ZETECH UNIVERSITY

NAME: RUTERE JOAN WANJA

REG. NO: DBIT-02-0035/2015

SCHOOL: I.T

COURSE: DBIT

UNIT NAME: OPERATING SYSTEMS

TASK: CAT 2

LECTURER: MR FRED

1. Context Switching

It is the process of storing of storing and restoring the state (context) of a process or thread so that the execution can be resumed from the same point at a later time.

There are three potential triggers for a context switch:

• Multitasking: Most commonly, within some scheduling scheme, one process needs to be switched out of the CPU so another process can run.This context switch can be triggered by the process making itself unrunnable, such as by waiting for an I/O or synchronization operation to complete. On a pre-emptive multitasking system, the scheduler may also switch out processes from being starved of CPU time, preemptive schedulers often configure a timer interrupt to fire when a process exceeds its time slice.This interrupt ensures that the scheduler will gain control to perform a context switch.

• Interrupt Handling

Modern architectures are interrupt driven.This means that if the CPU requests data from a disk, for example, it does not need to busy-wait until the read is over, it can issue the request and continue with some other execution. When the read is over, the CPU can be interrupted and presented with the read. For interrupts, a program called an interrupt handler that handles the interrupt from the disk.

When an interrupt occurs, the hardware automatically switches a part of the context (at least enough to allow the handler to return to the interrupted code). The handler may save additional context, depending on details of the particular hardware and software designs. Often only a minimal part of the context is changed in order to minimize the amount of time spent handling the interrupt. The kernel does not spawn or schedule a special process to handle interrupts, but instead the handler executes in the (often partial) context established at the beginning of interrupt handling. Once interrupt servicing is complete, the context in effect before the interrupt occurred is restored so that the interrupted process can resume execution in its proper state.

• User and Kernel mode Switching.

When a transition between user mode and kernel mode is required in an operating system, a context switch is not necessary; a mode transition is not by itself a context switch. However depending on the operating system, a context switch may also take place at this time

2. Give 3 examples of applications that could benefit from the OS support for random access to indexed files

Visual Basic C Java

3. Merits and demerits of supporting links to files that cross mount points.

The advantage is that there is greater transparency in the sense that the user does not need to be aware of mount points and create links in all scenarios.The disadvantage however is that the file system containing the link might be mounted while the file system containing the target file might not be and therefore one cannot provide transparent access to the file in such a scenario; the error condition would expose to the user that a link is a dead link and that the link and that the link does indeed cross file system boundaries.

4. Discuss the following terms giving examples

• Encryption
This is the process of encoding messages or information in such a way that only authorized parties can read it. (The sender and recipient)

Sender

[pic]

Cryptography

It refers to the practice and study of techniques for secure communication in presence of third parties. Symmetric Cryptography It includes methods of encryption that are best suited for processing large streams of data. It is distinguished the use of a single key for encrypting and decrypting messages by the sender and receiver. This type of cryptography is categorized by the use of stream or block ciphers.

Asymmetric Cryptography Asymmetric Cryptography encryption methods are best used for key exchange and user authentication. This type of cryptography is commonly used in digital signatures. It is distinguished by the use of a private and public key that are created with one-way functions using multiplication and exponentiation.

• Encoding This refers to the conversion of data into a coded form.

• System Security This refers to providing a protection system to computer system resources such as CPU, memory disk, software programs and most importantly data/information stored in the computer system. It can be achieved through installation of passwords, data encryption, use of firewalls etc

5. You are a new intern at Zetech University, the system manager keeps on getting debugs on the server Operating System. This debugs might delete important files on the server. The system Manager appoints you to do the following tasks

i. Suggest a suitable O.S for the server giving reasons, the advantages and give three examples of companies that use that O.S

A distributed operating system would be suitable for the school because it manages a group of distinct computers and makes them appear as a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine. When computers are in group work cooperation, they form a distributed system. Examples include: • IRIX OS- the implementation of UNIX System V, Release 3 for Silicon Graphics multiprocessor workstations. • AI OS for IBM RS/6000 computers. • Solaris Operating System for SUN multiprocessor workstations.

Distributed systems provide the following advantages: • Sharing of resources • Reliability • Communication • Computation speedup.

ii. Give 3 security measures to secure the O.S from access by intruders • Creating secure accounts with required privileges only (i.e. user management) • Scrutinizing all incoming and outgoing network traffic through a firewall •

References www.tutorialspoint.com en.wikipedia.org

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Office files Computer no: 3

Encrypt

6EB157O
O7A3O1

Office files Computer no:3

Decrypt

Recipient

Private key

Public Key