What is the difference between the MAR and the MDR?

The MAR (Memory Address Register) holds the location/address of the data in memory, while the MDR (Memory Data Register) holds the actual data or instruction that was found at that address.

How does the Opcode differ from the Operand?

The Opcode is the part of the instruction that specifies 'what' to do (the command), whereas the Operand is the 'item' being acted upon (either the data itself or the address where the data is stored).

When is the Program Counter (PC) updated during the cycle?

The PC is updated during the Fetch stage, immediately after its value is copied to the MAR. This ensures it points to the next instruction while the current one is still being processed.

What common error occurs when describing the data bus during a 'write' operation?

Students often forget that the data bus is bidirectional. While it brings instructions *to* the CPU during Fetch, it also carries data *from* the CPU (MDR) back to memory during certain Execute steps.

Why is it incorrect to say the PC always increments by 1?

While the PC usually increments by 1 to move to the next sequential instruction, 'branch' or 'jump' instructions in the Execute phase can change the PC to a completely different address.

What happens if a student confuses the Address Bus with the Control Bus?

They miss the distinction that the Address Bus only specifies 'where' to look, while the Control Bus specifies 'what action' to take (like Read or Write) and synchronizes the timing of the transfer.

Define the 'Current Instruction Register' (CIR).

The CIR is a special-purpose register that stores the instruction currently being decoded and executed, having been moved there from the MDR at the end of the Fetch stage.

What is the role of the Arithmetic Logic Unit (ALU) in the cycle?

The ALU performs the actual calculations (addition, subtraction) and logical comparisons (AND, OR, NOT) required by the instruction during the Execute phase.

What does the 'Accumulator' (ACC) store?

The Accumulator is a general-purpose register that stores the intermediate results of calculations and logical operations performed by the ALU.

Why must the CPU 'Decode' an instruction before executing it?

Instructions are stored as binary code; the Decode stage allows the Control Unit to interpret these bits to activate the specific hardware paths (like the ALU or specific registers) needed for that command.

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4. Processor Fundamentals

Fetch-Execute Cycle

Summary

The Fetch-Execute Cycle is the fundamental operational process of a computer's Central Processing Unit (CPU). It describes the continuous sequence of steps required to retrieve an instruction from memory, determine its meaning, and carry out the required actions, repeating billions of times per second to run software.

1. Definition & Core Concepts

The Fetch-Execute Cycle (also known as the instruction cycle) is the standard procedure used by the CPU to process a single program instruction.
It relies on the Von Neumann Architecture, where both data and instructions are stored in the same memory (RAM) and accessed via a shared set of buses.
The cycle is synchronized by the System Clock, which emits pulses; each step of the cycle typically corresponds to one or more clock ticks.

A circular flowchart showing the three main stages of the CPU cycle: Fetch, Decode, and Execute.

2. The Fetch Phase

The Program Counter (PC) holds the memory address of the next instruction to be processed. This address is copied to the Memory Address Register (MAR).
The CPU sends a signal via the Address Bus to the location in RAM stored in the MAR, while the Control Unit sends a 'read' signal via the Control Bus.
The instruction at that address is transferred from memory to the Memory Data Register (MDR) via the Data Bus.
Finally, the PC is incremented by 1 ( $PC \leftarrow PC + 1$ ) so it points to the next instruction, and the instruction in the MDR is copied to the Current Instruction Register (CIR).

3. The Decode Phase

4. The Execute Phase

5. Key Distinctions

6. Exam Strategy & Tips