Revision Notes "Covers everything from Theory to Program solving"
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computer science notes
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IGCSE Computer Science 0478 Revision Notes “Covers everything from Theory to Program solving”
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®IGCSE is the registered trademark of Cambridge Interna onal Examina ons. This text has not been through the endorsement process. © Copyright 2016 by smartnotesonline.com Data Transmission Simplex Data Transmission ‐ Data transmission in one direction only. Half Duplex Data Transmission ‐ Data transmission in two directions but not at the same time. Full Duplex Data Transmission ‐ Data transmission in two directions at the same time. Serial Data Transmission ‐ Data is sent one bit at a time over a single wire or channel. Parallel Data Transmission‐ Several bits of data are send down several wires or channels. Asynchronous Data Transmission ‐ Data is being transferred in an agreed bit pattern together with controls bits known as start bit & stop bit. Synchronous Data transmission – Data is being transferred in a continues stream accompanied by timing signals generated by an internal clock; timing signals are synchronized with each other to endure sender are receiver are synchronized with each other to ensure bits are reassembled into the correct bit pattern. USB – Universal Serial Bus; an asynchronous serial data transmission method which has become an industry standard What happens when a device is plugged into a computers using one of the USB ports? 1.) The device is automatically detected. 2.) A known device is recognized and a device driver is loaded. 3.) If a new device is plugged in, the computer looks for the correct device driver or the user will be prompted to download the driver. Error Checking Methods of Data Transmission What is parity Check? 1.) Parity Checks can be even (even number of bits) or odd (odd number of bits); one bit is known as Parity bit. 2.) Parity blocks allow an erroneous bit to be identified. 3.) If the parity of a byte of data hasn’t changed (following data transmission) but still an error has occurred, then it’s impossible using this method to identity where the error has occurred. What is Automatic Repeat Request (ARG)? 1.) ARG used acknowledgment to indicate data receiver correctly. 2.) A timeout is used which is the time allowed to elapse before an acknowledgment is received. What is Checksum? 1.) Data is sent in blocks. 2.) An additional value called Checksum, is end at the end of the block of data. 3.) The checksum is based on the number of bytes in the block. 4.) If the checksum at the receiver’s end doesn’t match the checksum at the end of the data block, then an error in transmission has occurred. What is Echo Check? 1.) Data is returned to the sender. 2.) The sender compares data with the data received back. 3.) If different error has occurred somewhere in the process. 4.) The problem could that the data was actually send correctly, but it was during the processing of sending back to the sender that the error occurred. Internet Technologies Internet Service Provider (ISP) – An ISP is company that provides the user with access to the internet, they usually charge a monthly fee. Internet Protocol (IP) Address – Each device is given an IP address as soon as it connects to the internet. The IP address will be different each time the device connects. Media Access Control (MAC) Address – MAC address uniquely identifies a device connected to the internet. HYPERTEXT MARK‐UP LANGUAGE (HTML) ‐ is used when writing and developing web pages. HTML Structure – is the essential part of the HTML document; it includes the semantics(meaning) and structural mark‐up of the document. HTML PRESENTATION ‐ is the style of the document; i.e. how the document will look (or even sound if it includes multimedia elements). Web Browser ‐ is software which allows a user to display a web page on their computer screen. Web browsers interpret or translate the HTML code from websites and show the result of the translation. Logic Gates & Logic Circuits Logic Gates: Truth tables: Logic Statements: Logic Circuits: Operating System Operating system – The software running in background of a computer system. It manages many of the basic functions. Interrupt – An interrupt is a signal sent from a device (or from somewhere) to processor requesting its attention. The interrupt will cause the processor to temporarily stop what it’s doing to serve the interrupt. Buffer – A buffer is temporary memory area in a device. It allows the processor to carry on with other tasks whilst. Computer Architecture Buses ‐ They move data around the computer and also end send out control signals to synchronize internal operations. Types of Buses: 1.) Address Bus – This carries signals relating to the address between the processor and memory; its uni‐directional. 2.) Data Bus – This sends data between the processor, memory unit and the input/output devices; its bi‐directional. 3.) Control Bus – This carries signals relating to control and coordination of all activates within the computer; it can be uni‐directional or bi‐directional due to internal connections. Registers – High‐speed storage areas within the computer. Types of Registers: 1.) Memory Address Register (MAR) 2.) Memory Data Register (MDR) 3.) Arithmetic & Logic Unit (ALU) 4.) Program Counter (PC) 5.) Current Instruction Register (CIR) Memory Unit – Made up to address and contents. Each address will uniquely identify each location within the memory map. Control Unit – Controls the operation of the memory, processor, and input/output devices; the control unit reads instructions, interprets them and sends out signals along the control bus to synchronize all computer components. What is Fetch Execute Style? Fetch – The next instruction is fetched from the memory address currently stores in program counter (PC) and is then stores in Current Instruction Register (CIR); The PC is then incremented so that the next instruction can be processed. Execute – The decoded instruction is the n passed as a set of control signals to the appropriate components of the computer system. Input Devices 2D Scanners – These convert hardcopy documents into an electronic form which can be storied in a commuter memory 3D Scanners – scan solid objects and produce a three‐dimensional image. Since solid objects have x, y and z coordinates, these scanners take images at several points along these three coordinates. A digital image which represents the solid object is formed. The scanned images can be used in COMPUTER AIDED DESIGN (CAD) or, more recently, sent to a 3D printer to produce a working model of the scanned image. They make use of lasers, magnetic resonance, white light, and so on. X‐rays: CT (computed tomography) scanners. Radio Waves: MRI (magnetic resonance imaging) scanners. Gamma rays: SPECT (single – photon emission computed technology) scanners. Barcodes ‐ A barcode is a series of dark and light parallel lines of varying thickness. The numbers 0 to 9 are each represented by a unique series of lines. So what happens when a barcode is scanned? • The barcode is first read by a red laser or red LED (LIGHT EMITTING DIODE). • Light is reflected back off the barcode; the dark areas reflect little or no light which allows the bars to be read. • The reflected light is read by sensors (photoelectric cells). • As the laser or LED light is scanned across the barcode, a pattern is generated which is converted into digital data – this allows the computer to understand the barcode. • For example: the digit ‘3’ on the left generates the pattern L D D D D L D (where L = light and D = dark); this has the binary equivalent of 0 1 1 1 1 0 1 (where L = 0 and D = 1). So the barcode has been read, then what happens? • The barcode number is looked up in the stock database (the barcode is known as the KEY FIELD in the stock item record); this key field uniquely identifies each stock item. • When the barcode number is found, the stock item record is looked up. • The price and other stock item details are sent back to the checkout (or POINT OF SALE TERMINAL (POS)). • The number of stock items in the record is reduced by one each time the barcode is read. • This new value for number of stock items is written back to the stock item record. • The number of stock items is compared to the re‐order level; if it is less than or equal to this value, more stock items are Download 1.58 Mb. Do'stlaringiz bilan baham: 
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