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

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1 Chapter 1.4 Hardware. 1.4 (a) Control Unit, Memory Unit, and Arithmetic Logic Unit The first thing to be said in this section, is what is not necessary. The three topics that are listed in the heading would normally lead to the fetch- execute cycle, this does not come into this module. Likewise, any complicated diagrams showing how the various parts of the processor are connected up and communicate with each other do not appear in the syllabus until next year. The requirements of this section are very simple; they are restricted to a basic understanding of what these three parts of the processor are meant to do. 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. Page 1 of 14

2 1.4 (b) Types of Primary Memory The first question that needs to be answered is What is Primary Memory? In section 1.4.a the concept of the processor needing to store data of all types in the memory was introduced. However, as those who have used a computer will know, there are plenty of other places that a computer can store data or programs e.g. a disk drive or a CDROM. These will be discussed in section 1.4.c. In the previous section it was stated that any data or program instructions that the processor was to follow had to be in the processor memory. Anything that is being held elsewhere, like on the surface of a disk drive, cannot be used by the processor until it is moved into the processor s memory. Because it is so important it is called the primary memory. The primary memory of the computer is the memory that is advertised as being 128 Mbytes of RAM. This simply describes the capacity of the 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. Notes: 1. There are many other types of primary memory that you may have heard of (PROM, EPROM, EAROM, SRAM, DRAM, ). They are all a type of either RAM or ROM and you have no need to learn about any of them, in fact it is probably better not to until the later parts of the course. 2. 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. Problems arise if a student answers that the operating system is stored in ROM because it can be, but in most micro systems it isn t for the reasons given above. The BIOS (if you don t know what that is, don t worry, it is not in the course) is another problem. The BIOS itself includes user defined parameters and hence is not stored in Page 2 of 14

3 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. This is past the level of this course and students are advised not to use this as an example of storage in ROM. Page 3 of 14

4 1.4 (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 Page 4 of 14

5 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. Solid state storage. Solid-state storage is a nonvolatile, removable storage medium that employs ICs or integrated circuits (all confined entirely within the solid materi.al) rather than magnetic or optical media. It is the equivalent of large-capacity, nonvolatile memory. Examples include flash memory Universal Serial Bus (USB) devices and various proprietary removable packages intended to replace external hard drives. The main advantage of solid-state storage is the fact that it contains no mechanical parts. Everything is done electronically. As a result, data transfer to and from solid-state storage media takes place at a much higher speed than is possible with electromechanical disk drives or optical drives. The absence of moving parts may translate into longer operating life, provided the devices are reasonably cared for and are not exposed to electrostatic discharge. Solid-state storage media lags behind electromechanical drives in terms of storage capacity. As of this writing, flash memory USB devices can store up to approximately 8 gigabyte (GB), whereas hard drives commonly hold 500 GB or more. The cost per megabyte is higher for solid-state storage devices than for electromechanical drives. However, the price gap appears to be narrowing, and the market for solid-state storage is growing. Many businesses and home users who have multiple computers favor solid-state storage devices for transferring data among their machines, because it is convenient, compact, and fast. Flash memory can wear off over a longer period of time and allows writing on it around million times. Some of the hard disks on the market are appearing as hybrid of solid state and magnetic mediums. Frequently used data on such hard disks is saved over flash memory and rest of the data on magnetic medium, which makes this hard disk real fast. 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 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. Notice that there are no figures quoted for access times. There is little point in doing this because access times vary according to the manufacturer and the point of development so far reached. If the author were to quote a number, not only would it be wrong as far as this work is concerned because there is such a wide range, but it would certainly be out of Page 5 of 14

6 date by the time you read it. 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. Page 6 of 14

7 1.4 (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. Every peripheral device that needs processor attention for its work raises interrupt and places its data into primary memory, which when turn comes up can be fetched, processed and placed back there in primary memory by the microprocessor. One of the peripheral can be the 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 travelling freely in both directions. Primary Memory 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 Primary Memory ALU Secondary Storage or other peripheral device. Secondary Storage or other peripheral device. 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. Page 7 of 14

8 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 Primary Memory Stage Two: Emptying the buffer to storage Primary Memory ALU ALU Buffer Buffer Secondary Storage or other peripheral device. Secondary Storage or other peripheral device. 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. 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. The system is not really quite as simple as suggested here, but further complications can wait until the second year of the course. Page 8 of 14

9 1.4 (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 1.5.b. 2. Storage devices. These devices, which provide for secondary storage in a computer system, have been described in section 1.4.c. 3. Input devices. There are too many different input devices to expect familiarity with all of them. However, by inclusion in the syllabus there are some that must be understood. While this list is not exhaustive, specific questions will not be asked about other input devices with the following exceptions. There are some devices that are so common that any candidate taking an exam in computing can be expected to have experience of using them, e.g. mouse, keyboard. Additionally, candidates should be encouraged to learn about a variety of contemporary devices so that they can use them when answering questions that ask for a hardware configuration for a specific application. 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. First, 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. The second is that 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 Page 9 of 14

10 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. Also pay attention to the other types of keyboards sometimes called designer keyboards and concept keyboards. 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 keyfield relating to a file of items that have been barcoded. The details of the contents of the barcodes are not of importance to us in this section, except to say that barcodes can easily be misread by the system, so one of the digits in the number is used to check that the rest of the code has been read properly. This digit is called the check digit, and will be discussed in more detail later in the course. 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 Page 10 of 14

11 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 There are too many output devices to be able to write notes on all of them. Again, the same thing is true about output as is true about input, that 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. 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. This is being 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 game program will need colour and many more pixels in Page 11 of 14

12 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 screen, 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 and the student should be aware of a number of them, their uses, advantages and disadvantages. However, there is no need to understand how they work. 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. There are many other peripheral devices and, as has been mentioned, 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 mentioned at the start of this section. In other words the mark will not be for a keyboard or a mouse, but for suggesting sensible methods of input to the system. Page 12 of 14

13 1.4 (g) Speed Mismatch A typical computer system will consist of the processor to do the necessary calculations or other processing, the peripheral devices to allow input, output and storage of data/information, and the human being that is using the computer. The processor can carry out its side of the task at great speed while the human being is very slow, both at providing input and making sense of the output. This difference in speed is known as the speed mismatch between the operator and the processor. Peripheral devices work at far greater speeds than the human being, but are considerably slower than the processor; consequently there are also speed mismatches on both types of communication concerning peripherals. If there are enough computer systems this speed mismatch does not matter so much, but if there is a shortage of processing time, then a method needs to be found to keep the slowest parts of the systems isolated from the processor. One way of doing this is to use batch processing (see 1.2). The speed mismatch causes more problems if the decisions of the computer need to be acted upon immediately. If a computer is being used to control the insertion of the graphite rods into a nuclear reactor it may become important to ignore the human being once the decision has been made and simply to report that an action has been taken after the event. In this way the speed mismatch has been overcome by cutting out the slowest part of the system. Page 13 of 14

14 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. Explain why speed mismatch would mean that the computer should be in control of the speed of the production line. (3) Page 14 of 14