Operating System IMP points

Data or instructions are the names.   Of symbolic terms... 
UNIX OS contains the shell which is core of the OS
In computer operating systems, paging is a memory management scheme by which a computer stores and retrieves data from secondary storage  for use in main memory.

In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages.
Paging is a memory management technique in which the memory is divided into fixed size pages.

Paging is used for faster access to data. When a program needs a page, it is available in the main memory as the OS copies a certain number of pages from your storage device to main memory.

Paging allows the physical address space of a process to be noncontiguous.
Need of paging :-

Paging allows the physical address space of the process to be non contiguous. The whole program had to fit into storage contiguously.

Paging is to deal with external fragmentation problem.

This is to allow the logical address space of a process to be noncontiguous, which makes the process to be allocated physical memory
Shell :- 
In computing, a shell is a user interface for access to an operating system's services.

In general, operating system shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer's role and particular operation.
What is a shell in Unix?

Shell is a UNIX term for the interactive user interface with an operating system.

The shell is the layer of programming that understands and executes the commands a user enters. In some systems, the shell is called a command interpreter.
What is the difference between kernel and shell?

The shell acts as an interface between the user and the kernel.

When a user logs in, the login program checks the username and password, and then starts another program called the shell.

The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out.
Kernel :-

In computing, the 'kernel' is the central component of most computer operating systems.

It is a bridge between applications and the actual data processing done at the hardware level.

The kernel's responsibilities include managing the system's resources (the communication between hardware and software components).
When using paging, the main memory is partitioned into equal fixed-size chunks that are relatively small, and each process is also divided into small fixed-size chunks of the same size.

Then, the chunks of a process, known as pages, are assigned to available chunks of memory, known as frames or page frames.
Page :-

Pages in virtual address space (which is what all code running under a virtual memory OS deals with) are called simply pages, or virtual pages. 

Page frames:-

The term "page frame" therefore simply means "a page of physical memory". It's important to understand that it NEVER refers to a page of virtual memory... even though the physical page (page frame) might be realizing one, or even several, pages of virtual memory at the time.
Demand paging :-

Demand paging is a system where in area of memory that are not currently being used are swapped to disk to make room for an application’s need.
Time sharing systems :-

In a Time sharing system, the CPU executes multiple jobs by switching among them, also known as multitasking. This process happens so fast that users can actually interact with each program while it is running.
SMP :-SMP is short for Symmetric MultiProcessing, and is the most common type of multiple-processor systems.

In this system, each processor runs an identical copy of the operating system, and these copies communicate with one another as needed.

Thread :-thread is a basic unit of CPU utilization.

In general, a thread is composed of a thread ID, program counter, register set and the stack.
Round Robin scheduling : -

RR scheduling algorithm is primarily aimed for time-sharing systems.

A circular queue is setup in such a way that the CPU scheduler goes around that queue, allocating CPU to each process for a time interval of up to around 10 to 100 milliseconds.
Enumerate the different RAID levels.

RAID 0 – Non-redundant striping

RAID 1 – Mirrored Disks

RAID 2 – Memory-style error-correcting codes

RAID 3 – Bit-interleaved Parity

RAID 4 – Block-interleaved Parity

RAID 5 – Block-interleaved distributed Parity

RAID 6 – P+Q Redundancy
RAID :-

Originally - redundant array of inexpensive disks,
Now - commonly redundant array of independent disks

is a data storage virtualization technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy, performance improvement, or both.
What are overlays?

Overlays are used to enable a process to be larger than the amount of memory allocated to it.

The basic idea of this is that only instructions and data that are needed at any given time are kept in memory.
Fragmentation :-

Fragmentation is memory wasted.

It can be internal if we are dealing with systems that have fixed-sized allocation units, or external if we are dealing with systems that have variable-sized allocation units.
When does thrashing occur?

Thrashing refers to an instance of high paging activity.

This happens when it is spending more time paging instead of executing.
How does swapping result in better memory management?

During regular intervals that are set by the operating system, processes can be copied from main memory to a backing store, and then copied back later.

Swapping allows more processes to be run that can fit into memory at one time.
What is Direct Access Method?

Direct Access method is based on a disk model of a file, such that it is viewed as a numbered sequence of blocks or records.

It allows arbitrary blocks to be read or written.

Direct access is advantageous when accessing large amounts of information.
What is the best page size when designing an operating system?

The best paging size varies from system to system, so there is no single best when it comes to page size.

There are different factors to consider in order to come up with a suitable page size, such as page table, paging time, and its effect on the overall efficiency of the operating system.
When designing the file structure for an operating system, what attributes are considered?

Typically, the different attributes for a file structure are naming, identifier, supported file types, and location for the files, size, and level of protection.
What is root partition?

Root partition is where the operating system kernel is located.

It also contains other potentially important system files that are mounted during boot time.
What are some pros and cons of a command line interface?

A command line interface allows the user to type in commands that can immediately provide results.

Many seasoned computer users are well accustomed to using the command line because they find it quicker and simpler.

The main problem with a command line interface is that users have to be familiar with the commands, including the switches and parameters that come with it.

This is a downside for people who are not fond of memorizing commands.