Week 1

Question 1

What is an OS?

Answer 1

An OS is a software that manages computer resources and offers abstractions

Question 2

What is the Kernel?

Answer 2

Central part of OS

Runs all time on the computer

Functionality depends on OS design

Question 3

What is the Kernel Mode? (Or privileged mode)

Answer 3

Where the kernel runs

CPU has full access to system resources

Question 4

What is User Mode?

Answer 4

It’s where applications run

CPU has restricted access to system resources

Question 5

How does the OS switch between kernel and user mode

Answer 5

To switch to kernel mode the trap mode bit is triggered and set to 0, after the task has been performed in kernel mode the return mode bit is set to 1 which brings it back to OS

Question 6

What is a Monolithic Structure?

Answer 6

Large kernel with most of OS services run in kernel mode

Question 7

What are the pros and cons of a monolithic structure?

Answer 7

Pros:

Fast performance

Cons:

Stability and Security

Maintenance and Debugging

Question 8

What is are the pros and cons of a Microkernel Structure?

Answer 8

Pros:

Stability and Security

Cons:

Poor Performance

Question 9

What are the pros and cons of a Microkernel Structure?

Answer 9

Pros:

Stability and Security

Cons:

Poor Performance

Question 10

What is a Layered Structure?

Answer 10

OS broken into hierarchy layers

Communication with above and below

Implemented into computer networks (TCP/IP)

Question 11

What are the pros and cons of a Layered Structure?

Answer 11

Pros:

Easy to maintain debug and test

Security and Stability

Cons:

Poor performance

Design challenges

Question 12

What is a Modular Structure?

Answer 12

Combines aspects of Monolithic and Microkernels

Start with small kernel

additional services are loaded on demand via modules

Question 13

What are the pros and cons of a Modular Structure?

Answer 13

Pros:

Fast Performance

Flexibility and Extensibility

Cons:

Monolithic as more modules loaded

Question 14

Fact

Answer 14

OS’s combine different structures

Question 15

What is a file?

Answer 15

A file is a named collection of related data stored on non-volatile storage

Question 16

What are magic numbers in files?

Answer 16

Sequence of bytes at beginning of file

Question 17

What are examples of meta data?:

Answer 17

Name

Identifier

Type

Location

Size

Timestamps

Permissions

Question 18

Where is Meta Data stored in linux?

Answer 18

In an Inode

Question 19

What are the 2 features of Linux Files?

Answer 19

Everything is a file

Files represented by tree-like inode pointer structures

Question 20

What are some examples of Linux file types?

Answer 20

Regular files

Special files (character of block)

Directories

Pipes

Links (soft or hard)

Question 21

What is a hard link?

Answer 21

Another name for an existing file

points directly to inode of file

If file is moved/deleted the link will still work

Question 22

what is a soft link?

Answer 22

A pointer to filename

If file is moved/deleted, the link will not work

Question 23

How is the storage space divided for files?

Answer 23

Filesystems divide storage space into fixed-size blocks

Typical block size is 4096 bytes (4KB)

Question 24

What can a file be broken down into?

Answer 24

into chunks that fit into blocks

Question 25

What are magnetic disks?

Answer 25

Use magnetic fields to store data eg. HDD or floppy disks

Question 26

What are magnetic disks organised into?

Answer 26

Platters - Circular disks Read/Write Heads For reading and writing the head senses/ changes the magnetism of a sector At bit level: magnetism in one direction represents a 1 magnetism in other direction represents a 0

Question 27

How are the platters of a Magnetic Disk organised?

Answer 27

Each platter is divided into circular tracks Each track is divided into sectors - each sector has fixed size Set of all tracks are immediately above one another make up a cylinder

Question 28

How does a magnetic disk work?

Answer 28

Read/ Written by moving the arms in and out of required cylinder All heads/ arms move together Platters rotate; rotation speed is related to data transfer rate

Question 29

What is a Solid State Disk?

Answer 29

No moving parts and instead stores data using flash memory

Question 30

What are the components in an SSD?

Answer 30

Controller (embedded processor) Buffer memory (volatile memory) Flash Memory - divided into pages that are grouped into blocks

Question 31

What are the read, write and erase speeds of an SSD?

Answer 31

Read - Fast Write to empty drive - Fast Erase - Slow

Question 32

How is an SSD read?

Answer 32

1. Copying flash memory page into the buffer 2. Reading data from the page in the buffer

Question 33

How is an SSD overwritten?

Answer 33

1. Copying memory block into buffer 2. Erasing block in the flash memory 3. Modifying block in the buffer 4. Writing block from the buffer to the flash memory

Question 34

What are the Pros and Cons of SSDs compared to magnetic?:

Answer 34

Pros: SSDs are faster SSDs more reliable SSDs are more power efficient Cons: SSDs more expensive SSDs deteriorate with every write

Question 35

What is a wear levelling?

Answer 35

A process that is designed to extend the life of solid state devices

Question 36

What is dynamic wear levelling?

Answer 36

New data written to least-recently-used block Avoid wearing out certain blocks by writing to the same ones again and again problem: "cold" data is not moved

Question 37

What is Static-wear-levelling?

Answer 37

Same as dynamic besides: Periodically moves existing data to least-recently-used block Therefore we avoid wearing out certain blocks while blocks with cold data is never moved

Question 38

What is the benefit of wear levelling?

Answer 38

A block will fail once it reaches a critical amount of writes Thanks to wear-levelling, we spread the writes evenly among the blocks