Basic Computer Organization and Design

Program Organization and Instruction Codes, Computer Registers, Computer Instructions, Timing and Control, Instruction Cycle, Memory-Reference Instructions, Input-Output, Interrupt

1. Program Organization and Instruction Codes

Definition

A program is a sequence of instructions stored in memory that directs the computer to perform specific tasks.
Instruction codes are binary patterns that define operations to be executed by the CPU.

Components of an Instruction

Opcode (Operation Code) – Specifies the operation (e.g., ADD, SUB, MOV).
Operand(s) – Data or memory locations on which the operation is performed.

Example

Instruction: ADD R1, R2
- Opcode: ADD (Addition)
- Operands: R1 (Register 1), R2 (Register 2)

Real-World Example

In a calculator program, instructions like ADD, SUB, MUL, and DIV are used to perform arithmetic operations.

Model Question (UGC NET)

Q: What is the function of an opcode in an instruction?
A: The opcode specifies the operation to be performed by the CPU (e.g., ADD, SUB, JUMP).

2. Computer Registers

Definition

Registers are small, high-speed storage locations inside the CPU used to hold data, addresses, and instructions temporarily.

Common Registers in a Basic Computer

Register	Purpose
Accumulator (AC)	Stores results of arithmetic/logic operations
Program Counter (PC)	Holds the address of the next instruction
Instruction Register (IR)	Holds the current instruction being executed
Memory Address Register (MAR)	Stores the address of memory to be accessed
Memory Buffer Register (MBR)	Temporarily holds data read from/written to memory

Example

Fetching an Instruction:
- PC → MAR → Memory → MBR → IR

Real-World Example

In a traffic light controller, registers store timing values and sequence data.

Model Question (UGC NET)

Q: Which register holds the address of the next instruction to be executed?
A: Program Counter (PC).

3. Computer Instructions

Types of Instructions

Data Transfer Instructions (e.g., MOV, LOAD, STORE)
Arithmetic Instructions (e.g., ADD, SUB, MUL)
Logical Instructions (e.g., AND, OR, NOT)
Control Instructions (e.g., JUMP, BRANCH, HALT)

Example

LOAD R1, [1000] → Loads data from memory location 1000 into Register R1.
ADD R1, R2 → Adds contents of R1 and R2, stores result in R1.

Real-World Example

In a banking system, instructions like ADD and SUB update account balances.

Model Question (UGC NET)

Q: Which instruction is used to transfer data from memory to a CPU register?
A: LOAD instruction.

4. Timing and Control

Definition

The Control Unit (CU) generates timing signals to synchronize operations.
Clock cycles determine instruction execution speed.

Control Signals

Fetch: PC → MAR → Memory → IR
Execute: Decode instruction → Perform operation

Example

Fetch-Decode-Execute Cycle:
1. Fetch instruction from memory.
2. Decode opcode.
3. Execute operation.

Real-World Example

A microwave oven uses timing signals to control heating duration.

Model Question (UGC NET)

Q: What is the role of the Control Unit in a CPU?
A: It generates control signals to coordinate instruction execution.

5. Instruction Cycle

Phases

Fetch: Retrieve instruction from memory.
Decode: Interpret the instruction.
Execute: Perform the operation.
Interrupt Check: Handle interrupts if any.

Example

Instruction: ADD R1, R2
- Fetch: Load instruction into IR.
- Decode: Identify ADD operation.
- Execute: Add R1 and R2, store result in R1.

Real-World Example

A washing machine follows an instruction cycle (fill → wash → rinse → spin).

Model Question (UGC NET)

Q: What are the main phases of the instruction cycle?
A: Fetch, Decode, Execute, and Interrupt Check.

6. Memory-Reference Instructions

Definition

Instructions that refer to memory locations for data (e.g., LOAD, STORE).

Example

STORE [2000], R1 → Stores R1’s content into memory location 2000.

Real-World Example

Database systems use memory-reference instructions to access records.

Model Question (UGC NET)

Q: Which instruction stores a register’s value into memory?
A: STORE instruction.

7. Input-Output (I/O) Operations

Definition

Transfer of data between CPU and external devices (keyboard, printer, etc.).

Methods

Programmed I/O (CPU polls devices)
Interrupt-Driven I/O (Devices interrupt CPU)
DMA (Direct Memory Access) (Devices access memory directly)

Example

Keyboard Input: When a key is pressed, an interrupt is sent to the CPU.

Real-World Example

Printers use interrupt-driven I/O to notify the CPU when printing is done.

Model Question (UGC NET)

Q: What is the advantage of interrupt-driven I/O over programmed I/O?
A: It reduces CPU idle time by allowing the CPU to perform other tasks until an interrupt occurs.

8. Interrupts

Definition

A signal that pauses the current process to handle a high-priority task.

Types

Hardware Interrupts (e.g., keyboard press)
Software Interrupts (e.g., system calls)

Interrupt Handling Steps

Save context (PC, registers).
Execute Interrupt Service Routine (ISR).
Restore context and resume execution.

Real-World Example

Emergency stop button in industrial machines triggers an interrupt.

Model Question (UGC NET)

Q: What happens when an interrupt occurs?
A: The CPU saves its current state, executes the ISR, and then resumes the original program.

Conclusion

Understanding Basic Computer Organization and Design is crucial for UGC NET aspirants. This guide covers instruction codes, registers, timing, instruction cycle, I/O, and interrupts with examples and model questions.

Final Model Questions (UGC NET)

Q: Which register holds the current instruction being executed?
A: Instruction Register (IR).
Q: What is DMA in I/O operations?
A: Direct Memory Access allows devices to transfer data directly to memory without CPU intervention.
Q: Explain the difference between opcode and operand.
A: The opcode defines the operation, while the operand specifies the data/memory location.

This document provides a structured and exam-oriented explanation of Basic Computer Organization for UGC NET preparation. 🚀