CS-Revision Guide4.pdf

318489_03_IGCSE_CO_SE_OL_025-048.indd Page 25 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Key objectives The objectives of this chapter are to revise:...

318489_03_IGCSE_CO_SE_OL_025-048.indd Page 25 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Key objectives The objectives of this chapter are to revise:  output devices: actuators, light projectors, computer architecture: printers, 3D printers, light emitting diode  the central processing unit (CPU) (LED) and liquid crystal display (LCD) microprocessor screens, speakers and sensors  Von Neumann architecture data storage:  arithmetic and logic unit (ALU), control unit  primary memory (random access memory (CU) and registers (RAM) and read-only memory (ROM))  system buses – control bus, address bus,  secondary storage (magnetic, optical and data bus solid state)  Fetch-Decode-Execute cycle  virtual memory  cores, cache and system clock  cloud storage  instruction sets for a CPU network hardware:  embedded systems  network interface cards (NICs) input and output devices:  MAC addresses  input devices: barcode scanners, QR code  IP addresses scanners, digital cameras, keyboards, – routers microphones, mouse, 2D/3D scanners and touchscreens 3.1 Computer architecture 3.1.1 The central processing unit The central processing unit (CPU) has the responsibility for execution and processing of all instructions and data in a computer. It consists of: control unit (CU) arithmetic and logic unit (ALU) registers and buses. A microprocessor is an integrated circuit (microchip) – also referred to as a processor or CPU. The microprocessor contains the ALU and control unit to enable it to interpret and execute all instructions and carry out arithmetic operations. The CPU also contains the system clock and primary memory. 3.1.2 Von Neumann architecture It is illegal to photocopy this page Von Neumann introduced the idea of the stored program computer, which has been the basis of many computers for a number of years. The von Neumann architecture had the following main novel features (none of which were available in computers prior to the mid-1940s). The concept of a central processing unit (CPU or processor). The processor was able to access the memory directly. Computer memories could store programs as well as data. Stored programs were made up of instructions which could be executed in sequential order. Hodder & Stoughton Limited © David Watson and Helen Williams 2022 25 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 26 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.1 Computer architecture The following diagram summarises the components of a typical CPU. The ALU allows all arithmetic (such as +, −) and logic (such as AND, OR) operations Arithmetic to be carried out. and Computers can have more than one ALU. logic unit The ALU allows multiplication and division using shifting operators. The control unit reads instructions from memory. Signals are generated during the Fetch-Decode-Execute cycle to control all Control unit components of the computer. The control unit synchronises data flow and program instructions throughout the computer. Registers can be general purpose or special purpose. High speed areas of memory within a microprocessor/CPU – used to store small amounts of data, address of next instruction to be executed or current instruction Registers being decoded. Registers include: current instruction register (CIR), accumulator (ACC), memory address register (MAR), memory data/buffer register (MDR), program counter (PC). Registers are used in the Fetch-Decode-Execute cycle. The system clock is used to produce timing signals on the control bus to ensure all functions are synchronised. System clock Central Changing clock speed can improve performance but there is always the risk of processing overclocking (which can lead to overheating and system crashes). unit Buses transfer data and control signals throughout a computer. They use parallel data transfer and the width of a bus can vary according to computer architecture (number of bits used can be 8 to 64). There are three common types of bus: address bus (unidirectional) carries addresses throughout the system System buses data bus (bidirectional) allows data to be carried to/from CPU/memory and to/from input–output control bus (bidirectional) carries signals from the control unit to all computer components. The width of the data bus and address bus determines the word length/size and the performance of the computer. The random access memory (RAM) holds data, programs and the part of the operating system currently in use. RAM is often known as the Immediate Access Store (IAS). All data being used in programs needs to be transferred to the RAM. Memory Memory is made up of an address and its contents; the address identifies the location of data and instructions. All memory locations are unique. It is illegal to photocopy this page Primary memory also includes read-only memory (ROM) which cannot be altered; ROM contains start-up routines for the computer. The Fetch-Decode-Execute cycle To carry out a set of instructions, the CPU first fetches data and instructions from memory and stores them in the appropriate registers. Both the address bus and data bus are used in the process. Each instruction needs to be decoded and finally executed – this forms the Fetch-Decode-Execute cycle. The following diagram summarises the cycle. 26 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 27 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Fetch-Decode-Execute cycle Fetch Decode Execute The The instruction PC containsisthe nowaddress decoded so of the The instruction is now decoded so that The CPU passes the decoded that it canlocation memory be interpreted in the of the next next instruction it can be interpreted in the next part of instructions as a set of control signals part to beoffetched the cycle. from memory (MAR). the cycle. to appropriate computer components. The address is copied from the PC to This allows each instruction to be the MAR using the address bus carried out in a logical sequence. The contents of the memory location contained in the MAR are then copied into the MDR. The contents of the MDR are then copied to CIR and the PC is incremented by one so the next instruction can be processed. 