U1

• The CPU (Central Processing Unit) is the main component of a computer, responsible for executing instructions from programs.
• A program is a set of instructions that tells the processor what to do.
• An instruction is an action or operation performed by the processor.
• CPU

  Central Processing Unit
• Purpose of the CPU

  To process data and instructions by constantly repeating the fetch-execute cycle
• FDE cycle

  1. Fetch

  2. Decode

  3. Execute
• CPU components


  • Control Unit
  • ALU
  • Registers
  • Cache memory
• Control Unit


  Sends control signals to direct the operation of the CPU, decodes instructions as part of the F-E cycle
• ALU


  Performs simple calculations and logical operations
• Registers


  Temporary storage space for one instruction or address, used during the FE cycle
• Cache memory


  Temporarily stores data that is frequently accessed, faster than RAM but slower than registers
• Computer architecture


  The way a computer is designed and structured
• Von Neumann architecture


  Stores both program instructions and data in the same memory (RAM)
• Before being able to store programs in memory, instructions had to be manually input by a human
• Buses


  Pathways used to transfer data between components
• Data is transferred between components on pathways called buses
• Registers


  Small storage spaces for temporary data in the CPU
• Registers


  • Each register has a specific role
• Essential registers used in the Fetch-Execute cycle

  • Program Counter (PC)
  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Current Instruction Register (CIR)
  • Accumulator (ACC)
• Program Counter (PC)


  A register that tracks the RAM address of the next instruction to be fetched
• Memory Address Register (MAR)


  A register that tracks the RAM address of the instruction that is to be fetched
• Memory Data Register (MDR)


  The MDR stores the instruction that has been transferred from RAM to the CPU
• Current Instruction Register (CIR)

  A register that stores the instruction that has been fetched from RAM, and is about to be decoded or executed
• Accumulator (ACC)


  The ACC stores the result of mathematical or logical calculations
• Fetch-Execute Cycle


  1. Instructions are fetched from RAM, to be decoded (understood) and executed (processed) by the CPU

  2. The Program Counter (PC) register displays the address in RAM of the next instruction to be processed

  3. The PC register is increased by 1 to prepare the CPU for the next instruction to be fetched

  4. The CPU checks the address in RAM which matches the address held in the MAR

  5. The instruction in RAM is transferred to the Memory Data Register (MDR)

  6. The instruction in the MDR is copied into the Current Instruction Register (CIR)

  7. The instruction in the CIR is decoded (understood) and executed (processed), with any result stored in the Accumulator (ACC) register

  8. The cycle repeats by returning to the first step and checking the program counter for the address of the next instruction
• Cache memory


  Temporary storage for frequently accessed data
• Cache memory


  • It is closer to the CPU than RAM, meaning that it can provide data and instructions to the CPU at a faster rate
  • A computer with more cache memory (e.g. 8GB instead of 4GB) should have a higher performance because repeatedly used instructions can be stored and accessed faster
• Cache memory is costly, so most computers only have a small amount
• Clock speed


  The measure of how quickly a CPU can process instructions, measured in Gigahertz (GHz)
• Higher clock speed


  The faster the computer can perform the FDE cycle, resulting in better performance because more instructions can be processed each second
• Overclocking


  When the computer's clock speed is increased higher than the recommended rate, making the computer perform faster but can lead to overheating and could damage the machine
• Underclocking


  When the computer's clock speed is decreased lower than the recommended rate, making the computer perform slower but will increase the lifespan of the machine
• Core


  A complete set of CPU components (control unit, ALU and registers), each able to perform its own FDE cycle
• More cores


  A computer with more cores will have a higher performance because it can process more instructions at once
• If one core is waiting for another core to finish processing, performance may not increase at all
• Some software is not written to make use of multiple cores, so it will not run any quicker on a multi-core computer
• Computer


  An electronic device that processes data
• Types of computers


  • Small, embedded computers (e.g. in watches and calculators)
  • Huge supercomputers (used for computationally intensive tasks like weather forecasting)
• General Purpose Systems


  • Programmed to perform a wide range of tasks
  • Can perform multiple tasks, including desktop publishing, sound and video editing, account tracking, email sending and Internet browsing
• Primary Storage


  • Very quick because it is directly accessed by the CPU
  • Typically smaller in storage size
  • Sometimes called 'main memory'
  • Includes RAM and ROM