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ArchitectureFundamentals

📚 Computer Architecture Fundamentals

Understand the basic principles of how computers work: Von Neumann architecture, Harvard architecture, and the fundamental components that make up a computer system.

What is Computer Architecture?

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Definition

Computer Architecture refers to the design and organization of computer systems. It defines the structure, behavior, and interaction of components within a computer. Architecture is about HOW the computer works at a fundamental level.

Hardware Organization

Physical layout and interconnection of components

Instruction Set

Set of commands CPU can execute

Memory System

How memory is organized and accessed

I/O System

How CPU communicates with peripherals

Von Neumann Architecture

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Key Concept

Proposed by John von Neumann in 1945. Most computers today follow this model: a single memory stores both instructions and data, and the CPU fetches, decodes, and executes instructions one at a time.

Von Neumann Architecture Components

CPU

Executes instructions

Memory

Stores data & instructions

I/O

Input/Output devices

Bus

Connects components

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Von Neumann Bottleneck

The single bus connecting CPU and memory limits performance. Both instructions and data must pass through the same bus, creating a bottleneck that restricts data flow rate.

Harvard Architecture

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Key Difference

Separate memory and bus for instructions and data. Allows simultaneous access to both, eliminating the Von Neumann bottleneck. Widely used in DSPs and embedded systems.

Von Neumann

Advantages:

  • ✓ Simple design
  • ✓ Less hardware

Limitations:

  • ✗ Memory bottleneck
  • ✗ Slower

Harvard

Advantages:

  • ✓ Parallel access
  • ✓ Faster
  • ✓ Better cache

Limitations:

  • ✗ More complex
  • ✗ More expensive

CPU Execution Cycle (Fetch-Execute Cycle)

Fetch
→
Decode
→
Execute
→
Memory
→
Write
1
1. Fetch

PC (Program Counter) holds address of next instruction. Instruction is fetched from memory into IR (Instruction Register).

2
2. Decode

Control Unit decodes the instruction and generates control signals. Operands are identified and fetched if needed.

3
3. Execute

ALU performs the operation specified by the instruction. This might be arithmetic, logic, or data movement.

4
4. Memory Access

If needed, data is read from or written to memory. Not all instructions need this step.

5
5. Write Back

Result is written back to a register or memory. PC is incremented to point to the next instruction.

Simple CPU Simulator (Python)

python

class CPU:
    def __init__(self):
        self.registers = [0] * 32
        self.pc = 0
        self.memory = [0] * 4096

    def fetch(self):
        instruction = self.memory[self.pc]
        self.pc += 1
        return instruction

    def execute(self, instruction):
        opcode = (instruction >> 24) & 0xFF
        rd = (instruction >> 16) & 0x1F
        rs = (instruction >> 8) & 0x1F
        imm = instruction & 0xFF
        
        if opcode == 0:  # ADD
            self.registers[rd] = self.registers[rs] + imm
        elif opcode == 1:  # LOAD
            self.registers[rd] = self.memory[imm]
        elif opcode == 2:  # STORE
            self.memory[imm] = self.registers[rs]

Interview Questions

What is the difference between Von Neumann and Harvard Architecture?

Von Neumann uses a single memory for both instructions and data with one bus, causing the Von Neumann bottleneck. Harvard uses separate memories and buses for instructions and data, allowing simultaneous access but requiring more hardware.

Explain the Fetch-Execute cycle in detail.

The CPU repeatedly: 1) Fetches instruction from memory address in PC, 2) Decodes it in control unit, 3) Executes using ALU, 4) Accesses memory if needed, 5) Writes back results and increments PC.

What is the Von Neumann bottleneck?

The single bus connecting CPU and memory in Von Neumann architecture can only transfer either an instruction or data at a time, not both. This limits throughput, known as the Von Neumann bottleneck.