3.1.3 Cores, cache and internal clock The system clock defines the clock cycle used to synchronise all computer operations. Timing signals are transmitted via the control bus. In general, increasing the clock speed increases the processing speed of the CPU (since it can carry out more processes per second). The width of the address bus and data bus increases the processing speed of the CPU (for example, a bus width of 16 bits can address 216 (= 65 536) memory locations, whereas a bus width of 64 bits can address 264 (almost 20 000 000 000 000 000 000 memory locations). Overclocking, caused by changing the clock speed in the Basic Input/ Output System (BIOS), can lead to overheating and to non-synchronised instructions which can cause the computer to crash. Caches, which store frequently used instructions and data, can speed up CPU performance. The larger the cache memory size the better the CPU performance. Using a different number of cores can also improve CPU performance. One core is made up of an ALU, a CU and registers. A quad core processor is a microprocessor with four independently operating units (cores) that can read and execute CPU instructions. Each core communicates with the other three cores using six channels. 3.1.4 Instruction set Instructions are a set of operations that need to be decoded in sequence; each operation is made up of an opcode and an operand. It is illegal to photocopy this page Operand is the data which needs to be acted on or refers to registers Opcode informs the CPU what operation needs to be done. in memory. A number of allowed opcodes exist and these are referred to as the instruction set. The Fetch-Decode-Execute cycle controls the sequence of steps to process each instruction in sequence. Instruction sets are low level language instructions that instruct the CPU how to carry out an operation. Hodder & Stoughton Limited © David Watson and Helen Williams 2022 27 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 28 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.1 Computer architecture 3.1.5 Embedded systems An embedded system is a combination of hardware and software designed to carry out a specific task. Hardware can be electronic, electrical or electro-mechanical. Embedded systems can be based on a microcontroller, microprocessor or system on a chip (SoC). Embedded system Microcontroller Microprocessor System on a chip Made up of a CPU with Integrated circuit consisting May contain a microcontroller RAM, ROM and peripherals of CPU only (no peripherals). as one of its components; all embedded on a single usually includes a CPU, chip to carry out a specific memory, input/output (I/O) task. ports and secondary storage all on a single chip. This is the general make-up of an embedded system: User interface Software and hardware on Analogue or digital input Output an embedded system Embedded systems can contain Modern systems use dual and sensor input, mechanical components, quad core CPUs and a variety of actuators and software. I/O connections. When installed either the operator inputs data manually (for example, pressing a button or turning a knob to select a temperature) or automatically (for example, reading from a sensor or from another computer system). Embedded systems can be programmable or non-programmable. There are definite benefits and drawbacks of devices being controlled using embedded systems: Benefits Drawbacks They are small in size and therefore easy to fit It can be difficult to upgrade some devices to take advantage of into devices. new technology. Compared to other systems, they are relatively Troubleshooting faults in the device is a specialist task. low cost to make. It is illegal to photocopy this page They are usually dedicated to one task making Although the interface can appear to be simple (such as a single for simple interfaces and often no requirement knob) in reality it can be more confusing (for example, changing for an operating system. the time on a cooker clock can require several steps). They consume very little power. Any device that can be accessed over the internet is also open to hackers viruses and so on. They can be controlled remotely using a mobile Due to the difficulty in upgrading and fault finding, devices are phone, for example. often just thrown away rather than being repaired (very wasteful). Very fast reaction to changing input (operate Can lead to environmental issues created by an increase in the in real time and are feedback orientated); with ‘throw away’ society if devices are discarded just because they mass production comes reliability. have become out of date. 28 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 29 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Examples of the use of embedded systems include: security systems (use sensors, such as temperature, acoustic and pressure, to monitor for intruders and sound an alarm if necessary) set-top box to record and play back television programmes (allow aerial, cable, satellite or Wi-Fi inputs and can be controlled remotely) lighting applications (to control lighting depending on time of day, whether a room is occupied and brightness of ambient light; makes use of sensors and actuators to monitor and control lighting levels) Vending machines vending machines (monitor selection, money entered, tilting of use microcontrollers to control a number of machine and delivery of items using actuators and motors; uses sensors functions. to detect tilting, temperature and to count money entered) washing machines (selection is via keypad which allows wash program to be selected) motor vehicles (fuel injection system, Global Positioning System (GPS) navigation, in-car entertainment, anti-lock braking system (ABS), and so on). Sample questions and answers a) Describe the role of registers and buses in a typical computer system. b) Discuss four ways to improve the performance of a CPU in a computer system. Sample high-level answer a) Registers are data holding places which form part of the CPU. They can hold an instruction, an address or data. Registers can be general or special purpose. The most common registers include:  current instruction register (CIR) which stores the current instruction being decoded and executed  accumulator (ACC) which is used when carrying out ALU calculations; it stores data temporarily during the calculations  memory address