National 5 Computing Science SUMMARY NOTES – Computer Systems Not to be used as your sole revision resource! Please check www.sqa.org.uk/sqa/56923.html for the up-to-date course spec. COMPUTER SYSTEMS – Data representation Binary Denary Floating point representation Extended ASCII A base 2 number system. Uses only 1 and 0. Place values are powers of 2. A base 10 number system. Uses only 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Place values are powers of 10. Real numbers are stored, using floating point representation, as a mantissa and an exponent. For example, in the number 0.333894 x 1036 the mantissa is 333894 and the exponent is 36. 8-bit code used to convert text into binary. Each character is represented by 8 bits. It includes 32 control characters such as null, escape, carriage return, etc. The rest are printable characters such as A, %, 8, etc. Vector graphics Stores attributes of individual objects. Resolution independent, small file size, can be manipulated at object level. SVG (Scalable Vector Graphics) A coding language used to define vector graphics. SVG objects SVG attributes Bitmap graphics <rect> <ellipse> <line> <polygon> Fill Stroke Coordinates – x, y, cx, cy, x1, y1, x2, y2, points Size – width, height, rx, ry Image is represented as a grid of pixels. The colour of every pixel is stored as a binary number. More appropriate than vector for real-life images, can be manipulated at pixel level. COMPUTER SYSTEMS – Security precautions Firewall Software which filters incoming and outgoing data packets and blocks any unwanted data. Encryption Scrambling data (using a public key) so it cannot be understood (without the correct private key). COMPUTER SYSTEMS – Computer structure CPU (Central Processing Unit) The main processor in a computer. Responsible for executing program instructions. Different processors can be compared using benchmark tests. ALU (Arithmetic and Logic Unit) Carries out arithmetical operations (*, -, +, -, ^) and logical comparisons (<, >, =, AND, OR, NOT). Control Unit Manages processor, ensures program instructions are executed in the correct sequence. Registers Temporary, fast-to-access storage locations on the CPU. Main memory Used to store data for programs which are currently running. Organised into individual memory locations each with a unique address. Connected to CPU by data and address buses. Data bus Carries data to and from memory. Bi-directional. Address bus Carries addresses to memory. Uni-directional. Low-level language Instructions relate to specific registers, memory locations, etc. Hard to learn and understand and difficult to debug but instructions can be executed quickly. Lowest level language is machine code (which just looks like binary). High-level language Instructions use English words and are easier to understand than low-level languages. One high-level instruction may equate to many low-level instructions. Makes coding and debugging programs easier but instructions must be translated into machine code before they can be executed. Translator Software which translates a high-level program into machine code. Interpreter Compiler A type of translator which translates then executes one line after the other. Program will run quite slowly (since every line needs to be translated before execution). Does not store the translated code. Helpful when testing since errors can be identified as they are encountered. Would typically be used while the program is being developed. A type of translator which translates the entire high-level program and saves it as a machine code version. Process of compiling may take some time. Compiled, machine code version of the program will run quickly. The original high-level program is called the source code. The machine code translation is called the object code. Would typically be used once the program is ready for distribution. COMPUTER SYSTEMS – Environmental impact The amount of energy a computer system uses can be reduced by: Leaving computers on standby or switching off when not in use. Adjusting monitor settings, lowering brightness for example. Adjusting Operating System power settings so computer waits less time before sleeping/hibernating. Energy use WEB DEVELOPMENT – Standard file formats and factors affecting file size Compression Reducing the size of a file so that it: takes up less storage space takes less time to transmit/upload/download from one computer to another. Lossy compression Compresses by removing data, so quality is lost. Lossless compression Compresses without removing data, quality is not affected. Resolution The number of pixels used to store an image. High resolution = larger file size, better quality. Colour depth Number of bits used to store the colour of each pixel. True colour = 24 bits. High bit depth = larger file size, more colours available. Audio sample A measurement of an analogue sound signal. Audio sample rate The frequency with which samples are taken. High sample rate = larger file size, better quality. JPEG GIF PNG Compression Lossy - adjustable compression Lossless (if used correctly) level Lossless Colour depth 24 bit (16 777 216 colours) 8 bit (256 colours) 24 bit (16 777 216 colours) Transparency None Basic (pixels can be transparent or opaque) 8-bit transparency – 256 different levels of opacity Animation None Yes! None MP3 WAV Compression Lossy - adjustable compression level Uncompressed Quality Dependent on compression level - from 128Kbps downwards you’re likely to notice the High quality difference in quality File size Small, hence its popularity Large (because it hasn’t been compressed!)
