The document discusses several topics related to operating systems: 1. It explains how virtual addresses are mapped to physical addresses using page tables, where a virtual address is split into a virtual page number and offset, and the page table converts the virtual page number to a physical page number. 2. It discusses the optimal page replacement algorithm, which replaces the page that will not be used farthest in the future. 3. It defines a file system as a method for organizing and storing computer files using a storage device divided into sectors organized into files and directories.

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Operating Systems
BT0070 Part-2
By Milan K Antony

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1. Explain how virtual addresses are mapped on to physical
addresses.
Let's assume that we have 1M of RAM. RAM is also called
physical memory. We can subdivide the RAM into 4K pages.
Thus 1M / 4K = 256 pages. Thus, our RAM has 256 physical
pages, weach holding 4K. Let's assume we have 10 M of disk.
Thus, we have 2560 disk pages. In principle, each program may
have up to 4 G of address space. Thus, it can, in principle,
access 2
20
virtual pages. In reality, many of those pages are
considered invalid pages.
Page Tables
How is an address translated from virtual to physical? First,
like the cache, we split up a 32 bit virtual address into a
virtual page (which is like a tag) and a page offset.
If this looks a lot like a fully-associative cache, but whose
offset is much much larger, it's because that's basically what it
is.

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We must convert the virtual page number to a physical page
number. In our example, the virtual page consists of 20 bits.
A page table is a data structure which consists of 2
20
page
table entries (PTEs). Think of the page table as an array of
page table entries, indexed by the virtual page number.
The page table's index starts at 0, and ends at 2
20
- 1.
Here's how it looks:
Translation
Suppose your program generated the following virtual address
F0F0F0F0hex (which is 1111 0000 1111 0000 1111 0000 1111 0000 two).
First, you would split the address into a virtual page, and a

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page offset (see below).
Then, you'd see if the virtual page had a corresponding
physical page in RAM using the page table. If the valid bit of
the PTE is 1, then you'd translate the virtual page to a
physical page, and append the page offset. That would give
you a physical address in RAM.
2. Discuss the Optimal Page Replacement algorithm.
The theoretically optimal page replacement algorithm ,
or Belady's optimal page replacement policy is an algorithm
that works as follows: when a page needs to be swapped in,
the operating system swaps out the page whose next use will
occur farthest in the future. For example, a page that is not
going to be used for the next 6 seconds will be swapped out
over a page that is going to be used within the next 0.4
seconds.

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This algorithm cannot be implemented in the general purpose
operating system because it is impossible to compute reliably
how long it will be before a page is going to be used, except
when all software that will run on a system is either known
beforehand and is amenable to the static analysis of its
memory reference patterns, or only a class of applications
allowing run-time analysis is allowed. Despite this limitation,
algorithms exist
]
that can offer near-optimal performance —
the operating system keeps track of all pages referenced by
the program, and it uses those data to decide which pages to
swap in and out on subsequent runs. This algorithm can offer
near-optimal performance, but not on the first run of a
program, and only if the program's memory reference pattern
is relatively consistent each time it runs.
3. Explain the concept of File.
A file system (often also written as filesystem) is
a method of storing and organizing computer files and their
data. Essentially, it organizes these files into a database for
the storage, organization, manipulation, and retrieval by the
computer's operating system. ost file systems make use of an
underlying data storage device that offers access to an array
of fixed-size physical sectors, generally a power of 2 in size
(512 bytes or 1, 2, or 4 KiB are most common). The file
system is responsible for organizing these sectors into files and
directories, and keeping track of which sectors belong to which
file and which are not being used. Most file systems address

