Operating System Short Question And Answer Set-4

1.What is virtual memory?
Virtual memory is a technique that allows the execution of processes that may not be completely in memory. It is the separation of user logical memory from physical memory. This separation provides an extremely large virtual memory, when only a smaller physical memory is available.

2.What is Demand paging?
Virtual memory is commonly implemented by demand paging. In demand paging, the pager brings only those necessary pages into memory instead of swapping in a whole process. Thus it avoids reading into memory pages that will not be used anyway, decreasing the swap time and the amount of physical memory needed.

3.Define lazy swapper.
Rather than swapping the entire process into main memory, a lazy swapper is used. A lazy swapper never swaps a page into memory unless that page will be needed.

4.What is a pure demand paging?
When starting execution of a process with no pages in memory, the operating system sets the instruction pointer to the first instruction of the process, which is on a nonmemory resident page, the process immediately faults for the page. After this page is brought into memory, the process continues to execute, faulting as necessary until every
page that it needs is in memory. At that point, it can execute with no more faults.This schema is pure demand paging.

5.Define effective access time.
Let p be the probability of a page fault (0£p£1). The value of p is expected to be close to 0; that is, there will be only a few page faults. The effective access time is Effective access time = (1-p) * ma + p* page fault time.ma : memory-access time

6.Define secondary memory.
This memory holds those pages that are not present in main memory. The secondary memory is usually a high speed disk. It is known as the swap evice, and the section of the disk used for this purpose is known as swap space.

7.What is the basic approach of page replacement?
If no frame is free is available, find one that is not currently being used and free it.
A frame can be freed by writing its contents to swap space, and changing the page table to indicate that the page is no longer in memory.Now the freed frame can be used to hold the page for which the process faulted.

8.What are the various page replacement algorithms used for page replacement?

  • FIFO page replacement
  • Optimal page replacement
  • LRU page replacement
  • LRU approximation page replacement
  • Counting based page replacement
  • Page buffering algorithm.

9.What are the major problems to implement demand paging?
The two major problems to implement demand paging is developing
a. Frame allocation algorithm
b. Page replacement algorithm

10.What is a reference string?
An algorithm is evaluated by running it on a particular string of memory references and computing the number of page faults. The string of memory reference is called a reference string.

11.What is a file?
A file is a named collection of related information that is recorded on secondary storage. A file contains either programs or data. A file has certain “structure” based on its type.

12.List the various file attributes.
A file has certain other attributes, which vary from one operating system to another, but typically consist of these:Name, identifier, type, location, size, protection, time, date and user identification

13.What are the various file operations?
The six basic file operations are

  • Creating a file
  • Writing a file
  • Reading a file
  • Repositioning within a file
  • Deleting a file
  • Truncating a file

14.What are the information associated with an open file?
Several pieces of information are associated with an open file which may be:

  • File pointer
  • File open count
  • Disk location of the file
  • Access rights

15.What are the different accessing methods of a file?
The different types of accessing a file are:

  • Sequential access: Information in the file is accessed sequentially
  • Direct access: Information in the file can be accessed without any particular order.
  • Other access methods: Creating index for the file, indexed sequential access method (ISAM) etc.

16.What is Directory?
The device directory or simply known as directory records information-such as name, location, size, and type for all files on that particular partition. The directory can be viewed as a symbol table that translates file names into their directory entries.

17.What are the operations that can be performed on a directory?
The operations that can be performed on a directory are

  • Search for a file
  • Create a file
  • Delete a file
  • Rename a file
  • List directory
  • Traverse the file system

18.What are the most common schemes for defining the logical structure of a
The most common schemes for defining the logical structure of a directory

  • Single-Level Directory
  • Two-level Directory
  • Tree-Structured Directories
  • Acyclic-Graph Directories
  • General Graph Directory

19.Define UFD and MFD.
In the two-level directory structure, each user has her own user file directory (UFD). Each UFD has a similar structure, but lists only the files of a single user. When a job starts the system’s master file directory (MFD) is searched. The MFD is indexed by the user name or account number, and each entry points to the UFD for that user.

20.What is a path name?
A pathname is the path from the root through all subdirectories to a specified file. In a two-level directory structure a user name and a file name define a path name.