Unit 5: Input / Output

INPUT / OUTPUT (I/O) ORGANIZATION & INTERRUPTS

Peripheral Devices

Peripheral devices are external devices connected to the computer system to:

Input data
Output results
Store data

They are slower than the CPU and main memory.

Types of Peripheral Devices

(a) Input Devices

Used to enter data into the system.

Examples:

Keyboard
Mouse
Scanner
Webcam

(b) Output Devices

Used to display results.

Examples:

Monitor
Printer
Speaker
Plotter

(c) Storage Devices

Used for secondary storage.

Examples:

Hard disk
Pen drive
CD/DVD

I/O Interface

An I/O interface is a hardware unit that connects peripheral devices to the system bus.

Why I/O Interface is Needed

Speed mismatch between CPU and devices
Data format differences
Control and status management

Functions of I/O Interface

Data buffering
Device control
Error detection
Synchronization

Components of an I/O Interface

Data register
Status register
Control register

I/O Ports

An I/O port is a logical address through which the CPU communicates with an I/O device.

Types of I/O Ports

(a) Input Port - Receives data from external devices
(b) Output Port - Sends data to external devices

I/O Addressing Methods

(a) Memory-Mapped I/O

I/O devices share address space with memory
Uses same instructions as memory

Advantage: Flexible
Disadvantage: Reduces memory space

(b) Isolated I/O (Port-Mapped I/O)

Separate address space for I/O
Uses special I/O instructions

Advantage: Memory space saved
Disadvantage: Limited instructions

Interrupts

An interrupt is a signal that temporarily stops the CPU from executing the current program and transfers control to an Interrupt Service Routine (ISR).

Need for Interrupts

Efficient CPU utilization
Avoids constant polling
Fast response to I/O devices

Interrupt Hardware

Interrupt Controller

Manages multiple interrupt requests
Determines priority
Sends interrupt to CPU

Example: - Programmable Interrupt Controller (PIC)

Interrupt Vector Table

Stores addresses of ISRs
Used in vectored interrupts

Interrupt Handling Steps

Interrupt request generated
CPU completes current instruction
CPU saves context (PC, registers)
Control transferred to ISR
ISR executed
Context restored
Program resumes

Types of Interrupts

Hardware Interrupts

Generated by external devices.

Examples:

Keyboard interrupt
Disk interrupt

Software Interrupts

Generated by program instructions.

Examples:

System calls
INT instruction

Maskable Interrupts

Can be disabled by CPU

Non-Maskable Interrupts (NMI)

Cannot be disabled
Used for critical errors

Vectored Interrupts

ISR address is predefined

Non-Vectored Interrupts

ISR address determined by software

Exceptions

An exception is an interrupt caused by internal CPU events during instruction execution.

Types of Exceptions

Exception	Description
Divide by zero	Arithmetic error
Overflow	Result exceeds limit
Invalid opcode	Undefined instruction
Page fault	Page not in memory

Interrupts vs Exceptions (Important Comparison)

Feature	Interrupt	Exception
Source	External device	CPU/internal
Timing	Asynchronous	Synchronous
Example	Keyboard	Divide by zero

Exam-Ready Summary Points

Peripheral devices enable I/O operations.
I/O interface bridges CPU and devices.
I/O ports provide communication addresses.
Interrupts improve CPU efficiency.
Exceptions handle internal CPU errors.

Frequently Asked Exam Questions

Explain I/O interface and its functions.
Compare memory-mapped and isolated I/O.
Explain interrupt hardware and handling.
Types of interrupts with examples.
Differentiate interrupts and exceptions.

MODES OF DATA TRANSFER

Programmed I/O, Interrupt-Initiated I/O, DMA, I/O Channels & I/O Processors

Data transfer refers to the movement of data between:

CPU
Main memory
Peripheral devices

Because I/O devices are much slower than the CPU, special transfer modes are required to ensure efficiency.

Programmed I/O

In Programmed I/O, the CPU directly controls the data transfer by repeatedly checking the status of the I/O device.

This method is also called Polling.

Working of Programmed I/O

CPU sends I/O command to device
CPU checks device status repeatedly
When device is ready, data is transferred
CPU continues program execution

Characteristics

CPU is fully involved
No interrupts used
Simple hardware

Advantages

Easy to implement
Low hardware cost

Disadvantages

CPU time wasted in polling
Very slow for large data transfer

Example: Reading a character from keyboard using polling.

Interrupt-Initiated I/O

In Interrupt-Initiated I/O, the CPU initiates the I/O operation and continues executing other tasks.
The device interrupts the CPU when it is ready.

Working of Interrupt-Initiated I/O

CPU sends I/O command
CPU continues other work
Device generates interrupt when ready
CPU saves context
Interrupt Service Routine (ISR) transfers data
CPU resumes normal execution

Characteristics

CPU not continuously waiting
Interrupt hardware required

Advantages

Better CPU utilization
Faster than programmed I/O

Disadvantages

Interrupt handling overhead
Not efficient for very large data blocks

Example:Printer interrupt after completing printing.

Direct Memory Access (DMA)

DMA allows an I/O device to transfer data directly to or from main memory without CPU involvement.

A special hardware unit called DMA Controller is used.