register (MAR) which stores the address of the memory location currently being read from or written to  memory data register (MDR) which stores data that has just been read from memory or data which is about to be written to memory  program counter (PC) which stores the address where the next instruction to be fetched can be found System buses are used in computers to carry out three functions:  address bus carries addresses throughout the computer system  data bus carries data to and from the CPU, memory and I/O devices (data can be an address, instruction or a numerical value)  control bus carries signals from the CPU to all other components to synchronize all operations b) The performance of a CPU can be improved by the following methods: It is illegal to photocopy this page  increasing the size of RAM; since data is constantly moved into and out of RAM, the larger the RAM the fewer data movements there needs to be which increases operational speed  increasing the width of the address bus and data; this allows more memory locations to be addressed at the same time thus increasing performance of the CPU  increasing the clock speed; however, this needs to be met with some caution, since it can lead to overheating and potential computer crash as the number of instructions cause the CPU to overheat and for operations to become out of synchronisation  making the processor quad core (or even higher) allows multiple functions to be carried out simultaneously which greatly improves CPU performance Hodder & Stoughton Limited © David Watson and Helen Williams 2022 29 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 30 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.1 Computer architecture Tips Teacher’s comments In part a) it is necessary to consider the main features of both registers The first answer has and buses in the context of a typical computer system. It is essential to state facts and expand on these facts as much as possible to ensure you provided an almost give a complete description; remember four marks implies at least four perfect answer for both facts fully explained to gain maximum credit. parts. The answer is a very comprehensive In part b) you are asked to discuss ways of improving CPU performance. description of both For each example you give to enhance CPU performance, you must include registers and buses, but a reason why it will have a positive effect or negative effect. also provided examples of their use. In part b) the four improvements are Sample low-level answer expanded to show why a) Registers are used in computers to store data during the Fetch Decode they improve performance. Execute Cycle. An example of a register is the Accumulator. A very informative answer. Buses allow data and instructions to be carried around the computer during processing. The second answer shows b) The performance of a computer can be improved by using more RAM, changing less understanding of the clock speed and using bigger buses. computer architecture. Part a) gains one mark for a mention of registers Exam-style questions being used in the FDE cycle and also a mark 1 Five statements are shown on the left, and five computer terms for the use of buses. on the right. The answer given is too By drawing lines, connect each statement to the correct brief to gain any further computer term. credit. In part b), there Ensures correct synchronisation of all computer is no mention of how the Core operations methods given would improve performance. Can improve the performance of the CPU by The question asked for Overclocking increasing its width a discussion of ways to improve performance; Temporary storage space that stores frequently this means it is necessary Address/data bus used data and instructions to explain why the methods mentioned would Can lead to overheating and non-synchronised work. The answer is far System clock too vague and needs operations in the CPU expanding (for example, Part of the processing chip that does the ‘changing clock speed’ Cache (memory) – how would this be processing work used; ‘use bigger buses’ 2 Give three benefits and three drawbacks of using embedded – what exactly does that systems in household devices. statement mean?). No 3 a) Explain what is meant by the Fetch-Decode-Execute cycle. marks would be gained in It is illegal to photocopy this page b) Describe three actions that take place during the Fetch- part b). Decode-Execute cycle. 30 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 31 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware 3.2 Input and output devices 3.2.1 Input devices Barcode readers Barcodes are a series of dark and light parallel lines of varying thickness. They are a unique series of lines representing the digits 0 to 9. Barcodes are read by a scanner that uses a red laser or red light emitting diode (LED) light source. Reflected red light is read by photoelectric cells (sensors) that create a digital sequence of dark and light; for example, LDDDDLD (or 0111101 in binary). Barcodes are used mainly on goods in a supermarket, but they can be used anywhere where an item can be scanned to identify it (for example, in a library). Barcode data is stored on a file in a products database; each barcode is used as a key field to uniquely identify the record which gives the data about the product – once the barcode is read, the product details (such as price) can be retrieved. They also allow for automatic stock control and sales reporting. Advantages of using barcodes to the management Much easier and faster to change prices on stock items. Much better, more up-to-date sales information/sales trends. No need to price every stock item on the shelves (this reduces time and cost to the management). Allows for automatic stock control. Possible to check customer buying habits more easily by linking barcodes to, for example, customer loyalty cards. Advantages of using barcodes to the customers Faster checkout queues (staff don’t need to remember/look up prices of items). Errors in charging customers is reduced. The customer is given an itemised bill. Cost savings can be passed on to the customer. Better track of ‘sell by dates’ so food should be fresher. QR codes Quick response (QR) codes are another type of barcode made up of a matrix of filled-in dark squares on a light background. QR codes hold considerably more