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data in fixed-sized units called "clusters" or "blocks" which
contain a certain number of disk sectors (usually 1-64). This is
the smallest amount of disk space that can be allocated to
hold a file.
However, file systems need not make use of a storage device
at all. A file system can be used to organize and represent
access to any data, whether it's stored or dynamically
generated
4. Explain the execution of RPC.
Remote procedure call (RPC) is an Inter-process
communication technology that allows a computer program to
cause a subroutine or procedure to execute in another address
space (commonly on another computer on a shared network)
without the programmer explicitly coding the details for this
remote interaction. That is, the programmer would write
essentially the same code whether the subroutine is local to the
executing program, or remote. When the software in question
is written using object-oriented principles, RPC may be
referred to as remote invocation or remote method
invocation.
A RPC is initiated by the client sending a request message to
a known remote server in order to execute a specified
procedure using supplied parameters. A response is returned
to the client where the application continues along with its

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process. There are many variations and subtleties in various
implementations, resulting in a variety of different
(incompatible) RPC protocols. While the server is processing
the call, the client is blocked (it waits until the server has
finished processing before resuming execution).
An important difference between remote procedure calls and
local calls is that remote calls can fail because of
unpredictable network problems. Also, callers generally must
deal with such failures without knowing whether the remote
procedure was actually invoked. Idempotent procedures (those
which have no additional effects if called more than once) are
easily handled, but enough difficulties remain that code which
calls remote procedures is often confined to carefully written
low-level subsystems.
5. Write a note on computer virus.
A computer virus is a computer program that can copy
itself and infect a computer. The term "virus" is also
commonly but erroneously used to refer to other types of
malware, including but not limited to adware and spyware
programs that do not have the reproductive ability. A true
virus can spread from one computer to another (in some form
of executable code) when its host is taken to the target
computer; for instance because a user sent it over a network
or the Internet, or carried it on a removable medium such as
a floppy disk, CD, DVD, or USB drive. Viruses can increase

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their chances of spreading to other computers by infecting
files on a network file system or a file system that is accessed
by another computer.
As stated above, the term "computer virus" is sometimes used
as a catch-all phrase to include all types of malware, even
those that do not have the reproductive ability. Malware
includes computer viruses, computer worms, Trojan horses,
most rootkits, spyware, dishonest adware and other malicious
and unwanted software, including true viruses. Viruses are
sometimes confused with worms and Trojan horses, which are
technically different. A worm can exploit security vulnerabilities
to spread itself automatically to other computers through
networks, while a Trojan horse is a program that appears
harmless but hides malicious functions. Worms and Trojan
horses, like viruses, may harm a computer system's data or
performance. Some viruses and other malware have symptoms
noticeable to the computer user, but many are surreptitious or
simply do nothing to call attention to themselves. Some viruses
do nothing beyond reproducing themselves.
6. Explain first-fit, best-fit and worst-fit allocation
algorithms with an example.
First Fit - A resource allocation scheme (usually for
memory). First Fit fits data into memory by scanning from the
beginning of available memory to the end, until the first free
space which is at least big enough to accept the data is

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found. This space is then allocated to the data. Any left over
becomes a smaller, separate free space.
Best Fit - A resource allocation scheme (usually for
memory). Best Fit tries to determine the best place to put
the new data. The definition of 'best' may differ between
implementations, but one example might be to try and minimise
the wasted space at the end of the block being allocated -
i.e. use the smallest space which is big enough.
worst fit The allocation policy that always allocates from
the largest free block. Commonly implemented using a size-
ordered free block chain (largest first).In practice, this tends
to work quite badly because it eliminates all large blocks, so
large requests cannot be met.
Given five memory partitions of 100 KB, 500 KB, 200 KB, 300
KB, and 600 KB (in order), how would each of the first-fit,
best-fit, and worst-fit algorithms place processes of 212 KB,
417 KB, 112 KB, and 426 KB (in order)? Which algorithm makes
the most efficient use of memory?
(a) First-fit:
(b) 212K is put in 500K partition
(c) 417K is put in 600K partition
(d) 112K is put in 288K partition (new partition 288K = 500K
- 212K)
(e) 426K must wait
(f) Best-fit:
(g) 212K is put in 300K partition
(h) 417K is put in 500K partition