Working of DMA

CPU initializes DMA controller
DMA controller takes control of system bus
Data transferred directly between device and memory
DMA controller sends interrupt after completion

DMA Modes

Burst Mode – transfers entire block at once
Cycle Stealing – DMA steals memory cycles from CPU
Transparent Mode – DMA works only when CPU is idle

Advantages

Very high speed
Minimal CPU involvement
Best for large data transfer

Disadvantages

Complex hardware
Higher cost

Example: Hard disk data transfer.

Comparison of I/O Data Transfer Modes (Very Important Table)

Feature	Programmed I/O	Interrupt I/O	DMA
CPU involvement	Full	Partial	Minimal
Speed	Slow	Medium	Very Fast
Hardware	Simple	Interrupt hardware	DMA controller
Efficiency	Poor	Better	Best
Data size	Small	Medium	Large

I/O Channels

An I/O channel is a specialized processor that controls I/O operations independently of the CPU.

Types of I/O Channels

Selector Channel - Controls one high-speed device at a time
Multiplexer Channel - Controls multiple low-speed devices

Advantages of I/O Channels

Offloads I/O work from CPU
Increases system performance

I/O Processors

An I/O Processor (IOP) is a dedicated processor that executes I/O programs and manages devices.

Functions of I/O Processor

Executes I/O instructions
Handles interrupts
Performs error checking
Controls data transfer

Difference Between DMA and I/O Processor

Feature	DMA	I/O Processor
Intelligence	Limited	High
Program execution	No	Yes
Control	Block transfer only	Complete I/O management

Exam-Ready Summary

Programmed I/O uses polling and wastes CPU time.
Interrupt I/O improves CPU efficiency.
DMA is best for large, high-speed transfers.
I/O channels and processors further reduce CPU workload.

Most Important Exam Questions

Explain Programmed I/O with diagram.
Compare Programmed I/O, Interrupt I/O, and DMA.
Explain DMA controller and its modes.
What are I/O channels and their types?
Differentiate DMA and I/O Processor.

SERIAL COMMUNICATION

Synchronous & Asynchronous Communication, Standard Communication Interfaces

Serial communication is a method of data transfer in which bits are transmitted one after another over a single communication line.

It is widely used because:

Requires fewer wires
Suitable for long-distance communication
Low cost and reliable

Examples:

USB
RS-232
Ethernet

Types of Serial Communication

Serial communication is broadly classified into:

Synchronous Communication
Asynchronous Communication

Synchronous Communication

In synchronous communication, data is transmitted continuously in blocks or frames with the sender and receiver sharing a common clock signal.

Working of Synchronous Communication

Sender and receiver are synchronized by a clock
Data is sent as a continuous stream
No start and stop bits are used

Characteristics

High data transfer speed
Clock synchronization required
Efficient for large data transfer

Advantages

Faster transmission
Less overhead
High efficiency

Disadvantages

Complex hardware
Clock synchronization required

Examples

SPI (Serial Peripheral Interface)
I²C (Inter-Integrated Circuit)
Ethernet

Asynchronous Communication

In asynchronous communication, data is transmitted one character at a time, and no common clock is shared.

Each character is framed with start and stop bits.

Working of Asynchronous Communication

Sender adds start bit before data
Stop bit is added after data
Receiver detects start bit to synchronize

Characteristics

Uses start and stop bits
Slower than synchronous communication
Simple design

Advantages

Simple hardware
No clock synchronization needed
Suitable for low-speed communication

Disadvantages

Extra bits increase overhead
Lower efficiency

Examples

RS-232
UART communication
Keyboard communication

Comparison Between Synchronous and Asynchronous Communication

(Very Important for Exams)

Feature	Synchronous	Asynchronous
Clock	Shared	Not shared
Data transfer	Continuous	Character-wise
Speed	High	Low
Overhead	Low	High (start/stop bits)
Complexity	High	Low
Examples	SPI, I²C	RS-232, UART

Standard Communication Interfaces

RS-232

Asynchronous communication standard
Used for serial ports
Point-to-point communication
Low speed and short distance

USB (Universal Serial Bus)

High-speed serial interface
Plug-and-play support
Supports multiple devices

SPI (Serial Peripheral Interface)

Synchronous communication
Master-slave architecture
High speed and full duplex

I²C (Inter-Integrated Circuit)

Synchronous communication
Uses only two lines (SDA & SCL)
Supports multiple devices

Ethernet

High-speed serial communication
Used in computer networks
Reliable and scalable

Summary Table of Standard Interfaces

Interface	Type	Speed	Usage
RS-232	Asynchronous	Low	Serial ports
USB	Both	High	Peripheral connection
SPI	Synchronous	Very High	Embedded systems
I²C	Synchronous	Medium	Chip-to-chip
Ethernet	Synchronous	Very High	Networking

Exam-Ready Key Points

Serial communication sends data bit by bit.
Synchronous communication uses a shared clock.
Asynchronous communication uses start and stop bits.
RS-232 is asynchronous, SPI and I²C are synchronous.
USB and Ethernet support high-speed data transfer.

Frequently Asked Exam Questions

Explain serial communication with advantages.
Differentiate synchronous and asynchronous communication.
Explain RS-232 interface.
What are standard serial communication interfaces?
Compare SPI and I²C.