information/data than barcodes (up to 7089 digits or 4296 characters). Three large squares at the corners are used to align the QR code when it is being scanned by a camera. It is illegal to photocopy this page QR codes can be used to advertise products, give automatic access to websites, store boarding passes electronically at airports and so on. QR codes are read by a camera on a smartphone or tablet using the following method. Camera on smartphone or tablet is pointed at the QR code. A stored app processes the image taken by the camera. Hodder & Stoughton Limited © David Watson and Helen Williams 2022 31 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 32 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.2 Input and output devices The browser automatically reads the data generated by the app and decodes any web addresses embedded in the QR code. Any weblinks are then sent to the smartphone or tablet. Advantages of QR codes compared to traditional barcodes They can hold much more information. There will be fewer errors; the higher capacity of the QR code allows the use of built-in error-checking systems – normal barcodes contain almost no data redundancy (data which is duplicated), therefore it isn’t possible to guard against badly printed or damaged barcodes. QR codes are easier to read; they don’t need expensive laser or LED scanners like barcodes – they can be read by the cameras on smartphones or tablet. It is easy to transmit QR codes either as text messages or images. It is also possible to encrypt QR codes which gives them greater protection than traditional barcodes. Disadvantages of QR codes compared to traditional barcodes More than one QR format is available. QR codes can be used to transmit malicious codes – known as attagging; since there are a large number of free apps available to a user for generating QR codes, this means anyone can generate QR codes. It is therefore relatively easy to write malicious code and embed this within the QR code. Digital cameras Digital images taken by cameras can easily be transferred to a computer (or other device) via USB port, Bluetooth (wireless transfer) or memory card reader. Microprocessors in the camera automatically control many camera functions, such as shutter speed, focus, aperture size, flash, ‘red eye’ removal and so on. Many modern cameras have tiny lenses which allow them to be used in, for example, car bumpers (as a parking aid), in drones (for aerial photos) and in endoscopes (to check the inside of pipes, for example). The image through the camera lens is captured on photodiodes (charge couple devices (CCD)) and converted into pixels which form an electronic matrix of the image which can be stored in memory. Keyboards Even with all the modern alternatives available, keyboards are still the most common data entry devices. They can be physical keyboards (connected via USB or Bluetooth) or virtual keyboards (for example, on a smartphone or keyboard using the touchscreen features). It is illegal to photocopy this page Each character on a keyboard has an ASCII value. The computer is able to detect which key is pressed, which allows it to determine the character that was selected. Entry of data via a keyboard is a slow process which is also prone to error and can lead to injuries such as repetitive strain injury (RSI). 32 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 33 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Microphones Microphones have a number of applications, for example, as a sensor (to detect sound in an intruder detection system), to input text into a computer (of particular benefit to a disabled person who cannot use a keyboard) or for doing voiceovers on presentation slides and so on. Microphones convert sound into electric currents of varying amplitude. A diaphragm in the microphone vibrates and a copper coil and cone attached to the diaphragm create an electric current as the coil vibrates back and forth. The electric current can be converted into digital data and then stored in a computer memory. Optical mouse A mouse is used to move an on-screen cursor which allows the user to take actions (for example, select an option from a menu).The optical mouse is an example of a pointing device. It uses tiny cameras and a red LED light source to allow the exact position of the mouse to be calculated. Unlike a mechanical mouse, the optical mouse can work on any surface and has no moving parts to wear out. They are connected to a computer using one of the USB ports or by Bluetooth connectivity. Advantages of an optical mouse compared to a mechanical mouse No moving parts therefore more reliable. Dirt can’t get trapped in any components. Scanning formats can be: No need for special software. PDF (portable document Advantages of a wired mouse (using USB) compared to a wireless format) mouse TIFF (tagged image file format) recommended Unlike Bluetooth, wired connections have no signal loss. for uncompressed Cheaper to use (no need for batteries). scanned images Fewer environmental issues (for example, disposal of batteries). JPEG (joint photographic experts group); Scanners standard file format if 2D scanners are used, for example, to scan in documents. The information a compressed image is on the document is converted into an electronic format that can be stored required. on a computer memory. Computers equipped with optical character recognition (OCR) software allow the scanned text to be converted into a text file format, which can then be imported into a word processor. The original text can then be manipulated as required. 2D scanners are used at airports, for example, to read passports enabling automatic border controls. The photo in the passport is scanned and then compared to an image taken by a camera at border control. Scanning It is illegal to photocopy this page faces is called facial recognition and is used in many security systems, for example, to control entry to a building or as a security device on a smartphone to prevent unauthorised use. 3D scanners scan solid objects and produce a 3D image that can then be used in computer-aided design (CAD) software or even sent to a 3D printer allowing the scanned object to be duplicated. 3D scanner technology uses lasers, X-rays, magnetic resonance or white light. 