CPU Architecture Overview

Overview

This lecture explains the architecture of the CPU, its key components, special registers, and the fetch-decode-execute cycle important for GCSE Computer Science.

CPU Overview and Role

• The CPU (Central Processing Unit) processes instructions using the fetch-decode-execute cycle.
• The CPU is considered the "brain" of the computer.

Main Components of the CPU

• The Arithmetic Logic Unit (ALU) processes mathematical operations, comparisons, and logical operations (AND, OR, NOT).
• The Control Unit decodes instructions and coordinates data movement within the CPU based on the clock cycle.
• Cache is very fast, small memory for recently or frequently used instructions.
• Registers are ultra-fast storage locations within the CPU holding instructions and data during processing.

Special Purpose Registers

• The Program Counter (PC) stores the address of the next instruction to fetch and execute.
• The Memory Address Register (MAR) temporarily holds the address of the next instruction or data to be fetched.
• The Memory Data Register (MDR) temporarily holds the actual instruction or data that has just been fetched.
• The Accumulator holds data values currently being processed or results from the ALU.

The Fetch-Decode-Execute Cycle

• At fetch: the PC sends the next instruction's address to the MAR, then increments to the next instruction.
• The instruction at the MAR’s address is fetched from RAM into the MDR.
• At decode: the Control Unit interprets the fetched instruction and determines if additional data is needed.
• If needed, data is fetched into the MDR; address is passed to the MAR.
• At execute: the instruction is performed, e.g., adding a value to the Accumulator.

Register Summary Table

• Program Counter — stores addresses.
• Memory Address Register — stores addresses.
• Memory Data Register — stores instructions or data.
• Accumulator — stores data.

Key Terms & Definitions

• CPU (Central Processing Unit) — The core processor executing instructions.
• ALU (Arithmetic Logic Unit) — Handles calculations and logic operations.
• Control Unit — Decodes instructions and controls data flow.
• Cache — Small, fast memory for frequently used data.
• Register — Fast storage area within the CPU.
• Program Counter (PC) — Holds the next instruction's address.
• Memory Address Register (MAR) — Holds address to fetch from memory.
• Memory Data Register (MDR) — Holds fetched instruction/data.
• Accumulator — Holds data being calculated or results.

Action Items / Next Steps

• Review resources at mrgoff.com for revision on CPU architecture.
• Prepare for next lesson on factors affecting CPU performance.