GCSE Computer Science - Topic 1.1

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  • Topic 1.1: Systems Architecture
    • Processor Functions
      • The processor is the "brains of the computer".
      • It processes data and carries out instructions (executes them)
      • It has a clock speed measured in Hertz (Number of cycles per second
      • Retrieves data/instructi-ons from the main memory
      • Stores Data in the Main memory once executes
    • Hertz
      • MHz
        • 1 million instructions per second
      • GHz
        • 1 billion instructions per second
      • 1 of these is equal to 1 instruction per second
    • Instruction
      • A instruction is data with context
    • Execute Cycle
      • Fetch-decode-execute cycle
        • Each tick of a computer's CPU completes this cycle.
        • 3GHz per second cpu completes this 3 billion times a second
          • FETCH
            • Program counter is incremented for each instruction of the program being executed
              • The contents of the program counter are put into MAR
                • The address is transferred along the address bud to the main memory (the address indicated where to fetch the data or instruction from.
                  • Data or the instruction, once found, is transferred back to the CPU via the data bus. Which is held in the Memory Data Register
                    • The instruction will then be transferred to the Current Instruction Register.
                      • DECODE
                        • The instruction to be decoded is held in the CIR.
                          • The instruction is split into a Op-code and Operand
                            • EXECUTE
                              • The instruction is then carried out by the ALU
        • It therefore needs a clock to regulate how many cycles it can do a second. Hence you can "Over-clock" your cpu
    • Embedded System
      • A computer system made up of both Hardware and Software
        • Usually for specialised tasks.
        • Doesn't usually contain an OS
        • An example of this is a Diswasher
    • Clock Speed
      • Amount of Hertz or instructions executed per second
    • Cache
      • Processors have "cache" which is a very fast, small amount of memory on the cpu. The cache acts as a intermediary between the cpu and main memory.
      • The cache holds the most commonly used instructions
        • Level 1: Fastest but smallest Level 2: A bit slower but larger and level 3 is the largest but slowest.
    • Core
      • Each processor has a "core" where instructions are executed.
      • Processors can be multi-core: Dual, quad or 8 core.
      • Each core executes instructions independently, dual core MAY execute up to twice as many instructions per second
    • What affects performance?
      • Cores
        • Clock Speed
          • Cache Size
            • Increasing the cache size will reduce the number of memory to disk transfers and thus may speed up the processing of the instructions
          • Doubling the clock speed will double the number of instructions executed per second
        • Quadrupling the number of cores may quadruple the number of instructions executed per second.
    • Moore's law
      • Number of transistors on a cpu doubles every 2 years, has been seen to be true for 50+ years
    • Processor:
      • Main Memory:
    • Von Neumann Architecture
      • In 1945 2 mathematicians proposed how to build a more flexible computer. One was Alan Turing and the other was John Von Neumann
        • Alan Turing had been involved in the German code breaking and "colossus" computer
        • Neumann had been working on the Manhattan project to build the first nuclear bomb and needed a lot of calculations to do so.
          • In WW2, the early computer ENIAC took 3 weeks to rewire to do other calculations, there had to be an easier way.
            • The new idea was that the program processing data and the data should be stored in the same memory, making it easier to rewire. This became known as "Von Neumann" architecture.
    • Components of the CPU and other essential parts
      • MAR (Memory Address Register)
        • The contents of the PC are copied here and then transferred along the Address bus to the Main memory
      • MDR (Memory Data Register)
        • Once instructions or data are brought from the memory address in the main memory via the data bus, they are placed in the MDR
      • PC (Program Counter)
        • Holds the location of the next instruction or data address in main memory.
      • Accumulator
        • Results of calculations are placed here from the ALU
      • ALU (Arithmetic Logic Unit)
        • Carries out calculations on data
          • Usually Add, Multiply, Divide, Subtract but also comparisons such as greater than, less than and equal to.
      • CU (Control Unit)
        • Manage the process of moving data and program in and out of memory and executes programs, one at a time.
          • Includes a "register" to hold immediate values and because it was one at a time it meant that the von neumann architecture is a sequential processing machine
      • Main Memory (RAM)
        • Holds all the data and instructions being used by the processor
        • Located on the motherboard
        • Directly Accessible by the CPU
        • Volatile so all data is lost once power is off
      • CIR (Current Instruction Register)
        • The instructions in the MDR are copied here
      • Buses
        • A set of parallel wires connection 2 or more independent components of a computer system in order to pass signals between them
          • Data Bus: Carries data or instructions from the main memory to the CPU (Or other secondary device). It's Bi-directional and data can be read/written
          • Address Bus: Carries addresses from the CPU to the main memory or other Input/output devices. It's Uni-directional and the CPU generates an address
          • Control Bus: Control signals are sent across here, such as memory read or memory write, it instructs which data will be travelling to/from memory.
      • Peripherals are any devices not directly connected to the CPU and are known as I/O devices.
      • I/O ports allow communicatio-n from an I/O device and the motherboard




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