3D scanners are used, for example, in medical applications to build up images of parts of the human anatomy (the part of the anatomy being scanned is divided up into thin Hodder & Stoughton Limited © David Watson and Helen Williams 2022 33 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 34 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.2 Input and output devices slices by the scanner producing a 3D image – this is known as tomography; it is effectively the reverse of what 3D printers do). Tomography uses X-rays to produce 3D images; magnetic resonance imaging (MRI) scanners use radio frequencies. Touchscreens All types of technology Touchscreens are now common input devices, used at ordering stations for permit use of bare fast food, information kiosks at airports and on smartphones and tablets. fingers or a special stylus. There are three common touchscreen technologies: capacitive, infrared and resistive. Touchscreen technology Capacitive Infrared Resistive Two main types: surface captive Uses a glass screen and an array of Made up of two layers of polymer screens or projective capacitive sensors and infrared transmitters. and glass. screens. Where the infrared beam is broken A voltage is applied across the two Relies on fingers touching the screen by a finger, the signal to some of the surfaces. causing a change in the electrostatic sensors is broken or reduced thus fields (change in current flow). allowing a microprocessor to calculate When the top polymer surface is where the screen was touched. touched it makes contact with the The point where the finger touches bottom layer completing a circuit the screen is calculated by a Allows multi-touch facility. allowing current to flow at that point. microprocessor. Has good screen durability. The point of contact is detected where If projective technology is used, a change in voltage occurred, which multi-touch facility (pinching and Not severely affected by scratches or enables a microprocessor to calculate sliding) is allowed. even cracked screens. the coordinates. Has good clarity in all lighting Can be sensitive to moisture on the Has good resistance to dust and water. conditions. screen. Low sensitivity and doesn’t allow Durable screens that tolerate scratches. Can be sensitive to light interference. multi-touch facilities. Technology can be sensitive to Vulnerable to scratches and cracked magnetic fields or microwaves. layers. 3.2.2 Output devices Actuators When a computer or microprocessor is used to control a process (such as a conveyer belt or a valve) it is usually necessary to use an actuator to start/stop the conveyor belt or open/close the value. An actuator is a mechanical or electromechanical device, such as a relay, solenoid or motor (see Section 3.2.3). It is illegal to photocopy this page Light projectors Light projectors are used to project computer output onto a larger screen or interactive whiteboard; the technology is quite complex since it has to convert a computer’s digital output into light which can then be projected. The two most common types are digital light projectors (DLP) or liquid crystal display (LCD) projectors. 34 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 35 17/07/22 12:58 PM F-0250 /145/HO02580/work/indd 3 Hardware Type of projector DLP LCD Uses many micro mirrors arranged on a A powerful beam of light is sent to a DMD chip. chromatic-coated mirror which splits the image into red, green and blue These mirrors can move according to the components. data sent to them from the computer. Three different versions of the same image Micro mirrors produce a greyscale image is produced; for example: of the light source (1024 shades of grey). A bright light is shone on the DMD chip passing through an RGB filter where the greyscale image is now converted into a The images are recombined using a full-colour image. prism which produces an enlarged full-colour image: The advantages and disadvantages of the two types of projector are summarised in this table. Advantages Disadvantages Digital light projector (DLP) Higher contrast ratios. Image tends to suffer from ‘shadows’ when showing a moving image. Higher reliability/longevity. Quieter running than LCD projector. Does not have grey components in the image. Uses a single DMD chip which equates to no The colour definition is frequently not as good as LCD projectors issues lining up the images. (that is, the colour saturation is not as good). Smaller and lighter than LCD projectors. It is better suited to dusty or smoky atmospheres than LCD projectors. Liquid crystal display (LCD) projector Gives a sharper image than DLP projectors. Although improving, the contrast ratios are not as good as DLPs. Has better colour saturation than DLP Has a limited life (that is, the longevity is not as good as DLPs). projectors. More efficient in its use of energy than DLP Since LCD panels are organic in nature, they tend to degrade technology – consequently it generates less with time (screens turn yellow and the colours are subsequently heat. degraded over time). It is illegal to photocopy this page Printers There are two common types of printer: inkjet and laser. Inkjet printers rely on spraying liquid ink droplets from a reservoir onto Some of the latest inkjet paper; they use either thermal bubble or piezoelectric technology to printers now use ink create the ink bubbles and droplets. Stepper motors move paper up a line reservoirs (instead of ink cartridges) which at a time and the print head moves across the page left to right. overcomes the problem The inkjet ink cartridges and paper trays are only suitable for relatively of running out of ink small print runs (for example, a one-off photograph or a few pages of during large print runs. high-quality colour printing). Hodder & Stoughton Limited © David Watson and Helen Williams 2022 35 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 36 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.2 Input and output devices Laser printers rely on using dry powder ink (known as toner); this solid ink is melted onto the paper using a fuser. The position where text or images is to be printed is charged negatively on a drum using a laser. Positively charged ink then sticks to the areas of negative charge on the drum which is then transferred to a sheet of paper as the drum rotates. The whole page is produced in one go. Ink/toner cartridges and paper trays are much larger than those used in inkjet printers; consequently, laser printers are more suitable for large print runs (for example, producing hundreds of advertising leaflets). 