OCR - GCSE - Computer Science - Systems Architecture

Central Processing Unit
1. Control Unit
  1. The Control Unit (CU) controls all of the other components of the CPU. It also contains the decoder.
    1. The decoder interprets program instructions and tells the ALU what operations to carry out.
2. Arithimetic Logic Unit
  1. The Arithmetic Logic Unit (ALU) performs all of the logical operations to carry out program instructions
    1. The ALU carries out the following activities: subtraction, addition, multiplication and division.
3. Clock
  1. The Clock controls the rate at which program instructions are carried out, by sending control electrical signals at regular intervals, called cycles.
  2. Clock Speed
    1. The clock is a vibrating quartz crystal and the faster it vibrates, the faster the instructions are processed, at least 1 per cycle.
    2. The faster the clock speed the hotter the processor gets, which causes it to malfunction.
      1. The processor must have a heat sink and a fan to dissipate this heat.
        Multi-core Processors
        A multi-core processor contains more than one CPU.
        Multi-core Processors can give faster processing speeds than single-core processors and can work on different tasks at the same time.
        Positives: The cores can work together on the same program - parallel processing. The Cores can work on different programs at the same time - multitasking.
        Negatives: Not all programs will run at twice the speed with a dual-core processor. The programs may be sequential so that one task requires output from a previous task and so the second task cannot start until the first has finished.
4. Buses
  1. These are electrical conductors that carry electrical signals between components in the CPU and between the CPU and other components on the motherboard.
5. Cache
  1. This is very fast Random Access Memory (RAM).
    1. The Cache memory speeds up processing by storing recently or frequently used instructions so that they do not have to be fetched from the main memory which is much slower.
      1. As the cache memory becomes larger it takes longer to find the data and so it becomes slower.
        Therefore, it is split into different levels L1, L2 with the smallest nearest to the CPU.
6. Registers
  1. These are memory locations. Some perform special functions in the fetch-decode-execute cycle.
    1. The Fetch-Decode-Execute cycle is what the CPU uses when carrying out the program instructions.
      1. Fetch Stage: The next instruction to be executed is transferred from the RAM to the CPU.
      2. Decode Stage: The CU interprets the instruction.
      3. Execute Stage: The CU then carries out the instruction. It instructs the ALU if calculations need to be performed.
    2. Program Counter is one of the registers which holds the address of the next instruction to be fetched.
    3. The Memory Data Register (MDR) temporary store (buffer) for anything copied from memory.
    4. The Memory Address Register (MAR) holds the address of the memory location currently being read (fetched) or written to.
    5. Accumulator is a register which stores the results of the calculations carried out by the ALU.
Embedded Systems
1. Components within an embedded system are on a Single Printed Circuit Board (PCB) and have a processor, memory, input and output interfaces.
2. Each embedded system is built for a small range of specific tasks.
3. Embedded Systems are called real-time systems because they must ensure an immediate response in order for the system to react to different situations.

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OCR - GCSE - Computer Science - Systems Architecture

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