What is the 'System Clock'?

The system clock is a timing device that sends out regular electrical pulses to synchronize all the components of the CPU and the wider computer system.

Why are registers located inside the CPU rather than in the RAM?

Placing registers inside the CPU minimizes the distance data must travel, allowing for near-instantaneous access speeds required for active processing tasks.

What is the primary difference between the Address Bus and the Data Bus?

The Address Bus is unidirectional, carrying memory locations from the CPU to RAM, while the Data Bus is bidirectional, carrying actual data and instructions between the CPU and memory.

How does the role of the MAR differ from the MDR?

The MAR (Memory Address Register) holds the location of the data in memory, whereas the MDR (Memory Data Register) holds the actual data value that is being transferred to or from that location.

What is the functional difference between the Control Unit (CU) and the Arithmetic Logic Unit (ALU)?

The CU manages the execution of instructions and coordinates data flow, while the ALU performs the actual mathematical and logical operations on the data.

What error in logic occurs if the Program Counter (PC) fails to increment?

The CPU would repeatedly fetch and execute the same instruction from the same memory address, causing the program to enter an infinite loop or stall.

Why is it a mistake to assume the Address Bus is bidirectional?

The CPU is the sole component that specifies which memory address is being accessed; memory does not send addresses back to the CPU, making a bidirectional path unnecessary and inefficient.

What happens if the System Clock frequency is set higher than the components can handle?

This leads to synchronization errors where data may not arrive at a component before it is processed, resulting in system crashes or data corruption.

Define the function of the Accumulator (ACC).

The Accumulator is a register that stores the results of calculations performed by the ALU, acting as a temporary holding area for data during processing.

What is the purpose of the Current Instruction Register (CIR)?

The CIR holds the instruction that is currently being decoded or executed, allowing the CPU to keep the instruction stable while the Control Unit processes it.

Components of the CPU | Cambridge International Examinations AS-Level Computer Science

Revision Notes

AS-Level

Cambridge International Examinations

Computer Science

4. Processor Fundamentals

Components of the CPU

Summary

The Central Processing Unit (CPU) is the primary component of a computer system responsible for processing data and executing instructions. It consists of several specialized internal components, including the Control Unit, Arithmetic Logic Unit, and various registers, all interconnected by a system of buses that facilitate the movement of data and control signals.

1. Core Processing Units

Control Unit (CU): This component acts as the manager of the CPU, directing the flow of data between the processor and other devices. It decodes instructions and sends control signals to coordinate the actions of the ALU, memory, and input/output devices.
Arithmetic Logic Unit (ALU): The ALU is responsible for performing all mathematical calculations (such as addition and subtraction) and logical operations (such as AND, OR, and NOT). It is the 'engine' where the actual data processing occurs.
System Clock: This component generates continuous electrical pulses at a fixed frequency to synchronize all internal operations. Each pulse represents a 'tick' or state change, ensuring that components operate in perfect timing with one another.

Diagram showing the internal architecture of a CPU including the Control Unit, ALU, and Registers.

2. Internal Memory: Registers

Program Counter (PC): A specialized register that holds the memory address of the next instruction to be fetched. It automatically increments after each fetch to point to the subsequent instruction in the sequence.
Memory Address Register (MAR): This register stores the specific memory location (address) of the data or instruction that the CPU needs to read from or write to in the main memory.
Memory Data Register (MDR): Acting as a buffer, the MDR holds the actual data or instruction that has just been fetched from memory or is waiting to be written to memory.
Current Instruction Register (CIR): This register holds the instruction that is currently being decoded and executed by the Control Unit.
Accumulator (ACC): A general-purpose register used by the ALU to store the intermediate and final results of calculations.

3. The Bus System

Data Bus: A bidirectional path used to transport the actual data and instructions between the CPU, memory, and input/output devices. Its width (number of wires) determines how much data can be moved simultaneously.
Address Bus: A unidirectional path that carries the address of the memory location or I/O device that the CPU wants to access. Because it only carries addresses from the CPU to memory, it does not need to be bidirectional.
Control Bus: A bidirectional path used by the Control Unit to transmit command signals (like 'read' or 'write') and receive status signals (like 'interrupts') from other hardware components.

4. Key Distinctions

5. Exam Strategy & Tips