Microprocessors & Architecture: Basics, Components & Systems

TAKEAWAYS:

• A microprocessor is a single-chip CPU that executes instructions, performs arithmetic/logic operations, and manages data flow in digital systems.
• The von Neumann architecture (stored-program concept) is the foundation of modern microprocessors, where data and instructions share the same memory.
• Key internal components include the Arithmetic Logic Unit (ALU), Control Unit (CU), registers, and clock system, each playing a distinct role in instruction execution.
• The external bus structure (address, data, control buses) enables communication between the microprocessor and other system components like memory and I/O devices.
• Microprocessors are classified based on word size (8-bit, 16-bit, 32-bit, 64-bit), architecture (RISC/CISC), and application domain (general-purpose, embedded).
• Understanding microprocessor systems is critical for designing computers, embedded systems, and optimizing performance in real-world applications like banking transactions, mobile apps, and IoT devices.

1. What is a Microprocessor?

A microprocessor is an integrated circuit (IC) that contains all the functions of a Central Processing Unit (CPU) on a single chip. It is the "brain" of a computer or embedded system, responsible for executing instructions, performing arithmetic and logic operations, and managing data flow.

Key Characteristics of a Microprocessor:

• Single-chip design: Unlike older CPUs that required multiple chips, a microprocessor integrates all components (ALU, CU, registers, cache) into one chip.
• Programmable: Can execute different programs by loading instructions into memory.
• General-purpose or specialized: Used in computers, calculators, washing machines, and even cars.
• Clock-dependent: Operates synchronously with a clock signal to coordinate operations.

Evolution of Microprocessors

The development of microprocessors followed Moore’s Law, leading to exponential growth in performance and complexity:

• 1971: Intel 4004 (4-bit, 2,300 transistors).
• 1974: Intel 8080 (8-bit, 6,000 transistors).
• 1978: Intel 8085 (8-bit, widely used in early PCs).
• 1980s–Present: 16-bit (80286), 32-bit (Pentium), 64-bit (Core i7), and modern multi-core processors.

2. Von Neumann Architecture: The Foundation

Most modern microprocessors follow the von Neumann architecture, proposed by John von Neumann in 1945. This architecture defines how a computer processes data and instructions.

Key Features:

1. Stored-Program Concept:
   • Both data and instructions are stored in the same memory (RAM).
   • The CPU fetches instructions sequentially unless redirected (e.g., by jumps or interrupts).
2. Five Main Components:
   • Input Unit: Receives data from external sources (keyboard, sensors).
   • Memory Unit: Stores data and instructions (RAM, ROM).
   • Arithmetic Logic Unit (ALU): Performs calculations and logical operations.
   • Control Unit (CU): Manages instruction execution and data flow.
   • Output Unit: Displays or transmits results (monitor, printer).

How It Works (Fetch-Decode-Execute Cycle)

stateDiagram-v2
    [*] --> Fetch: Instruction from memory
    Fetch --> Decode: Interpret opcode
    Decode --> Execute: Perform operation (ALU/CU)
    Execute --> Memory: Store result if needed
    Memory --> [*]

Example: When you press a key on your keyboard (e.g., typing in eSewa to pay a bill), the following happens:

1. The input unit detects the keypress and sends a signal to the microprocessor.
2. The CU fetches the corresponding instruction from memory (e.g., "store character in buffer").
3. The ALU processes the data, and the output unit displays it on the screen.

3. Internal Organization of a Microprocessor

The microprocessor consists of three main functional units:

A. Arithmetic Logic Unit (ALU)

• Performs arithmetic operations (addition, subtraction, multiplication, division).
• Performs logical operations (AND, OR, NOT, XOR).
• Example: Calculating the total bill in Daraz when adding multiple items.

B. Control Unit (CU)

• Fetches instructions from memory.
• Decodes instructions to determine the operation.
• Coordinates data movement between ALU, registers, and memory.
• Example: Managing the login process in Khalti by verifying credentials step-by-step.

C. Registers

Small, high-speed memory locations inside the CPU used for temporary storage. Common registers in the 8085 microprocessor:

Example: In Ncell’s billing system, registers might temporarily store a customer’s phone number (B register) and balance (A register) during a transaction.

4. External Bus Structure

Microprocessors communicate with external components (memory, I/O devices) via three buses:

A. Address Bus

• Unidirectional: CPU → Memory/I/O.
• Determines the memory location or I/O device to access.
• Width = Maximum addressable memory.
  • 8-bit address bus → $2^8=256$ locations.
  • 16-bit address bus → $2^{16}=64$ KB.

B. Data Bus

• Bidirectional: CPU ↔ Memory/I/O.
• Transfers actual data (instructions, operands, results).
• Width = Data transfer capacity (e.g., 8-bit, 16-bit, 32-bit).

