Week 2

Question 1

What are I/O devices?

Answer 1

allows humans or devices to communicate with computers

Question 2

What are examples of I/O devices?

Answer 2

Human interface devices: keyboard, mouse, monitor, speaker and printer, game controllers

Storage Devices: HDD’s, SSD’s, USB flash devices

Communication devices: network interface cards

Question 3

What is a bus used for with I/O devices?

Answer 3

The communication with I/O devices is performed over a bus

Question 4

What are controllers used for with I/O devices?

Answer 4

The I/O devices are operated using controllers

Question 5

How does the processor communicate with a controller?

Answer 5

The processor communicates with the controller by reading and writing to the controller’s registers

Question 6

What are the 4 controller registers?

Answer 6

Data register - temporal storage of data

data-in register - data coming from device

Status register - indicating status of device

Control register - commands to device

Question 7

What is a Port-mapped I/O (PMIO)?

Answer 7

Isolated

A separate IO address space

CPU uses special IO instructions to access IO devices

Question 8

What is a Memory-mapped I/O (MMIO)?

Answer 8

Device registers are treated as part of memory space

CPU access registers using standard memory instructions

Question 9

What are the 2 I/O Control Methods?

Answer 9

Polling

Interrupts

Question 10

What is Polling (SR)?

Answer 10

CPU repeatedly checks the status register to see if it needs attention

Question 11

What is an example of Polling?

Answer 11

Writing to a disk

CPU repeatedly checks the status register to determine if disk is ready

If status is ready, CPU can write to the data-out register

Once the write operation is complete, the disk controller updates the status register

Question 12

What are interrupts?

Answer 12

a signal from an I/O device to notify CPU it requires attention

CPU temporarily halts it current progress

Performs necessary action then resumes previous task

Question 13

When may polling be preferable?

Answer 13

If controller and device are fast

I/O rate is high

Some I/O data can be ignored

CPU has nothing better to do

Question 14

How are large data transfers done on devices?

Answer 14

Offloading this work to a special purpose processor (DMA controller)

Question 15

What is a DMA controller (Direct memory access)?

Answer 15

DMA allows IO devices to bypass CPU and directly read/write from/to memory

Enables CPU to spend more resources on other tasks

Question 16

What is a Device Driver?

Answer 16

Lets the OS and a device communicate

Hides differences between various device controllers by defining interface between OS and I/O devices for specific class of I/O devices

Question 17

What is a character device + examples?

Answer 17

transfers bytes one by one

examples: keyboards, computer mice, microphones, speakers

Question 18

What does the character I/O interface include?

Answer 18

get - read()

put - write()

other operations, line by line access ,seek ,editing services (eg. backspace)

Question 19

What are Block I/Os + examples?

Answer 19

Block devices handle I/O in blocks (chunks) of data

examples: HDD, SSD, CD-ROM, and DVD drives

Usually high volume devices

Question 20

What does the Block I/O interface include?

Answer 20

read - reading blocks of data

write - writing blocks of data

Question 21

What do Network I/O’s do?

Answer 21

sending and receiving packets over the connection

Question 22

What is the main difference between network I/Os and other I/O devices?

Answer 22

things routinely go wrong (missing packets etc.)

Therefore there need to be system functions for:

Checking whether data transfer was successful

recovering gracefully from unsuccessful transfers

Question 23

What is a Program?

Answer 23

A set of instructions for performing a specific task (passive)

Stored on disk

Question 24

What is a Process?

Answer 24

Instance of a program in execution (active)

Requires CPU resources, memory and I/O

There can be multiple process instances of one program

Question 25

What are the benefits of Processes?

Answer 25

Modularity - simplifies OS development and maintenance Speedup through Parallelism - Execute multiple processes in parallel using multiple CPU cores Security and Stability through Isolation - Usually, each process operates in its own memory space Minimize inter-process disruptions - problems in a process may not affect others

Question 26

What is a Process Control Block (PCB)?

Answer 26

Each process is represented by a PCB A PCB is a data structure that stores all info about a process PCB is the kernels representation of a process

Question 27

What is context switching?

Answer 27

Stop running process 1 and start process 2 1. Change the state of process 1 from running to ready 2. Save the context of process 1 3. Save the context of process 2 4. Change process scheduling state of process 2

Question 28

Whats the tradeoff of Context Switches?

Answer 28

With too few context switches: No fairness between processes - some processes may have to wait for a long time With too many context switches: The processor will spend too much resources on overhead Aim : Optimise the number of context switches so that: No process is waiting too long Associated overhead does not slow down performance

Question 29

What are the properties of The Stack and its operations?

Answer 29

Data Structure - LIFO (Last in first out) Operations: Push - adding to stack Pop - removing from the stack Function call: Activation record (parameters, local variables, return address) pushed to stack Function return: Activation record popped from stack

Question 30

What are the properties of The Heap?

Answer 30

Dynamically allocated like the stack Blocks of memory are allocated and removed in an arbitrary order Used when you need: To store a large block of memory for a longer period of time Variables that can change size dynamically

Question 31

What is process spawning?

Answer 31

Method by which a process (parent) creates a new process (child) In Linux there are four system calls for process spawning

Question 32

What is a Fork? (Part of process spawning)

Answer 32

Fork creates a child process with unique process ID Child is a copy of the parent process For returns: 0 to child process Process ID of the child process to the parent process

Question 33

What is a Exec? (Part of process spawning)

Answer 33

Replaces the program executed by a process

Question 34

What is a Wait? (Part of process spawning)

Answer 34

Parent process is put in waiting state until child process finishes

Question 35

What is an Exit? (Part of process spawning)

Answer 35

When done, child process issues the exit system call, and the parent process can resume

Question 36

What is Interprocess Communication (Share data)?

Answer 36

Processes may need to share data with each other Examples: Multiples processes interested in same info Subdivide a process into multiple subtasks that are solved simultaneously

Question 37

What are the 2 ways Interprocess Communication can be implemented?

Answer 37

1. Shared memory 2. Message passing

Question 38

What is Shared Memory?

Answer 38

Memory space is shared between 2 processes To communicate processes read or write into shared region Establishment of shared memory typically requires system calls Processes have to manage simultaneous access to same memory space

Question 39

What is Message Passing?

Answer 39

Processes communicate by sending messages to each other The messages are sent to/received from an agreed "mailbox" that is not part of the address space of any of the processes

Question 40

What are the pros of Shared Memory?

Answer 40

Faster than Message Parsing Message parsing systems typically require system calls Shared memory communication only requires system calls to set up shared memory space

Question 41

What are the pros of Message Passing?

Answer 41

Good for small amounts of data Avoids issue of setting up the shared memory space