Control Unit, Memory Unit, and Arithmetic Logic Unit. The CPU or Central Processing Unit, is the central part of a computer.
Author: James Leong Mook Seng
Education Officer
Control Unit, Memory Unit, and Arithmetic Logic Unit
The CPU or Central Processing Unit, is the central part of a computer. Because the CPU performs many different functions, there is a need to divide it up into its components. There are 3 components namely Arithmetic & Logic Unit (ALU), the Control Unit and the Immediate Access Store (known as the IAS or primary memory)
==> Arithmetic Logic Unit (ALU)
The ALU is where things are actually done in the processor.
It can perform arithmetic calculations on data. For example, it can add two numbers together (in binary)
2. It perform logic decisions to be made on data (If the value is negative then ..., the part of the processor that makes the decision is the ALU). These are computations that involve the use of AND, OR, NOT, >,<,>=,<=,<>, =
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.
==> 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 four jobs
. 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.
4 The control unit is to be able to find the information stored there when it is to be used. It knows where the next instruction in memory is because there is a register called the Program Counter that holds the memory address of the next instruction.
==> Memory Unit
The IAS is the place in the CPU where programs and the data that is needed by programs are held, ready to be fetched then decoded and executed by the CPU. The CPU will also use this place to store the results of any processing it does. It is made up of individual memory locations, each capable of storing a byte of data. The parts of the operating system, which the computer is using at the time, also need to be stored in memory.
Performance of Computer System
The materials used in a processor will affect the reliability, speed and performance of that processor. Importantly, the speed of the slowest component might slow right down an otherwise fast computer system.
Processors perform differently because of their clock speed. Every computer has a 'clock' which generates 'pulses' that are used to control how the different components of a computer system work together. A 800 MHZ computer means that the system clock is generating approximately 800 million pulses every second. The faster the clock, the more fetch-decode-execute cycles the CPU can perform in a second, and the faster programs will run.
The size of the address and data buses will affect the performance of the computer. A bus is simply a 'highway' used to move binary data around a computer. Big data and address buses mean that data can be moved quickly compared to smaller buses.
Types of Primary Memory
There are a number of different types of memory in the processor:
. Read Only Memory (ROM)
ROM is memory that cannot be altered. This means that even switching the computer off will not effect the contents of the ROM. There is very little that needs to be stored in ROM so it tends to be very small. This type of memory holds a special program that starts running when the computer is turned on. It holds a part of a program called the BIOS. This program does 2 things
* It checks that the computer hardware is present and correctly working
* It runs a routine that looks for another special program called the boostrap program. Its job is to locate the OS on the hard drive and then load it into RAM and run it. Starting up a computer from a power-off situation to where the OS has been loaded up known as 'booting up' the computer.
The BIOS itself includes user defined parameters and hence is not stored in ROM entirely. In fact the BIOS tends to be stored in a special type of RAM which is refreshed using battery power when the system is switched off. You are advised not to use this as an example of storage in ROM.
A typical question will ask for an example of what is stored in ROM and RAM. The safest answers are the bootstrap being stored in ROM and user software and data being stored in RAM.
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. A computer with 128 MB of RAM has approximately 128 million memory locations in which to store applications and data. The more RAM a computer have, the more applications can be opened at the same time and the more data files can be opened as well.
Why the operating system is not stored in the ROM as the bootstrap program?
Other types of memory
==> Registers
These are part of the design of the CPU. They are memory circuits and are very very fast as they are constantly accessed by the CPU. Examples of registers are the Accumulator, the status register and the Program Counter.
==> Cache
It is provided in computer systems to speed up processing and like all memory it is measured in bytes. Programs are made up of instructions. These are fetched from memory using the fetch-decode-execute cycle. The data that instructions need is also fetched from memory and some data might need to be fetched over and over again.
For example, a constant that is held in memory and is used lots of time in lots of calculations. Fetching data from IAS takes time. Fetching the same data time after time is a waste of time. Processing can be speeded up by storing constantly needed data in some very fast-access memory. This will reduce the 'fetch' time. This fast-access memory is called 'cache'. It is much faster than RAM but slower than registers. It is also very expensive.
How to compare different types of memory
Memory can be classified in a whole variety of ways:
. Primary memory and secondary memory(more commonly referred to as 'secondary storage')
One way to deal with memory is to split it up into two ways: primary memory and secondary memory. Primary memory is the memory that stores data and programs temporarily. Secondary storage devices are connected to the CPU as peripherals such as floppy disk or a hard disk which store data and programs permanently. Primary memory is also known as 'immediate access storage' or IAS
2. READ devices and READ/WRITE memory devices.
Registers, cache, RAM, floppy disks, hard drives and CD-R/W are READ/WRITE devices. That means that data in these devices can be read but also new data can be written to them. ROM, DVDs and some CDROMS are READ ONLY. Data is burnt(written to once) onto these devices and cannot be changed. They are sometimes called WORM devices(Write Once Read Many times)
3. Volatile memory and non-volatile memory.
Another way to split memory types is to divide them between volatile and non-volatile memory. RAM is volatile. That means that when the power is turned off, its ...
