CPU

• Purpose of the fetch-execute cycle :

  1. Data / Instructions are fetched from the main memory.
  2. Data / Instructions are translated into a series of commands.
  3. Data /Instructions are carried out by the CPU.
  4. Cycle repeats.
• Purpose of the ALU (Arithmetic Logic Unit)

  • Performs mathematical calculations and logic operations
  • E.g = addition, subtraction, division
• Purpose of the Control Unit (CU)


  • Sends signals to direct operations
  • Controls the flow of data
  • Follows the fetch-execute cycle
• Purpose of Cache

  • High speed memory in the processor
  • Stores frequently used data and instructions
• Purpose of Register

  • Small piece of memory in processor
  • Holds one instruction / address
• Registers of the CPU

  1. Program Counter
  2. Memory Address Register
  3. Memory Data Register
• Purpose of the Program Counter

  • Stores the address of the next instruction to be fetched from memory.
  • Increments in each fetch-execute cycle.
• Purpose of MAR


  • Stores the address of the data to be fetched from.
• Purpose of MDR


  • Stores the address of the data being currently fetched
• Von Neumann Architecture 


  • All data and program instructions stored in the same memory.
• The CPU 


  • Central Processing Unit
  • Processes and executes program data and instructions, by the fetch-decode-execute cycle
• CPU Performance : 3 Main factors

  1. Clock Speed
  2. Number of Cores
  3. Cache Size
• What else can affect the CPU performance?

  1. RAM
  2. Graphics Card
• Clock speed

  1. Number of instructions that the CPU can carry out per second
  2. Measured in GigaHertz
• Greater Clock Speed

  1. More instructions carried out per second
  2. Instructions executed more quickly
  3. More programs able to run at the same time
• Disadvantages of high clock speed

  1. Overheating of machine
  2. Potential for errors
• CPU Cores

  • Single processors
• The greater the number of cores :

  • More instructions carried out simultaneously (parallel processing).
  • More instructions at the same time
  • More programs can be run at the same time
• Disadvantages of great number of cores

  1. Some software does not support multiple cores
  2. Some instructions are required to be executed before other instructions, in a specific order
• Effects of large cache size

  1. More space for frequently used instructions
  2. Faster access to the CPU
  3. Faster fetching of instructions from main memory
• Embedded systems


  • A computer system built into an another device
• 3 examples of Embedded Systems

  1. Dishwasher
  2. Washing Machine
  3. Microwave
• Characteristics of embedded systems

  1. Specific hardware required to function
  2. Limited functions / not general-purpose computer
  3. difficult to change the OS
• Primary Storage

  • RAM
  • ROM
• RAM


  • Random Access Memory
  • Stores currently in use program data and instructions
  • Directly accessed by the CPU
• Volatile v Non-volatile : RAM

  • RAM is volatile = data is lost when the computer is powered off.
• Difference between RAM and ROM

  1. RAM is volatile whereas ROM is non-volatile
  2. Contents of RAM changed frequently, whereas ROM is read only / never be changed.
• Purpose of ROM 


  • Stores the BIOS
  • This starts the computer + loads the operating system
• Purpose of Virtual Memory

  1. Used when RAM is full
  2. Part of secondary storage used as temporary RAM
  3. Data from RAM moved to VM to free up space in RAM
  4. RAM can be filled with new data
  5. When data in VM is needed, moved back to RAM
• Disadvantages of excessive use of VM

  • Slower to access than directly accessing from RAM
  • Moving data between VM and RAM takes processor time
  • Decreases performance of CPU
• Importance of secondary storage

  • Long-term / non-volatile storage of data when computer is turned off.
  • Larger storage space
• How does more RAM improve CPU performance?

  • Will speed up access to data currently in use
  • Improves the CPU performance