"Describe the fetch/execute cycle in a typical CPU system"

Introduction
Think of when you start your car!. To produce motion, the engine must go through four unique cycles: Intake Fuel / Compression / Ignition / Expel Exhaust .If this order were to altered or ignored, the engine would stop operations quite quickly.

The same may be said of a CPU. There are three ( or four ) stages which must occur for information to pulled across the address and data bus.

Fetch / Execute Cycle
Every computer system must retrieve and replace data through it's bus systems based on a carefully sequenced set of actions ( or "states" ) This process is called the fetch/execute cycle, and is fundamental to the operation of the computer system.

A READ operation uses three T-states :

T1: CPU puts the address on the address bus
T2: CPU Asserts the READ lead
T3: Data is put on the data bus ( by Memory or IO ) and is read into the CPU

For a WRITE operation also need three T-states::

T1: CPU puts the address on the address bus
T2: CPU puts data on the data bus and asserts write lead
T3: Device being accessed ( memory or IO ) accepts data

An INSTRUCTION FETCH ( where the CPU gathers and instruction from memory ) needs four T-states:

T1: CPU puts the address on the address bus
T2: CPU Asserts the READ lead
T3: Data is put on the data bus ( by Memory or IO ) and is read into the CPU
T4: Instruction is now inside the CPU and is interprated

Each one of these T-states occur for exactly one clock cycle, and the three or four states combined is called a "machine cycle".