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Registers, cache, RAM, floppy disks, hard drives and CD-R/W are READ/WRITE devices. That means that data in these devices can be read but also new data can be written to them. ROM, DVDs and some CDROMS are READ ONLY. Data is burnt(written to once) onto these devices and cannot be changed. They are sometimes called WORM devices(Write Once Read Many times)
3. Volatile memory and non-volatile memory.
Another way to split memory types is to divide them between volatile and non-volatile memory. RAM is volatile. That means that when the power is turned off, its contents is lost. Motherboards have a small amount of battery-backed RAM which keeps the details of the BIOS password, date and time when the power is switched off.
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 storage 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 memory 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.
* Whether the media is read-only or read-write
* The cost of the media
How the capacity of secondary storage media is measured
Data storage is measured in 'bytes'.
* 1 kilobyte(1 KB) equals 1024 bytes or 210 bytes exactly
* 1 megabyte(1 MB) equals 1048576 or 220 bytes exactly
* 1 gigabyte (1 GB) equals 1074741824 or 230 bytes exactly
. Magnetic tape
Magnetic tape is still widely used, particularly in the form of tape streamers. They are used typically to back-up data files on networks.
Advantages
* Large volumes of data can be held
* The medium is freely transportable.
These qualities make it valuable for producing back up copies of files stored on a computer system.
Disadvantage
Access to the data stored is, by necessity, sequential which makes it largely unsuitable for most data handling applications.
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.
Advantages
* 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.
Disadvantage
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.
Advantage
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.
Disadvantage
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.
Advantages
* 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. It is now possible to write to CDs by using a special peripheral device called a CD writer and a special CD, but the above points are still true.
5.Others
There are many other types of secondary storage:
(1) 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.
(2) 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.
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 encyclopedia, 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 (unless it is a special re-writable). 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 encyclopedias.
Data Transfer using buffers and interrupts
When two devices working at different speeds try to communicate, they have to do so at the speed of the slowest device. This is not good, especially when the CPU is involved because the CPU gets tied up managing the transfer of a constant stream of data. That means it can't work on other tasks that may be more urgent than mere printing!
A buffer is simply some fast memory that improves the efficiency of data transfer between two devices working at different speeds by allowing big blocks of data to be collected together and then sent at once rather than a constant stream of data that needs constant CPU management.
A buffer can also be an external piece of hardware designed specifically for the purpose of buffering.
. The CPU, via the ALU, fills the buffer with as much of the file as it can. When the buffer is full, it sends an interrupt to the CPU to say 'stop'.
2. The buffer then communicates with the printer, sending it the data it needs as it needs it. The CPU is free to get on with another job.
3. When the buffer is nearly empty, it sends an interrupt signal to the CPU, to say 'Give me some more of the file, please'
4. When the CPU gets the I/O interrupt, it stops doing and then pushes the contents of its registers onto the stack. It then sends the buffer some more of the file out of the ALU.
5. This is repeated until the whole file has been sent.
The transfer of data between two devices that work at different speeds can be made far more efficient by using a buffer because large blocks of data can be sent rather than in lots of small quantities. This example could be extended to transferring of data from memory to a secondary storage device such as a floppy disk or a hard disk.
Notes:
A buffer is a small amount of fast memory outside the processor that allows the processor to get on with other work instead of being held up by the secondary device.
An interrupt is the message which the secondary device can send to the processor which tells the processor that all the data in the buffer has been used up and that it wants some more.
The example used here was communication between primary and secondary storage, but the same is true for any communication between the processor and a peripheral device, e.g. a printer or a keyboard.
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.
. Communication devices.
These devices allow for communication between machines and will be covered in detail in 'Data Transmission and Networking'
2. Storage devices.
These devices, which provide for secondary storage in a computer system, have been described above.
3. Input devices.
a) Keyboard. A standard keyboard uses keys that stand for the different characters that the computer recognises in its character set. 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 determined by the use to which the machine is to be put. Keyboards are the most common form of input device to a system because they are universally available and understood.
The common keyboard is known as the QWERTY keyboard because those are the first six characters on the top line. The design is not very good for two reasons.
