GCSE Computer Science Revision

GCSE Computer Science Revision

1. Architecture of the CPU

   • CPU
     • The CPU is the Central Processing Unit
     • The brains of the Computer
     • It uses the fetch decode cycle
     • It processes data and instructions to make the system work
   • The 3 main parts of the CPU
     • CU
       • The control unit is in overall control of the CPU. Its main job is to manage the fetching, decoding and execution of program instructions by following the fetch execute cycle It controls the flow of data inside the CPU and outside the CPU
       • The Control Unit
       • Has overall control over the CPU
       • manages the fetch, execute cycle
       • Controls the flow of data inside and outside the CPU
     • ALU
       • the ALU basically does all the calculations. It completes simple addition and subtraction, compares the size of numbers and can do multiplication and division using repeated addition and subtraction. It preforms logic operations such as and,or and not and binary shifts  Computers process binary data 'It contains the accumilator register
       • The Arithmetic logic unit
       • Does the calculations
         • Addition
         • Subtraction
         • Multiply
         • Divide
       • logic operations
         • AND,OR,NOT
         • Binary Shift
     • The Cache
       • The cache is very fast memory in the CPU. Its slower than the register but faster than RAM. It stores regularly used data so that the CPU can access it quickly the next time its needed. When the CPU requests data, it checks the cache first to see  if the data is there. If not it will fetch it from the RAM 
       • Very fast memory
         • Faster them RAM
         • Only regular used data
       • Slower than the registers
       • Have low capacity
       • Very expensive
       • 3 levels of cache
         • L1
           • quickest
           • Lowest capacity
         • L2
           • Slower than L1
           • More capacity than L1
         • L3
           • Slowest form of Cache
           • Most storage in all of cache
   • Von Neumann architecture
     • The von Neumann architecture d 
   • Central Processing Unit

2. Embedded systems

3. GCSE Computer Science

   • Memory and Units
     • Memory
     • units
       • Bit
         • Binary digit 1 or 0
           • 4 bits half a byte
       • Nibble
         • 4 bits, half a byte
       • Byte
         • 8 bits
         • can represent 1 character in ASC11
         • 1024 bytes
           • approximated to 1000 bytes
         • 1,000,000 bytes
       • Kilobyte
         • KB
         • 1024 kilobytes
           • Can be approximated to 1000 kilobytes
       • Megabyte
         • MB
         • 1024 megabytes
           • can be approximated to 1000 megabytes
             • 1,000,000,000 bytes
       • Gigabyte
         • GB
       • Terabyte
         • TB
         • 1024 gigabytes
           • 1000 gigabytes
             • 1,000,000,000,000
       • Petabyte
         • PB
         • 1024 terabytes
           • 1000 terabytes
             • 1,000,000,000,000,000 bytes
     • Arithmetic overflow
       • When there are insufficient bits to store the result from addition due to a final carry bit
     • Binary shift
       • a left shift multiplies by 2 right shift divides by 2
     • Check digit
       • A calculation on data to create a number included with the data for error checking
         • When the number is input the check digit is recalculated to check it matches. Therefore valid data can be assumed
     • Characters
       • Character set
         • A defined list of characters recognised by comuter hardware and software. Each character is represented by a number. Two common charater sets are ASCII and Unicoide.
     • Images
       • How bitmap pictures are stored
         • each pixel has a binary number The more colours the more bits are required per pixel
           • Images are stored as a series of pixels. Pixel colour is represented by binary. The more bits the more colours available. Black and white images only have 2 colours so only use 1 bit for each pixel
   • Wired and wireless networks, protocols and layers
     • Layers
       • Advantages of layers
         • Reduces the complexity into manageable sub problems
         • Devices can be manufactured to work at different levels
         • Products from different vendors will work together
     • Wireless encryption
       • Wireless networks are identified with a unique Service Set Identifier (SSID).
         • The SSID must be used by all devices that want to connect to the network and can be set to automatically broadcast to any device in range of a WAP.
       • Protecting your wireless network
         • The SSID is set automatically, but you can change it if you wish
         • It can also be hidden to make it harder to detct
         • Additionally it can be sceured with a password
         • Encrytion
           • Wireless networks networks broadcast data that must be encrypted to be secure
             • this is done by scrambling the data into ciphertext using a master key created from the network's SSID and password
               • data is decrypted by the receiver using the same master key the key is not transmitted
                 • Wireless encryption protocols include WEP, WPA and WPA2
                   • A handshake protocol is used to ensure the receiver has a valid master key before transmission begins
           • Although wired networks are more secure, encryption can still be used and is often a sensible precaution
       • Encrytion
     • Modes of connection
       • Wired
         • Fibre Optic
         • Ethernet
           •    •A standard for networking technologies. •Used for communicating on a wired local area network. •Includes a number of associated protocols (rules for governing communications). •Provides reliable, error-free, fast communication between two points. •Originally used in old-style bus networks. •Still used today in more modern star and mesh networks. •Data is transmitted in frames, which include: •Preamble of bits used to synchronise transmission. •Start frame deliminator to signify the start of the data part of the frame. •Source and destination MAC address. •The actual data. •Error-checking information (cyclic redundancy check or CRC). •User location is limited by the need for a physical cable connection. •An Ethernet setup relies on lots of cables, connections, ports and physical hardware, raising costs.   
           • Ethernet is a standard that has been around for many year
           • It has evolved as network topologies has evolved
         • Twisted pair copper
       • Wireless
         • Wi-FI
         • Bluetooth
         • Wireless connectons are popular because