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CPU Components & Von Neumann Architecture

Everything you need to know for your GCSE exam

📑 Contents

1. CPU Components

The CPU (Central Processing Unit) is the "brain" of the computer. It contains four main components that you need to know:

ALU (Arithmetic Logic Unit)

What it does: Performs all calculations and comparisons

The ALU handles two types of operations:

Arithmetic Operations

  • Addition (+)
  • Subtraction (-)
  • Multiplication (×)
  • Division (÷)

Logical Operations

  • AND
  • OR
  • NOT
  • Comparisons (>, <, =)
Remember ALU = Arithmetic + Logic. That's two types of operations!

CU (Control Unit)

What it does: Controls and coordinates all CPU operations

The Control Unit is like a conductor in an orchestra - it doesn't play instruments (do calculations) but tells everyone else what to do and when.

The Control Unit:
  • Fetches instructions from memory
  • Decodes instructions (works out what they mean)
  • Controls timing and data flow
  • Sends signals to other components
The CU coordinates but doesn't calculate - that's the ALU's job!

Cache

What it does: Provides fast storage for frequently used data

Cache is small, fast memory inside the CPU. It stores copies of data and instructions that are used frequently.

Why cache matters:
  • Cache is faster than RAM but smaller
  • CPU checks cache first before going to slower RAM
  • Cache hit = data found in cache (fast!)
  • Cache miss = data not in cache, must get from RAM (slower)
More cache = better performance, because less waiting for slow RAM.

Registers

What they do: Store small amounts of data being processed

Registers are tiny, super-fast storage locations inside the CPU.

Key facts about registers:
  • The fastest type of memory
  • Very small (typically 32 or 64 bits each)
  • Different registers have specific purposes
  • Some store addresses, some store data

2. Von Neumann Architecture

Most modern computers use Von Neumann architecture, named after mathematician John von Neumann who described it in 1945.

Key principle: Programs and data are stored in the same memory (RAM) and share the same bus system.

Components of Von Neumann Architecture

The key point is that programs and data share the SAME memory. Before Von Neumann, computers had separate memory for programs!

3. The Four Key Registers

You MUST know these four registers for OCR J277:

Register Full Name Purpose Stores
PC Program Counter Holds the address of the NEXT instruction to be fetched ADDRESS
MAR Memory Address Register Holds the address of the memory location being accessed NOW ADDRESS
MDR Memory Data Register Holds the data that has been fetched from (or is being written to) memory DATA
ACC Accumulator Stores the results of calculations from the ALU DATA
PC and MAR store ADDRESSES (where to look). MDR and ACC store DATA (what we found or calculated).

4. Data vs Address - The Key Difference

This is a crucial distinction for your exam. You must know which registers store data and which store addresses.

📍 ADDRESS

A location in memory

Like a house number - tells you WHERE something is

Registers: PC, MAR

Example: 100, 101, 256

📦 DATA

The actual content stored

Like who lives in the house - the ACTUAL information

Registers: MDR, ACC

Example: "ADD 5", 42, "Hello"

Quick Memory Aid

5. The Fetch-Execute Cycle

Every instruction goes through this cycle. Learn what happens at each stage and which registers change.

🔄 FETCH Stage

Purpose: Get the next instruction from memory

  1. PC → MAR
    The address in the Program Counter is copied to the Memory Address Register
    MAR = PC
  2. MAR sends address to RAM
    The address is sent via the address bus to RAM
  3. RAM → MDR
    The instruction at that address is sent via the data bus to MDR
    MDR = [contents of that address]
  4. PC increments
    The Program Counter increases by 1, ready for the next instruction
    PC = PC + 1
PC increments DURING fetch, not at the end of the cycle!

🔍 DECODE Stage

Purpose: Work out what the instruction means

  1. Instruction sent to Control Unit
    The instruction in MDR is passed to the Control Unit
  2. CU decodes the instruction
    The Control Unit works out what operation is needed
  3. CU prepares control signals
    Control signals are prepared for the execute stage

⚡ EXECUTE Stage

Purpose: Carry out the instruction

What happens depends on the instruction type:

Example: LDA 200 (Load from address 200)

  1. Address 200 is placed in MAR
  2. Data at address 200 is fetched to MDR
  3. Data is copied from MDR to ACC

Example: ADD 150 (Add value from address 150)

  1. Address 150 is placed in MAR
  2. Data at address 150 is fetched to MDR
  3. ALU adds MDR value to ACC
  4. Result stored back in ACC

6. Summary & Exam Checklist

✅ What You Need to Know

CPU Components:
  • ☐ ALU - performs arithmetic AND logical operations
  • ☐ Control Unit - fetches, decodes, coordinates
  • ☐ Cache - fast memory for frequently used data
  • ☐ Registers - tiny, fast storage in CPU
Von Neumann Registers:
  • ☐ PC stores ADDRESS of NEXT instruction
  • ☐ MAR stores ADDRESS being accessed
  • ☐ MDR stores DATA being transferred
  • ☐ ACC stores DATA (calculation results)
Fetch-Execute Cycle:
  • ☐ FETCH: PC→MAR, RAM→MDR, PC++
  • ☐ DECODE: CU interprets instruction
  • ☐ EXECUTE: Instruction carried out (may use ALU)
Data vs Address:
  • ☐ PC and MAR store ADDRESSES
  • ☐ MDR and ACC store DATA
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