Fetch-Execute Cycle

1. Fetch

• The address in the Program Counter (PC) is copied to the Memory Address Register (MAR). The CPU then sends a signal along the address bus to RAM, and the instruction at that location is returned via the data bus to the Memory Data Register (MDR). Finally, the PC is incremented by 1.

2. Decode

• The instruction held in the MDR is sent to the Control Unit (CU). The CU interprets the instruction's opcode (what to do) and operand (what data to use) to determine which hardware components need to be activated.

3. Execute

• The CPU carries out the instruction. This might involve the ALU performing a calculation, data being moved between registers, or the CPU jumping to a different memory address by updating the PC.

• Address vs. Data: It is critical to distinguish between the location of information (Address) and the information itself (Data). The MAR handles the 'where,' while the MDR handles the 'what.'

• Trace the Registers: When asked to describe the cycle, always mention the specific movement of data between registers (e.g., PC $\rightarrow$ MAR $\rightarrow$ RAM $\rightarrow$ MDR).

• The PC Increment: Remember that the Program Counter increments during the fetch stage, not at the very end of the cycle. This ensures the CPU is ready for the next cycle even if the current execution takes multiple steps.

• Bus Identification: Always specify which bus is being used. Addresses travel on the Address Bus, while instructions and data travel on the Data Bus.

• Confusing MAR and MDR: Students often swap these. Remember: Address (MAR) is the map coordinate; Data (MDR) is the treasure found at that coordinate.

• Execution is not just Math: While the ALU is vital, the 'Execute' stage also includes simple data movements or branching (changing the PC), not just arithmetic.

• Clock Speed Misinterpretation: A higher clock speed ($3.0$ GHz vs $2.0$ GHz) means more cycles per second, but it does not necessarily mean the computer is 'faster' if other factors like cache or cores are inferior.