C. Control Bus

• Bidirectional: CPU ↔ Memory/I/O.
• Carries control signals like:
  • Read (RD): CPU reads data.
  • Write (WR): CPU writes data.
  • Interrupt Request (INTR): Signals from I/O devices.
  • Clock Signals: Synchronizes operations.

Bus Structure in 8085

Real-World Example: When you transfer money via eSewa:

1. The address bus locates the recipient’s account in the bank’s database.
2. The data bus transfers the transaction amount (e.g., Rs. 500).
3. The control bus ensures the transaction is validated and completed.

5. Microprocessor System Organization

A complete microprocessor system includes:

1. Microprocessor (CPU)
2. Memory Units:
   • ROM (Read-Only Memory): Stores permanent data/instructions (e.g., BIOS in PCs).
   • RAM (Random Access Memory): Temporary storage for running programs (e.g., WhatsApp messages).
3. I/O Devices: Keyboard, monitor, sensors, etc.
4. Support Chips: Timer, DMA controller, interrupt controller.

Example: Traffic Light Control System (NTC)

How It Works:

1. Sensors detect cars waiting at a junction (input via data bus).
2. The microprocessor processes the data and decides the light sequence (e.g., green for 30 seconds).
3. Control signals are sent to the traffic lights (output via control bus).

6. Classification of Microprocessors

Microprocessors are categorized based on:

Example:

• Pathao’s ride-hailing app uses a 64-bit ARM processor (RISC) for efficient mobile performance.
• ATMs (Nabil Bank) use 32-bit CISC processors for secure transaction processing.

7. Applications of Microprocessors

Microprocessors are ubiquitous in modern life:

A. Computing Devices

• PCs/Laptops: Intel Core i7 (multi-core, 64-bit).
• Servers: AMD EPYC (high-performance, multi-socket).

B. Embedded Systems

• Smartphones: Apple A15 (ARM-based, RISC).
• Washing Machines: Microprocessor controls water levels, temperature.
• Cars: Engine control units (ECUs) use microprocessors for fuel injection.

C. Communication Systems

• Routers (NTC): Manage internet traffic using microprocessors.
• Mobile Phones (Ncell): Process calls, messages, and data.

D. Industrial Automation

• Robotics: Control arms in factories.
• Medical Devices: Pacemakers use microprocessors to regulate heartbeats.

In the Real World

1. eSewa Payments:

   • Uses microprocessor-based servers to validate transactions.
   • The control unit ensures secure data transfer between user accounts and banks.
   • Example: When you pay Rs. 1000 for electricity via eSewa, the microprocessor in the server calculates the deduction, updates your balance, and sends a confirmation.

2. Khalti Mobile App:

   • The ARM processor in your smartphone executes instructions to encrypt your payment details (using ALU for cryptographic operations).
   • The address bus locates your account data in the app’s memory.

3. Daraz Order Processing:

   • When you place an order, the microprocessor in Daraz’s server manages:
     • Data bus: Transfers order details (product ID, quantity, address).
     • Control bus: Coordinates with inventory and shipping systems.
     • ALU: Calculates total cost (including tax and discount).

4. NTC Traffic Management:

   • Microprocessors in traffic light controllers use sensors (input) to detect congestion and adjust timings (output).
   • The program counter (PC) ensures the correct sequence of operations (e.g., green → yellow → red).

5. Bank ATMs (Nabil, Global IME):

   • The CISC processor executes complex instructions like:
     • Validating PIN (logical operations in ALU).
     • Dispensing cash (control signals to the cash dispenser).
   • The address bus accesses customer account records in the bank’s database.

Exam Tip

This unit is conceptual and diagram-based. Expect questions on:

1. Definitions: What is a microprocessor? Von Neumann architecture?
2. Diagrams: Draw and label the 8085 internal block diagram or bus structure.
3. Components: Explain the role of ALU, CU, and registers.
4. Real-world applications: Relate microprocessor functions to eSewa, Khalti, or NTC systems.
5. Comparisons: Differences between RISC and CISC (save for Unit 7, but basics are tested here).

Common Pitfalls:

• Forgetting that the address bus is unidirectional while the data bus is bidirectional.
• Confusing ROM (permanent storage) with RAM (temporary storage).
• Not labeling diagrams properly (e.g., missing "16-bit address bus" in the 8085 figure).

Scoring Tips:

• Use bullet points for classifications (e.g., types of buses).
• For worked examples, tie them to Nepali contexts (e.g., Daraz orders, Khalti payments).
• Memorize the 8085 pin diagram (especially DMA pins: HOLD, HLDA).
• Practice short-answer questions on von Neumann architecture and bus types.