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1. Speed/clock speed of a clock signal

   • Processor runs to the timings of a clock signal
   • Measured in hertz
   • Processors work by continually fetching instructions from memory, decoding them and executing them. This is known as the fetch-decode-execute cycle

2. Central processing unit (CPU)

   • CPU = 'Brain of the computer'
     • Carries out instructions given to it
   • Billions of transistors inside a processor
     • Transistor = effectively electronic switches

3. Registers

   • Memory locations within the processor itself
   • Work at extremely fast speeds (so that it can be used by the processor without causing a bottleneck)
   • Program Counter (PC)
     • Keeps track of the memory location of the machine code being executed
     • Gets incremented to point to the next instruction
     • Changed by instructions that alter the flow of control
       • Branch if zero (BRZ)
       • Branch always (BRA)
       • Branch if positive (BRP)
   • Memory data register (MDR)
     • Stores the data that has been fetched from or stored in memory.
   • Memory Address register (MAR)
     • Stores the address of the data or instructions that are to be fetched from or sent to.
   • Current instruction register (CIR)
     • Stores the most recently fetched instruction, waiting to be decoded and executed.
   • Accumulator (ACC)
     • Stores the result of calculations made by the ALU
   • General purpose registers
     • Used temporarily to store data being used rather than sending data to and from comparatively much slower memory.
   • Buses
     • Are the communication channels through which data can be sent around the computer
     • USB(Universal serial bus) - Used to transfer data between the computer and external devices
     • Fetch-decode-execute cycle
       • When looking at the fetch-decode-execute cycle, there are three buses inside the computer that must be considered:
         • The data bus - carries data between the processor and memory
         • The address bus - carries the address of the memory location being read from or written to
         • The control bus - sends control signals from the control unit
   • Arithmetic logic unit (ALU)
     • Carries out calculations and logical decisions. The results of its calculations are stored in the accumulator.
   • Control unit (CU)
     • Sends out signals to co-ordinate how the processors works
     • Controls how data moves around parts of the CPU and how it moves between the CPU & memory.
     • Instructions are decoded in the control unit.

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