2.1.4 Explain the machine instruction cycle

Machine Instruction Cyclemachine-cycle.jpg

  • 4 process cycle
    • 1) reads and 2) interprets machine language
    • 3) executes and 4) stores the code
  • performed by CPU

The 4 Stepson the machine instruction cycle

Diagram of the Machine Instruction Cycle

  1. Fetch
    • instructions pulled from memory
    • instructions stored in main memory
    • CPU requests instructions from secondary memory
      • instructions stored in main memory
      • the Program counter (also instruction pointer) stores the address, which is the location of storage
    • Via data bus the content of address contained in the program counter is fetched from the main memory & stored in the CIR (Current Instruction Register)
    • instruction register: part of CPU that holds instructions executed
    • Program counter = incremented by 1
  2. Decode
    • The instruction decoder interprets the instruction
    • All required data = fetched from main memory & put in data registers
  3. Execute
    • CPU performs the actions required
  4. Store
    • Results of calculations in CPU = stored in main memory or sent to output devices
    • Program Counter could be updated to a new address


1. Fetch cycle
  • Steps 1 & 2 are called the Fetch Cycle
  • same for each instruction
  • processes the instruction from the instruction word (fixed number of digits), which contains an opcode and operand
    • instruction word: fixed number of digits for a data value (binary)
    • opcode: operation code - part of instruction naming the operation to be executed
    • operand: part of instruction specifying which data values to manipulate
2. Execute cycle
  • Steps 3 & 4 are called the Execute Cycle
  • steps change with each instruction
  • 1st step
    • Process-Memory
      • data is transferred between the CPU and I/O module
  • 2nd step
    • Data-Processing
      • mathematical & logical operations used in reference to data
  • 3rd step
    • Central alterations
      • sequence of operations
        • e.g. jump operation
  • Last step
    • combined operation
      • combines all steps

First video example of the Machine Instruction Cycle:

Second video example of the Machine Instruction Cycle:

Additional links:
http://www.it.jcu.edu.au/Subjects/cp1300/resources/lectnotes/system/fde.html (A more detailed explanation)
Machine Instruction Cycle (Interactive Flash animation from www.ib-computing.com)

Written by: Jocbe
Edited by: Lucie Magister
Last updated: 05. November 2014