I/O, Interrupts, and Device Drivers

Question 1

I/O overview

Answer 1

Look at I/O mechanisms
– How we implement and control access to devices
– Clocks and Timers
* Used to control and synchronise operations

Question 2

Memory Mapped I/O

Answer 2

I/O devices visible as memory addresses
– Easily accessed using C point

Question 3

Isolated I/O

Answer 3

Separate bus for I/O devices
– Easier interfacing: distinct from high-speed memory bus
– Accessed via special instructions
* Generally not supported by compilers (must use assembler)

Question 4

Optimising Memory Use

Answer 4

Buffers used to smooth I/O operations.

Input/Output buffers allow CPU and devices to work asynchronously.

Status registers track readiness and availability.

Question 5

Direct Memory Access

Answer 5

• Alternative to programmed I/O
• Relieves CPU of data transfer (improving speed and reducing load)

Question 6

Clocks & Timers

Answer 6

RTC (Real-Time Clock): Tracks real-world date/time.

PIT (Programmable Interrupt Timer): Generates regular interrupts for periodic tasks

Question 7

Efficient Timer Use

Answer 7

Instead of constant intervals, set timers to fire when the next task is due.

Saves CPU time, improves responsiveness.

Used in OS scheduling and simulation systems.

Question 8

What is an Interrupt?

Answer 8

A signal to the CPU that an event needs attention.

Temporarily halts the current process, handles the event, then resumes.

Question 9

Types of Interrupts

Answer 9

Hardware
Triggered by I/O devices (e.g., keyboard, disk)
Software
Triggered by programs (e.g., divide by zero)
Timer
From a system clock (e.g., every 10ms)
System Calls
Controlled interrupts to switch to kernel mode

Question 10

Why Interrupts Matter

Answer 10

Efficient I/O: Devices notify CPU when ready.

Multitasking: Timer interrupts help switch processes.

Responsiveness: Quick reaction to user input or errors.

Question 11

Maskable vs Non-Maskable

Answer 11

Maskable: Can be disabled (e.g., during critical sections).

Non-Maskable: Always handled (e.g., serious hardware failures).

Question 12

Context Switching

Answer 12

Caused by timer or I/O interrupt.
CPU saves current process and loads another (for multitasking)

Question 13

Polling

Answer 13

CPU repeatedly checks (polls) each device to see if it needs attention.

cons: Inefficient – wastes CPU time checking idle devices.
pros:
No need for interrupts

Question 14

Nested Interrupts

Answer 14

A higher-priority interrupt can interrupt a currently running ISR (Interrupt Service Routine).
cons: More complex to manage.
pros:
Improves system responsiveness to critical tasks.

Question 15

Device drivers

Answer 15

Extensible model
– Create new drivers as needed to control hardware/ devices
– Dynamically loadable modules allow run-time c

Question 16

Unix Device Driver Interface

Answer 16

Unified, file-like interface using standard functions:

open(), read(), write(), ioctl(), mmap(), close(),

Question 17

Device Driver Table

Answer 17

Maintains function pointers for each supported operation.

Devices may leave unsupported operations as null or error.

Question 18

Tasks Performed by Drivers

Answer 18

I/O scheduling (e.g. reordering disk operations).

Buffering, spooling, caching.

Error handling.

I/O protection (prevent unauthorized access).

Question 19

Single vs Multi-instance Devices

Answer 19

Single-instance: Only one process can use the device at a time (e.g. printer); use spooling.

Multi-instance: Can serve many processes concurrently (e.g. keyboard input).

Question 20

Unix I/O System Architecture

Answer 20

Treats everything as a file: plain files, devices, sockets.

Uses cooked (processed) and raw (direct) modes:

Cooked: e.g. line-based input editing.

Raw: e.g. direct disk block access including metadata.