3D printers 3D printers are used to produce solid objects that actually work; the printers are based on inkjet and laser printer technology. A solid object is built up from very thin layers of material; the printing medium can be powdered resin, powdered metal, paper or ceramics (but this list is by no means exhaustive). 3D printing is an example of additive printing (that is, an object is built up layer by layer). Direct 3D printing uses a print head moving left to right and up and down as it builds up the thin layers (typically 0.1 mm thick). Binder 3D printing works in a similar way to direct printing, except there are two passes of the print head for each layer; the first pass is dry powder and the second pass is a binding agent (which holds the powder layers together). There are many uses of 3D printing: in medicine (for example, prosthetic limbs and reconstructive surgery), in aerospace to make light-weight parts, fashion and art to create one-off dresses and sculptures (and to make copies of rare paintings which are indistinguishable from the originals) and making parts for items (for example, vintage and veteran cars) which are no longer available. Many televisions are LED and LCD screens advertised as LED when An LED screen is made up of many tiny light emitting diodes. Each LED in fact they are LCD is red, green or blue. By varying the electric current to each diode, its screens backlit with LEDs. brightness is controlled which results in millions of different colours. LED screens are used in large outdoor advertising displays and large scoreboards at sporting events. LCD screens are made up of millions of tiny liquid crystals arranged as a matrix (array) of pixels. By varying the electric field to the liquid crystals their properties change. Since LCDs do not produce any light, they need to be backlit with a light source, such as LEDs. LEDs are used to backlight LCD screens because: they reach maximum brightness immediately It is illegal to photocopy this page they produce a very white light which gives good colour definition they last almost indefinitely and consume very little power. LCD–LED screens are being replaced with organic light emitting diodes (OLED) screens, which generate their own light thus requiring no backlighting. This is because OLED screens: allow very thin screens (2 mm or less in thickness), which means they can be formed into almost any shape provides brighter colours than LED backlit LCD screens allow for true black, unlike LCD consume very little power. 36 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 37 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Speakers Loudspeakers produce sound from varying electric currents. If sound is stored digitally on a computer, it needs to pass through a digital to analogue converter (DAC) first to turn the digital signals into an electric current to drive the speakers. Speakers are made up of a paper/plastic cone, permanent magnet and a coil of wire wrapped around an iron core. Varying electric currents cause the iron core to vibrate; this is connected to the paper/plastic cone which therefore also vibrates producing sound. 3.2.3 Sensors Sensors are input devices that read physical properties from their surroundings (for example, temperature or light). Sensors constantly send signals to a microprocessor (or computer); they usually pass through an analogue to digital converter (ADC) first since computers can only understand binary (digital) data. If the computer or microprocessor is only monitoring a process, then the sensor data is compared to stored data and a warning sound or screen output is produced to alert the user. Examples include monitoring a patient’s vital signs in a hospital, an intruder alarm system, and pollution levels in a river or the atmosphere. If the computer or microprocessor is controlling a process, then the sensor data is again compared to stored/set data and action is taken if the sensor data indicates the measured parameter is out of range. Signals are then sent out to alter the process until the sensor readings are back in range again (for example, a valve may need to be open, or a heater switched off). This is called feedback. Examples of control include: automatically turn on a car’s headlamps when it gets dark, controlling the temperature in a greenhouse or controlling a chemical process (for example, to maintain correct temperature, acidity (pH) or pressure). Control data from the computer or microprocessor is often sent through a DAC first so that it can operate the actuators. The following table summarises some common sensors. Sensor Description Example applications Temperature Measures temperature of the surroundings control central heating system by sending signals; these signals will change control/monitor chemical processes as the temperature changes. control/monitor temperature in a greenhouse Moisture Measures water levels in, for example, soil control/monitor moisture levels in soil (it is based on the electrical resistance of the monitor moisture levels in a food processing sample being monitored). factory Humidity Slightly different to moisture; measures monitor humidity levels in a building It is illegal to photocopy this page the amount of water vapour in, for example, monitor humidity levels in a factory a sample of air (based on the fact that the manufacturing microchips conductivity of air will change depending on monitor/control humidity levels in the air in a amount of water present). greenhouse Light Use photoelectrical cells which produce an switch street lights off or on depending on light output (in the form of an electric current) levels depending on the brightness of the light. switch on car headlights automatically when it gets dark Hodder & Stoughton Limited © David Watson and Helen Williams 2022 37 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 38 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.2 Input and output devices Sensor Description Example applications Infrared Use an invisible beam of infrared radiation turn on car windscreen wipers automatically (active) picked up by a detector; if the beam is broken, when it detects rain on the windscreen