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(i) 112K is put in 200K partition
(j) 426K is put in 600K partition
(k) Worst-fit:
(l) 212K is put in 600K partition
(m) 417K is put in 500K partition
(n) 112K is put in 388K partition
(o) 426K must wait
In this example, best-fit turns out to be the best.
7. Explain demand paging virtual memory system.
Demand paging follows that pages should only be brought into
memory if the executing process demands them. This is often
referred to as lazy evaluation as only those pages demanded
by the process are swapped from secondary storage to main
memory. Contrast this to pure swapping, where all memory for
a process is swapped from secondary storage to main memory
during the process startup.
When a process is to be swapped into main memory for
processing, the pager guesses which pages will be used prior
to the process being swapped out again. The pager will only
load these pages into memory. This process avoids loading
pages that are unlikely to be used and focuses on pages
needed during the current process execution period.
Therefore, not only is unnecessary page load during swapping
avoided but we also try to preempt which pages will be needed
and avoid loading pages during execution.

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Commonly, to achieve this process a page table implementation
is used. The page table maps logical memory to physical
memory. The page table uses a bitwise operator to mark if a
page is valid or invalid. A valid page is one that currently
resides in main memory. An invalid page is one that currently
resides in secondary memory
8. Explain different operations possible on Files.
Files on a computer can be created, moved, modified,
grown, shrunk and deleted. In most cases, computer programs
that are executed on the computer handle these operations,
but the user of a computer can also manipulate files if
necessary. For instance, Microsoft Word files are normally
created and modified by the Microsoft Word program in
response to user commands, but the user can also move,
rename, or delete these files directly by using a file manager
program such as Windows Explorer (on Windows computers).
Although the way programs manipulate files varies according to
the operating system and file system involved, the following
operations are typical:
 Creating a file with a given name
 Setting attributes that control operations on the file
 Opening a file to use its contents
 Reading or updating the contents

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 Committing updated contents to durable storage
 Closing the file, thereby losing access until it is opened
again
9. Explain how applications and data can be distributed.
Here are two main reasons for using distributed systems and
distributed computing. First, the very nature of the application
may require the use of a communication network that connects
several computers. For example, data is produced in one
physical location and it is needed in another location.
Second, there are many cases in which the use of a single
computer would be possible in principle, but the use of a
distributed system is beneficial for practical reasons. For
example, it may be more cost-efficient to obtain the desired
level of performance by using a cluster of several low-end
computers, in comparison with a single high-end computer. A
distributed system can be more reliable than a non-distributed
system, as there is no single point of failure. Moreover, a
distributed system may be easier to expand and manage than a
monolithic uniprocessor system
 The graph G is the structure of the computer network.
There is one computer for each node of G and one
communication link for each edge of G. Initially, each
computer only knows about its immediate neighbours in
the graph G; the computers must exchange messages with

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each other to discover more about the structure of G.
Each computer must produce its own colour as output.
 The main focus is on coordinating the operation of an
arbitrary distributed system.
While the field of parallel algorithms has a different focus than
the field of distributed algorithms, there is a lot of
interaction between the two fields
10. How the protection is implemented using the concept
of domains?
System resources need to be protected. Resources
include both hardware and software. Different mechanisms for
protection are as used.
The operating system defines the concept of a domain .A
domain consists of objects and access rights of these objects.
A Subject then gets associated with the domain and access to
object in the domain .A domain is a set of access rights for
associated objects and a asystem consists of many such
domains.A user process alwaya executes in any one of the
domians. Domain switching is also possible.In reality domain is a
user with a specific id having different access right for
different objects such as files,directories and devices.Processes
created by the user inherit all access rights for that user.

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On Windows Server Systems, a domain controller (DC) is a
server that responds to security authentication requests
(logging in, checking permissions, etc.) within the Windows
Server domain.[1] A domain is a concept introduced in Windows
NT whereby a user may be granted access to a number of
computer resources with the use of a single username and
password combination.