peripheral device.

Peripheral devices can be categorised under four headings.

1. Communication devices.

These devices allow for communication between machines and will be covered in detail in section 1.6.b.

2. Storage devices.

These devices provide for secondary storage in a computer system.

3. Input devices.

a) Keyboard.

One problem with normal keyboards is that they are particularly prone to damage from dirt or liquids because of the gaps between the keys. A different type of keyboard, where the keys are simply part of a continuous surface which has areas on it, which are sensitive to pressure, can overcome this problem. Called touch-sensitive keyboards, or concept keyboards, they are ideal for use outside because rain will not damage them like a normal keyboard.

A type of keyboard not yet mentioned is a musical keyboard. Normally arranged like a piano keyboard these need a special piece of hardware to allow them to work properly, known as a MIDI (musical instrument digital interface) this connects the musical keyboard to the processor and allows data to be passed between the instrument and the processor.

b) Mouse. 

c) Barcode readers. 

d) MICR (magnetic ink character reader). This is a device that reads characters that are printed on an original document at the time of it being created. The characters are printed using magnetic ink. The value is that the characters are readable by humans and by machines. The only common use for such characters is the data printed on the bottom of cheques containing account identification.

e) OCR (optical character reader). This is a device that reads characters and can distinguish between the different characters in a given character set. It works by comparing the shape of a scanned character with a library of shapes that it is intended that it should recognise. OCR tends to be an unreliable form of input and works more effectively when it is restricted to having to recognise a standard character set produced by printing rather than by using hand writing. OCR is used for reading post codes on printed documents and also for reading documents for blind people, the contents of which can be output using a voice synthesizer.

f) OMR (optical mark reader).

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

g) Scanners..

h) Graphics Tablet. 

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

4. Output Devices

a) Screens. Monitor screens are categorised according to the obvious colour/monochrome, also according to the number of pixels that there are on the screen. The more pixels there are, the better the picture will be, known as the screen resolution.

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

The first type is a dot matrix printer. These tend to be slow, and the output is particularly poor quality. The big advantage is that the output is produced by using pins to strike at the surface of the paper. Because of the physical nature of the way that the printout is produced, it is possible to obtain multiple copies by using carbon paper or self carbonating paper. A good example of this is the receipt that a shopper is presented with if buying something using 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 with them.

Ink jet printers, which produce output by spraying ink on to the paper could not produce the two copies that the dot matrix can, but it can produce much better quality and in colour, 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 is necessary to give a good impression, for instance sending letters from a solicitor’s office to clients.

Plotters are a type of printer designed for drawing lines and geometric designs rather than for producing characters. The image is created by pens being moved across a piece of paper, under the command of the processor. Plotters tend to be used for drawing blueprints, perhaps in an architect’s office to produce detailed drawings of buildings for builders to follow.

c) Speakers. Used to output sound from a computer system.

Example Questions.

1.         State three functions of the arithmetic logic unit.                              (3)

2. a)     State two ways that RAM and ROM memory differ.                                   (2)

    b)     Explain what types of data would be stored in each of RAM and ROM memory, giving reasons for your answers.                                                          (4)

3.         A student has a home computer system.

            State what storage devices would be used on a home computer system and justify the need for each one.                                                                         (8)

4.         Describe how buffers and interrupts can assist in the transfer of data between primary memory and a secondary storage device.                               (4)

5.            A department store decides to place a computer system by the main entrance to the store   so that customers can find out whereabouts in the store items are available. The different departments remain in the same places, but the articles available in each department change on a regular basis. State a sensible hardware design for such a computer system, giving reasons for your choices of hardware.         (6)

6.            Distinguish between the various types of printers. (9)

7.            Give an application of the following devices:

·                                                        > OMR

·                                                        >MICR

·                                                        >Scanner

·                                                        > Bar code reader     (8)

8. Briefly explain 2 peripheral devices that can be used by disabled persons (4)

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