then there will be a change in the amount security alarm system (intruder breaks the of infrared radiation reaching the detector infrared beam) (sensor). Infrared Measure the heat radiation given off by an security alarm system (detects body heat) (passive) object; for example the temperature of an monitor the temperature inside an industrial intruder or the temperature in a fridge. freezer or chiller unit Pressure A transducer that generates different electric weigh lorries at a weigh station currents depending on the pressure applied. measure the gas pressure in a nuclear reactor Acoustic/ Basically microphones that convert detected pick up the noise of footsteps in a security sound sound into electric signals/pulses. system detect the sound of liquids dripping at a faulty pipe joint Gas Most common ones are oxygen or carbon monitor pollution levels in the air at an airport dioxide sensors; they use various methods to monitor oxygen and carbon dioxide levels in a detect the gas being monitored and produce greenhouse outputs which vary with the oxygen or carbon monitor oxygen levels in a car exhaust dioxide levels present. pH Measure change in voltages in, for example, monitor/control acidity levels in soil soil depending on how acidic the soil is. control acidity levels in a chemical process Magnetic field Measure changes in magnetic fields – detect magnetic field changes (for example, in the signal output will depend on how the mobile phones and CD players) magnetic field changes. anti-lock braking systems in cars Accelerometer Measure acceleration and motion of an measure rapid deceleration in cars, and apply application, that is, the change in velocity (a airbags in a crash piezoelectric cell is used whose output varies change between portrait and landscape mode according the change in velocity). in mobile phones Proximity Detect the presence of a nearby object. detect when a face is close to a mobile phone screen and switch off screen when held to the ear Flow (rate) Measure the flow rate of a moving liquid or gas in respiratory devices and inhalers in hospitals and produce an output based on the amount of measure gas flows in pipes (for example, liquid or gas passing over the sensor. natural gas) Level Use ultrasonics (to detect changing levels monitor levels in a petrol tank in a car in, for example, a tank) or capacitance/ in a pharmaceutical process where powder conductivity to measure static levels levels in tablet production need to be (for example, height of water in a river) – monitored note level sensors can also be optical or leak detection in refrigerant (air conditioning) mechanical in nature. Sample questions and answers a) Explain the term sensor. It is illegal to photocopy this page b) Describe an application that uses magnetic field sensors. c) A chemical process only works correctly if the temperature is above 70 °C and the pH is below 3.5. Sensors are used as part of the control process. A heating element is used to heat the reaction if necessary and valves are opened or closed to add acid to maintain the reaction pH. Describe how sensors, actuators and a microprocessor are used to control the conditions in this chemical process. 38 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 39 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware Tips Teacher’s comments In part a), a simple definition is required with some expansion for the The first set of answers is second mark. Part b) requires the description of a sensor application. You are not asked to explain how the sensor works, simply the very clear and concise for application it refers to. Part c) is also a description. However, this time all three parts. The use of you are asked to explain the interaction between sensors, actuators and a diagram of the control a microprocessor. If at all possible, it is worth drawing a simple diagram system is particularly of the process which will help in your description. helpful in the explanation of how sensors, actuators and a microprocessor Sample high-level answer interact. In part b), the candidate has given a a) A sensor is an input device that measures physical properties from the ‘what’ (ABS), a ‘why’ (to environment. detect when wheels lock b) An application of magnetic field sensors is in anti-lock brakes used in vehicles up) and a ‘how’ (applies (ABS). When a wheel locks up, this is detected by the microprocessor (which brake force and alleviates is constantly monitoring the sensor readings). The rotational speed of each brake force as necessary). wheel is carefully controlled by applying braking force wherever necessary to prevent locking up. The driver feels a judder through the brake pedal as brakes The second answer is are rapidly applied and released. a very brief answer for c) The following diagram of the process will help in the description that follows. part a), but gains a mark for input device. Part Sensors b) is interesting as it Valve DAC is a very good example of the use of magnetic sensors (mobile phones Reactor ADC Microprocessor and gaming controllers). DAC ADC The description is short but clear including a ‘what’ (used in gaming controller), a ‘why’ (to  Temperature and pH sensors constantly collect data and send it to the measure orientation of microprocessor (via an ADC). the device) and ‘how’ (by  The microprocessor compares incoming data with pre-set data values measuring twisting and stored in memory. rotational movement).  