Unit 1 - CPU


The CPU - (Components)


  • Central Processing Unit
  • Responsible for all of the computer's processing
  • Much like a human brain
  • Arguabley the most important part of a computer

Control Unit

  • Coordinates all the activities that take place within the CPU


  • Arithmetic Logic Unit
  • Responsible for all the calculations that take place in the CPU
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The CPU - (Registers)


  • Program Counter
  • Holds the address of the next instruction that will be executed


  • Memory Address Register
  • Holds the memory address of current instruction
  • Holds the memory address of the data that will be used


  • Memory Data Register
  • Holds the instruction
  • Holds the data that has be fetched from the memory


  • Holds the result of an instruction before it is transfered back to memory
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The CPU - (Fetch, DeCode,Execute)


  • Causes the next instruction the be fetched
  • Any data needed for the instruction will also be fetched
  • Fetched from the main memory


  • Decodes the instruction once it has been fetched


  • Carries out the instruction
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The CPU - (Performance affectors)

Clock Speed

  • Everything in the computer acts on the impulse of an internal clock
  • The faster the clock speed the faster the computer
  • Because more instructions can be processed in the same time

Processor Speed

  • 1 Hz = 1 instruction completed a second
  • The higher the Hz the faster it is as more instructions can be completed faster
  • Hz = Hertz

Multi-core processing

  • The more cores a CPU has the faster it can process
  • A quad core under ideal circumstances can performe four times faster than a single core
  • This only really has a affect when more than one programme is being run
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The CPU - (Cache memory)

Cache memory

  • Small amount of memory
  • Really fast
  • Expensive

Cache levels

  • Level 1 - Extremely fast, small, located on the CPU
  • Level 2 - fairly fast, medium-sized, fairly close to the processor

Benefits of Cache memory

  • Data most often used is held in Level 1 and can be accessed instantly
  • In most systems, 95% of the time data is accessed through level 2
  • Greatly reduces time for an instruction to get from the main memory
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The CPU - (embedded systems)

Embedded system

  • A single microprocessor
  • Includes RAM, ROM and a CPU
  • Used to perform only one function.
  • Example - Dishwashers have Embedded systems to perform simple tasks
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The CPU - (RAM)

What is RAM

  • Random Access Memory
  • Can be read from
  • Can be writen to
  • Very fast
  • Normally stores the operating system, software currently in use and data required by the software

What happens if you run out of space

  • If you run ot of space on the RAM, it will go onto the virtual memory.
  • This is an extension of the RAM which takes a place on the hard disk.
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The CPU - (ROM)

What is ROM

  • Read Only Memory
  • Stores data permenantly, even when the computer is off
  • Can only be read
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