Computer science paper 1

• Purpose of the CPU


  To process data and instructions by constantly repeating the fetch, decode and execute cycle
• CPU


  The central Processing unit
• Fetch-Execute Cycle


  The cycle the CPU runs through billions of times per second to make a computer work
• Fetch-Execute Cycle stages


  1. Fetch

  2. Decode

  3. Execute
• Fetch stage


  • The next instruction or data must be fetched from the computer's memory (RAM). Instruction or data is brought back to the CPU.
• Decode stage


  • The CPU needs to work out what is required from the instruction
• Execute stage


  • The CPU will carry out the instruction that was fetched
• CPU components


  • Arithmetic Logic Unit (ALU)
  • Control Unit (CU)
  • Cache
  • Registers
• Arithmetic Logic Unit (ALU)


  Performs arithmetic operations and logical decisions
• Control Unit (CU)


  Coordinates how data moves around the CPU, decodes instructions fetched from memory
• Cache


  Very small, very fast memory located in the CPU which is used to provide quick access to frequently used instructions and data
• Registers


  Extremely small, extremely fast memory located in the CPU, each with a specific purpose
• Von Neumann Architecture


  A design of the CPU proposed by John Von Neumann in the 1940s, which most general-purpose computers are built upon
• Von Neumann Architecture registers

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


  Holds the memory address of the next instructions to be executed, increments by 1 as the fetch-decode-execute cycle runs
• Memory Address Register (MAR)


  Holds the memory address of where data or instructions are to be fetched from
• Memory Data Register (MDR)


  Stores the data or instruction which has been fetched from memory
• Accumulator (ACC)


  Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU)
• Common CPU characteristics


  • Clock Speed
  • Cache Size
  • Number of Cores
• Clock speed


  Measures the number of fetch-decode-execute cycles that can take place in 1 second
• Cache size


  The larger the cache size, the more frequently used instructions or data can be stored, speeding up performance
• Number of cores


  Multiple cores can fetch, decode and execute instructions at the same time, increasing performance
• Embedded system


  A computer system with a single function, inside a larger mechanical unit
• Properties of embedded systems

  • Small in size
  • Use less power than a general-purpose computer
  • Have a lower cost
• Registers


  Extremely small, extremely fast memory located in the CPU
• Registers


  • Each register has its very own specific purpose
• Von-Neumann-architecture


  A computer architecture that stores data and instructions in the same place.
• Main registers in Von-Neumann-architecture


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


  • Holds the memory address of the next instructions to be executed
  • Increments by 1 as the fetch-decode-execute cycle runs
• Memory Address Register (MAR)


  • Holds the memory address of where data or instructions are to be fetched from
• Memory Data Register (MDR)


  • Stores the data or instruction which has been fetched from memory
• Accumulator (ACC)


  • Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU)
• Common characteristics of the CPU


  • Clock Speed
  • Cache Size
  • Number of Cores
• Each of these characteristics has a significant impact on the performance of the CPU
• Clock speed


  Measured in Hertz (Hz), the number of fetch-decode-execute cycles that can take place in 1 second
• The faster the clock speed


  The more instructions can be fetched and executed per second
• Modern computers have a clock speed in Gigahertz (GHz), meaning billion
• A clock speed of 3.5GHz can perform up to 3.5 billion instructions per second
• Cache

  Very small, very fast memory on or close to the CPU, used as temporary storage to provide quick access to a copy of frequently used instructions and data
• The larger the cache size


  The more frequently used instructions or data can be stored