If the temperature < 70 °C then a signal is sent to the heater from the Full marks would be microprocessor (a DAC may be needed) to switch on. gained. Part c) was  If pH > 3.5 then the microprocessor sends a signal to open the valve (a DAC not well answered. One may be needed) to add acid. mark for naming sensors and the data is sent to a microprocessor. But Sample low-level answer there is the very common a) A sensor is an input device. error of stating sensors only send data when b) Magnetic sensors can measure position sensing and angular sensing; for these they detect a change reasons, these sensors are used in mobile phones and gaming controllers. It is illegal to photocopy this page in conditions – this is When used in these devices they can estimate the orientation of the device incorrect, since sensors (for example, control of a screen character in a game by rotating or twisting constantly send data to the controller). the microprocessor. They c) The system uses temperature and pH sensors. When the temperature or pH also incorrectly claim that changes, the sensors send signals to the microprocessor. The sensors control the sensors control the the actuators which are used to switch on a heater or open a valve to add acid. process. Only one mark would be awarded for the whole part. Hodder & Stoughton Limited © David Watson and Helen Williams 2022 39 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 40 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.2 Input and output devices Exam-style questions 4 A pharmaceutical company uses synthetic 6 Name a suitable output device for each of the gloves to protect scientists when carrying out following applications. work in a glovebox. The gloves used have a a) Produce a working model of a new toy limited operational life (a ‘use by’ date) and must during its development stages. be regularly checked and changed if necessary. b) Advertising a new product on a billboard As part of the monitoring system, each glove on the approach road to a large city. The has a unique barcode stamped on it which billboard will show moving images. contains the part number (0123111) and use by c) Very thin screen which can be attached date (22/10/2025): to a curved tunnel wall at a Metro station. The screen will show an animated advertising display. d) Projection of a computer image onto a 012311122102025 large screen to be shown to a large audience. e) Production of 10 000 high quality colour a) Describe how the barcode is read and how flyers to advertise a new product. it could be used to indicate whether the f) Production of a one-off calendar using glove needs to be changed. 12 photos from a user’s photo library. The b) The barcode has the number calendar will be printed on semi-gloss 012311122102025. photographic paper. Modulo-11 is used to generate a check g) Computerised control of a water valve in an digit. Calculate the check industrial process. Opening and closing of digit for this barcode, showing all your the valve is under computer control. working. h) Output device that allows a partially– c) Give two advantages to the site managers sighted person to check if a word processed of using barcodes to track the age of document has been entered correctly. gloves. 7 a) Explain the difference between d) To improve the system further, the monitoring and control. managers are considering the introduction b) An apartment has an air conditioning of QR codes to replace barcodes. system which is controlled by sensors and i) Describe the differences between the a microprocessor. The temperature and methods used to read barcodes and to humidity levels are constantly measured read QR codes. and, if either fall outside a set range, the air ii) Suggest additional information the conditioning unit is switched on. QR could hold. Give a reason for Name suitable sensors for this application. your answer. Describe how sensors, actuators and a iii) Give one disadvantage of using QR microprocessor interact to control the air codes. conditioning unit. You may wish to draw a 5 Mobile phones use touchscreen technology. diagram of your system. a) Name one type of touchscreen technology. b) Describe the advantages and disadvantages of the touchscreen technology named in part a). It is illegal to photocopy this page 40 Cambridge IGCSE™ and O Level Computer Science Study and Revision Guide Second Edition 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 41 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3 Hardware 3.3 Data storage Memory and storage can be split into two groups: primary memory secondary storage. Random access memory (RAM) and read-only memory (ROM) are usually referred to as primary memory. Both memories are fairly small and relatively expensive per GiB, so it is necessary to also have secondary storage (see later). ROM chips are typically 4 to 8 MiB in size. RAM chips are typically 1 to 256 GiB in size. Secondary storage is typically 2 TiB or larger. Unlike secondary storage, primary memory is directly addressable by the CPU. If data on secondary storage is needed by the CPU, it must first be loaded into RAM before the CPU can use the data. Secondary storage devices can be external or internal to the computer. Primary memory RAM ROM Internal secondary External secondary Secondary storage storage storage devices: Hard disk drive (HDD) are not directly DVD/CD and DVD-RAM addressable by Solid-state drive (SSD) the CPU are non-volatile Blu-ray disc USB memory stick/ flash memory The larger the RAM Removable hard drive the faster the CPU will operate; as RAM becomes ‘full’, the CPU has to continually access 3.3.1 Primary memory secondary storage to The following diagram shows how primary memory is split into RAM and overwrite old data on ROM and the two types of RAM commonly used. The cache memory is also RAM with new data – shown; this is an extension to primary memory. Cache is high speed RAM increasing RAM size that holds a copy of only the most frequently used data or instructions – reduces this requirement cache is first checked when the CPU wants to access data/instructions making the CPU more from the primary memory. efficient. It is illegal to photocopy this page Cache memory Primary memory RAM ROM SRAM DRAM Hodder & Stoughton Limited © David Watson and Helen Williams 2022 41 318489_03_IGCSE_CO_SE_OL_025-048.indd Page 42 12/07/22 1:10 PM F-0250 /145/HO02580/work/indd 3.3 Data storage Computers need both RAM and ROM. RAM is the working area of the computer where data currently being used is temporarily stored; ROM contains data that can’t be changed and needs to be stored permanently (for example, start-up routines). The following table summarises the differences between RAM and ROM. RAM ROM Temporary memory device. Permanent memory device. Volatile memory. Non-volatile memory device. Can be written to and read from. Data stored cannot be altered. Used to store data, files, programs, part of operating system (OS) Always used to store BIOS and other data currently in use. needed at start-up. Can be increased in size to improve operational speed of a computer. RAM can be two types: dynamic RAM (DRAM) or static RAM (SRAM). DRAM SRAM Consists of a number of transistors and capacitors. Uses flip flops to hold each bit of memory.