November 2025

Definition

A Bus is a communication pathway used to transfer data, addresses, and control signals between different components of a computer system — such as the CPU, memory, and input/output devices.

Think of a bus like a highway that connects various parts of the computer and allows information to move between them.

Need for a Bus

Without a bus, every component would need a separate connection, which would make the system complex and inefficient.
The bus system simplifies this by providing a shared pathway for communication.

Functions of a Bus

1. Transfers data between CPU, memory, and I/O devices.
2. Carries memory addresses so that CPU can access specific locations in memory.
3. Transmits control signals that coordinate the operations of the computer.
4. Ensures communication and synchronization between components.

Types of System Buses

The system bus is divided into three main types:

1. Data Bus

• Used to transfer actual data between the CPU, memory, and I/O devices.
• It is bi-directional — data can flow both ways (to and from CPU).
• The width of the data bus (like 8-bit, 16-bit, 32-bit, or 64-bit) determines how much data can be transferred at once.

Example:
If the CPU has a 32-bit data bus, it can transfer 32 bits (4 bytes) of data in one operation.

2. Address Bus

• Used to carry the address of memory locations that the CPU wants to read or write.
• It is uni-directional — addresses flow only from CPU to memory or I/O.
• The width of the address bus determines how many memory locations can be accessed.

Example:
A 16-bit address bus can access 216=65,5362^{16} = 65,536216=65,536 (or 64 KB) memory locations.

3. Control Bus

• Carries control and timing signals from the CPU to other components.
• It helps coordinate operations like reading, writing, and interrupt handling.
• It is bi-directional because control signals can be sent in both directions.

Common Control Signals Include:

• Read (RD): Instructs memory or I/O device to send data to CPU.
• Write (WR): Instructs memory or I/O device to store data.
• Interrupt (INT): Indicates a request for CPU attention.
• Clock Signals: Synchronize all operations.

Comparison Table

Bus Type	Direction	Purpose	Example Signal
Data Bus	Bi-directional	Transfers data between CPU, memory, and I/O	Data bits
Address Bus	Uni-directional	Carries memory addresses from CPU	Memory location (e.g., 2000H)
Control Bus	Bi-directional	Carries control and timing signals	Read/Write, Interrupt

Importance of Bus in Computer System

• Provides a shared communication path for all components.
• Reduces hardware complexity.
• Ensures faster and organized data transfer.
• Allows CPU, memory, and I/O devices to work together efficiently.

Summary

• A bus connects all the main parts of a computer.
• It is divided into Data Bus, Address Bus, and Control Bus.
• Together, they enable data transfer, address communication, and control operations — ensuring smooth system functioning.

Microprocessor (CPU)

What is a Microprocessor?

A Microprocessor is the central processing unit (CPU) of a computer system, built on a single integrated circuit (IC) chip.
It performs all computational tasks, decision-making, and control operations.

You can think of it as the “brain” of the computer that processes all data and executes instructions stored in memory.

Examples:

• Intel 8085, 8086
• Intel Core i5, i7
• AMD Ryzen 5
• ARM processors (used in smartphones)

Main Components of CPU

1. ALU (Arithmetic Logic Unit)
2. CU (Control Unit)
3. MU (Memory Unit) or Registers

1. Arithmetic Logic Unit (ALU)

Definition:
The ALU is the part of the CPU responsible for all mathematical and logical operations. It takes input data from registers or memory, processes it, and sends the result back.

Functions of ALU:

Arithmetic Operations:

• Addition → 5 + 3 = 8
• Subtraction → 9 - 4 = 5
• Multiplication → 2 × 6 = 12
• Division → 12 ÷ 3 = 4
• Increment / Decrement → X = X + 1, X = X - 1

Logical Operations:

• AND – Returns 1 if both bits are 1.
  Example: 1 AND 1 = 1, 1 AND 0 = 0
• OR – Returns 1 if any bit is 1.
  Example: 1 OR 0 = 1
• NOT – Inverts the bit.
  Example: NOT 1 = 0
• XOR – Returns 1 if bits are different.
  Example: 1 XOR 0 = 1
• Comparison – Checks if A > B, A = B, or A < B.

Example

If you want to add two numbers stored in registers R1 = 10 and R2 = 20,
→ The ALU performs: R3 = R1 + R2
→ Result (30) is stored in register R3.

2. Control Unit (CU)

Definition:
The Control Unit manages and coordinates all the operations inside the CPU.
It tells the ALU, Memory Unit, and Input/Output devices what to do and when to do it.

It does not process data itself but ensures that all components work together correctly.

Main Functions of CU:

1. Fetching:
   CU fetches instructions from memory (using Program Counter).
   Example: Fetch instruction ADD R1, R2.
2. Decoding:
   CU interprets the fetched instruction — e.g., it understands that this means add contents of R1 and R2.
3. Execution Control:
   CU sends control signals to ALU to perform the addition.
4. Storing Result:
   CU directs the result to be stored in the specified register or memory location.

Example

Suppose you write a small program to calculate A + B:

1. CU fetches the instruction ADD A, B from memory.
2. CU decodes it (understands what to do).
3. CU signals ALU to perform addition.
4. ALU adds A and B.
5. CU then stores the result in memory or a register.

3. Memory Unit (MU)

Definition:
The Memory Unit stores data, instructions, and results temporarily or permanently.
It acts as the storage space for everything the CPU needs to process.

Types of Memory:

Type	Description	Example
Primary Memory (Main Memory)	Directly accessible by CPU	RAM, ROM
Secondary Memory	Long-term data storage	Hard disk, SSD
Cache Memory	High-speed temporary storage	L1, L2, L3 cache
Registers	Small, fastest memory inside CPU	AX, BX (in Intel CPUs)

Example

When you run a program:

1. Instructions are stored in RAM.
2. CU fetches them one by one.
3. ALU executes them.
4. Intermediate results may be stored in registers temporarily.
5. Final output is stored back in RAM or disk.

4. How CPU Works (Step-by-Step Cycle)

This process is known as the Instruction Cycle or Fetch–Decode–Execute Cycle.

Step	Description	Example
Fetch	CU fetches instruction from memory.	Fetch “ADD R1, R2”
Decode	CU interprets what needs to be done.	Understand that it’s an addition command.
Execute	ALU performs the operation.	Add the data in R1 and R2.
Store	Result is saved in register or memory.	Store sum in R3.

Other CPU Components

Component	Description
Registers	High-speed storage for temporary data (like R1, R2, Accumulator).
Program Counter (PC)	Holds the address of the next instruction to execute.
Instruction Register (IR)	Stores the current instruction being executed.
Bus System	Transfers data and signals between CPU, memory, and devices (Data Bus, Address Bus, Control Bus).

Summary Table

Unit	Full Form	Function	Example
ALU	Arithmetic Logic Unit	Performs arithmetic & logical operations	Addition, Comparison
CU	Control Unit	Controls and coordinates CPU activities	Fetch–Decode–Execute
MU	Memory Unit	Stores data, instructions, and results	RAM, Cache
Registers	—	Temporary data storage inside CPU	AX, BX
PC	Program Counter	Tracks next instruction	Next step pointer

Example (Real-Life Analogy)

Imagine a teacher (CU) in a classroom:

• CU (teacher) reads the question (instruction) from the book (memory).
• The teacher asks a student (ALU) to solve it.
• The student (ALU) does the calculation and gives the answer.
• The teacher writes the answer back in the notebook (memory).

This is exactly how your CPU works!

Primary and Secondary Memory