How (all) machines work...

All machines that have been invented by humans operates on this common property:

  • A machine will always repeat the same steps over and over again

Example: the combustion engine (= machine !!!) used in automobiles

The steps in the combustion engine "machine"

  1. Intake step:

      • The engine suck in a mixture of gasoline and air

  2. Compression step:

      • The engine compress a mixture of gasoline and air (higher pressure is needed for combustion)

  3. Combustion step (see the spark !):

      • The spark plug ignites the compressed fuel mixture (and the explosion will generate energy)

  4. Expell step:

      • The burned gas is expelled

After step 4, the engine is back to its original (= starting) state and the engine is ready to repeat the steps

Observe that the 4 steps in the combustion engine repeat continuously

Underlying Principle:   the last step in a machine cycle will always ends in the initial machine state

The Instruction Execution cycle of the CPU - Introduction

  • The CPU is an (electronic) machine !!!

  • The task performed by a CPU (machine):

      • Fetch and (then) execute a computer instruction      

  • CPU is machine:

    • A CPU (machine) will repeat a (fixed) number of "steps" (or phases)

    • The steps/phases that a CPU (machine) go through will fetch and execute the next instruction in the computer program !!

  • The Instruction Execution Cycle of the CPU:

      • The steps/phases that the CPU goes through to fetch and execute the next instruction is called:

        • The Instruction Execution Cycle (of the CPU)

The 4 steps in the CPU's Instruction Execution Cycle

The steps/phases of the instruction execution cycle are:

  1. The instruction fetch step/phase

      • The CPU will fetch the next instruction from memory and store it in the Instruction Register (IR)

  2. The instruction decode step/phase

      • The CPU finds out what the instruction is.

  3. The operand fetch step/phase

      • The CPU fetches the operands to perform the instruction

  4. The instruction execute step/phase

      • The CPU performs the arithmetic/logic operation

Next:   example of Instruction Execution Cycles for normal and out-of-order program flow