(1) The arrangement of characters comes from the original typewriter whose keyboard was arranged to be the most difficult to use in order to slow typists down so that they did not jam the mechanism of the old machines.
(2) The keyboard itself is difficult to use comfortably because of the way that the keys are arranged in rigid rows, making it awkward to keep your arms comfortable while using it.
The first problem is very difficult to solve because of all the experienced operators that can use the present keyboard so well. Retraining them to use a different arrangement of keys would not be feasible.
Various attempts have been made to address the second problem by arranging the keys in curves that fit the palm of the hand rather than in straight lines. These are called natural or ergonomic keyboards.
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.
Another type of keyboard 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. A mouse is a device designed to be used with a pointer on the screen. It is particularly useful because it mimics the natural human reaction of being able to point at something. A mouse is really two input devices in one.
One is the movement around the screen created by actually moving the mouse in a physical way, and the other is the ability to select, which is done by using a switch (the mouse buttons).
Variations have been developed which use the same basic principles but are designed for particular applications. An example is the tracker ball used in many laptop computers. This is like an upside down mouse where the user moves the ball directly rather than moving an object around a flat surface. This is necessary because when using a laptop there may not be a flat surface available.
c) Barcode readers. A barcode reader is a laser scanner that reads the reflected laser light from a series of dark and light coloured lines of varying thickness. The different widths of pairs of lines make up a code that can be converted into a number. This number can then be used as the key field relating to a file of items that have been barcoded.
Barcodes are particularly useful because they do not rely on human beings to input the data, although, if the barcode is damaged so that the laser scanner cannot read it properly, the digits represented by the code are printed underneath so that they can be input by a user at a keyboard. Barcodes are used where the data does not change, and so can be printed on original packaging.
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). This device can recognise the presence of a mark on a sheet of paper. The position of the mark conveys information to the machine.
For example a school register may consist of a list of names of pupils in a class together with two columns of small rectangles, one for present and one for absent. The same action (shading in a rectangle) stands for both being present and being absent. The difference is the position that the mark occupies on the paper. Printing in the sensitive areas of the sheet is done using a special type of ink which the optical scanner does not see, that is why OMR documents tend to be printed in a light blue or pink colour. The other standard use for OMR documents is as multi choice examination answer sheets.
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. A scanner is a device that converts a document into a series of pixels (picture elements - these are small squares that, when put together, form a picture). The larger the number of pixels, or conversely the smaller each individual pixel, the better the definition of the final picture. There are different types of scanner, but all use the same principle to create the image.
A typical use for a scanner would be to input a picture of a house so that it could be included with the details of a house that is for sale in an estate agent's publication.
A scanner is an input device, not to be confused with a plotter which is an output device.
h) Graphics Tablet. A graphics tablet is a flat surface on which a piece of paper is placed. The user can then draw on the paper and the tablet will sense where the pencil is pointing and transfer the line to the screen.
i) Microphones. Used to input sound to a computer system.
4. Output Devices
a) Visual Display Units. VDUs 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, this is known as the screen resolution.
These notes can be typed using a very low resolution, monochrome screen. If you consider the contents, there is no reason for any further sophistication to be necessary. However, a computer system running a modern game program will need colour and many more pixels in order to produce a satisfactory picture. The more pixels that there are on the screen, the higher the resolution is said to be.
A particular type of VDU, called a touchscreen, acts as both an input device and an output device. Information is output by the system onto the screen and the user is invited to answer questions or make choices by pointing at a particular area of the screen. The device can sense where the user is pointing and can report the position to the processor. The processor can then deduce what the user's reply was according to the position that was pointed to.
Touchscreens 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 other input devices, e.g. very young children who want to be able to draw on a screen.
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. There are many different types of printer:
(1) 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.
(2) 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.
(3) 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.
(4) 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.
Examiner Notes
There are many other peripheral devices and, as has been mentioned, a knowledge of some others will not come amiss, however that is enough to be able to answer questions in the exam. The questions will normally take the form of presenting a scenario and then asking for a description of the hardware required. The important thing to remember is how the marks will be awarded. There will not be a mark for every device mentioned, but the candidate will be expected to give sensible suggestions for each of the four areas of peripherals (communication, input, output and storage). In other words the mark will not be for a keyboard or a mouse, but for suggesting sensible methods of input to the system.
There are too many input and output devices to be able to write notes on all of them. It is important to know about those devices stated in the syllabus and also a range of devices that will allow for sensible decisions about peripheral devices to be made for a given scenario in a question.
Example Questions.
