Performance of the CPU

• FDE Cycle

  The process a computer uses to fetch instructions, decode them, and then execute them in a continuous repetitive cycle billions of times a second
• Computer


  • An electronic device that takes an input, processes data, and delivers output
• Computer operation


  1. Take input

  2. Process data

  3. Deliver output
• Computer program


  A set of instructions that a computer follows to carry out its function
• Computer components


  • Memory that stores the program
  • Central processing unit (processor) that carries out the instructions
• Fetch-decode-execute cycle

  1. Fetch instruction from memory

  2. Decode instruction

  3. Execute instruction
• Fetch stage


  1. Check program counter for address of next instruction

  2. Copy address to memory address register

  3. Send address along address bus to main memory

  4. Receive read signal from control bus

  5. Copy content from memory to memory data register

  6. Copy instruction to current instruction register

  7. Increment program counter
• Instruction


  Made up of opcode (what to do) and operand (what to do it to)
• Opcode


  Part of an instruction that specifies the operation to perform
• Decode stage


  Decode the opcode and operand of the instruction
• Execute stage


  1. Send address from operand to memory address register

  2. Receive data from memory along data bus

  3. Copy data to accumulator register
• The program counter is updated to point to the next instruction to be executed
• Branching


  When a program doesn't strictly follow a sequential order of execution, but can jump around to different parts of the program based on certain conditions
• Branching process


  1. Fetch instruction

  2. Decode instruction

  3. If branch instruction, update program counter with operand value

  4. Fetch next instruction from new location
• The fetch-decode-execute cycle is repeated billions of times per second by a processor
• The simple program executed performs the calculation 3 + 1, stores the result 4 in memory, and outputs the result