LECTURE 4 IN COMPUTER SCIENCE ENGINEERING

Lecture 4: Computer Architecture & Organization

To understand how computers function, we must study Computer Architecture and Computer Organization. While programming teaches us how to give instructions to a computer, architecture and organization explain how the computer actually carries out those instructions internally.

1. Difference Between Architecture & Organization

Computer Architecture – Defines the design, structure, and functionality of a computer system (what the computer can do). Example: instruction set, addressing modes.
Computer Organization – Refers to the operational structure and physical components that implement the architecture (how the computer does it). Example: control signals, memory hierarchy, hardware details.

2. Basic Structure of a Computer

The modern computer system is based on the Von Neumann Architecture, consisting of:

Input Unit – Devices that send data to the computer (keyboard, mouse, scanner).
Output Unit – Devices that display results (monitor, printer, speakers).
Memory Unit – Stores instructions and data.
Control Unit (CU) – Directs the flow of data and instructions.
Arithmetic Logic Unit (ALU) – Performs calculations and logical operations.
Central Processing Unit (CPU) – Combination of CU + ALU; the “brain” of the computer.

3. The CPU: Heart of the Computer

The Central Processing Unit (CPU) executes instructions and controls other components. It works in a cycle called the Instruction Cycle:

Fetch – Retrieve the instruction from memory.
Decode – Interpret the instruction.
Execute – Carry out the instruction (calculation, data movement, etc.).
Store – Save the result (if needed).

4. Memory Hierarchy

Computer memory is organized in levels to balance speed and cost:

Registers – Fastest, located inside the CPU, stores small temporary values.
Cache – High-speed memory close to CPU (L1, L2, L3).
Main Memory (RAM) – Stores programs and data currently in use.
Secondary Storage – Hard drives, SSDs; slower but larger capacity.
Tertiary Storage – External storage (DVDs, USB drives, cloud).

Rule of thumb: The closer the memory is to the CPU, the faster but smaller it is. The farther, the slower but larger.

5. Types of Computer Architecture

Von Neumann Architecture – Single memory for data and instructions.
Harvard Architecture – Separate memory for data and instructions (used in embedded systems).
RISC (Reduced Instruction Set Computer) – Simple, fast instructions (e.g., ARM processors).
CISC (Complex Instruction Set Computer) – Richer instruction set, fewer lines of code (e.g., Intel, AMD).

6. Example: Simple Machine Instruction

# Pseudo assembly code
LOAD R1, 2000   ; Load data from memory address 2000 into register R1
ADD R1, R2      ; Add contents of R2 to R1
STORE R1, 3000  ; Store result into memory address 3000

This shows how instructions are fetched, executed, and results stored by the CPU.

7. Importance of Studying Computer Architecture

Helps in writing efficient programs by understanding hardware limitations.
Guides system design in embedded systems, mobile devices, and supercomputers.
Enables performance optimization (cache use, memory allocation).
Forms the basis for careers in hardware engineering and low-level software development.

“Software runs on hardware. To build truly efficient systems, an engineer must understand both.”

8. Summary of Key Points

Computer Architecture = design & functionality; Organization = implementation details.
Von Neumann model forms the basis of most modern computers.
CPU executes instructions via the Fetch–Decode–Execute–Store cycle.
Memory is structured in a hierarchy: Registers → Cache → RAM → Storage.
Architectural models: Von Neumann, Harvard, RISC, CISC.

✦ Lecture 4 prepared under the authority of English Master Institute (EMI) Worldwide ✦

LECTURE 5 ON COMPUTER SCIENCE ENGINEERING