. 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)
A. 1. Any arithmetic is carried out in the ALU. (This normally takes the form of addition, and the result is stored in the ALU temporarily before being returned to the memory)
2. Logical decisions can be made by the processor. (The processor can use the ALU to decide whether one value is greater than another, or if one is negative,..., and then make decisions based on the answers)
3. Communication with peripheral devices is carried out via the ALU. (All data which is to be used by the processor has to be stored in the memory. Any item of data which is bound for the memory must be stored temporarily in the ALU while the processor decides where it is going to be stored)
Notice that the question asked for three answers. The sensible response would seem to be to number them. The expected answers are the three single line responses. The contents of the brackets give extra information which would not be expected in this question. However, if the question had said 'Describe...' or 'Explain...' instead of 'State...', then the question would have been worth 6 marks, and some extra detail, for instance the contents of the brackets, would have been necessary.
2 A. a) -RAM is volatile (will lose its contents when the power is switched off), while ROM is not.
-The contents of RAM can be altered, whereas the data stored in ROM cannot be altered.
b) -RAM would contain user files and software that is in current use.
-RAM is used for these files because they have to be alterable, and the user will want to use different software from time to time, implying that the original software will need to be erased so that it can be replaced. Losing these files when the machine is switched off is not a problem because they will have been saved on secondary storage.
-ROM is used to store the bootstrap program.
-ROM is used because when the computer is initially switched on there must be a program present that can be used to load up the rest of the operating system from secondary storage. This program must not be altered because without it the computer cannot start to work.
Notice the way that the answers have been phrased in part (a). The question asked for a comparison of ROM and RAM so both need to be mentioned when you are giving an answer. To say that RAM is volatile is true, but does not answer the question until you say that ROM is not.
Part (b) is asking for reasons to be given. Notice that the reasons given related to the answers given in part (a). The part (b) answer is not, "user files because there is more space in RAM", although that could be right if phrased a little more carefully.
3 A. -Hard disk...
-to store software and user files (e.g. word processor and essays)
-Floppy disk drive...
-to enable work to be transported between school and home so that it can be continued in the evening
-CDROM...
-to allow the import of new games to the PC.
-Zip drive...
-to back up the files, so that important work is not lost if the hard disk crashes.
Notice that the question does not specify how many devices should be mentioned. However, each device has to be named and justified (2 marks) and there are 8 marks for the question, so four devices seems sensible. What happened to tape? After all that is the first secondary storage that was mentioned in the text. Tape is not sensible in this example, there is not a big enough volume of data to make a tapestreamer worthwhile, and the old cassette tape is an antique by now. Consequently, a tape device does not fit the application given. Notice, also, the way that the justification for each of the devices was linked to the application, for a floppy drive, the answer did not just say that it could be used to transport files, but it gave a good reason for wanting to do so in this case.
4 A. For the purposes of this answer I will assume that data is being transferred from the processor to the secondary storage.
-Buffer is an area of fast access storage..
-which can be filled by the processor and then emptied at slower speed by the secondary storage device...
-allowing the processor to continue with other tasks.
-When the secondary device has used the contents of the buffer...
-it needs to tell the processor that it requires more data...
-this is done by sending a message to the processor, called an interrupt.
Notice that the answer describes transfer of data from the processor to the peripheral. Transfer in the other direction is equally valid and gives rise to a similar answer. However, if the two answers are mixed together then it produces a confused, and often, wrong, answer. This is why the first statement is important as it has set the parameters by which the question will be answered. You don't have to say this in your answer, but it makes clear what is being done.
Also, there are more mark points than there are marks available. Hopefully, you are getting used to this because it is generally true in most questions. In this question there are obviously two points for interrupts and two for buffers, but in order to get the marks you must make sure that the definition appears somewhere. So, for the interrupt, you need to earn the last mark point, otherwise you haven't defined what an interrupt is.
5 A. Input:
-Touchscreen
-Some protection against vandalism/restricts user access to contents of menu system displayed/simple to use because it uses human reaction of pointing so no training necessary
Output:
-(Touchscreen)/printer
-Touchscreen outputs choices for user to select from/printer available for producing hardcopy as a permanent record for the customer
Storage:
-Hard drive/CDROM
-Hard drive necessary to store details of the products on sale because of the large number of changes that occur in this file. CDROM used to keep the store plan and the location of the departments as these do not change.
Notice the large number of possible mark points. Also the way that the marks are split up. In this type of question you can't expect a lot of marks for simply writing down a long list of peripheral devices, the marks are split up according to the four types of peripheral mentioned in section 1.5.e. In reality, this question would probably have talked about such systems being available throughout the store, but this would mean that they would be able to communicate with one another. As communication comes in the next section it was left out of this example question, but such a question would normally have 8 marks, 2 marks for each of the types of peripheral.
.5 Hardware James Leong Mook Seng
AS