COMP 3411 Operating Systems

Assignment

Operating Systems

Discussion about Virtual Memory

Table of Contents

Introduction

The laptop can handle extra reminiscence than the real quantity programmed into the machine. This additional memory is known as virtual memory and is part of a hard disk designed to imitate laptop RAM.

The biggest seen advantage of this program is that the structures may be larger than the physical memory. Visual memory has purposes. First, it lets in us to expand the use of physical reminiscence via disk. Second, it lets in us to have reminiscence safety, because every visible deal with is translated into a local address.

The following are conditions, in which the whole system does not want to be absolutely loaded into the main memory.

• User written mistakes control procedures are simplest used in case of data mistakes or calculation.
• Certain alternatives and machine features can be used sparingly.
• Most tables are assigned a limited address space despite the fact that simplest a small wide variety of tables are used.
• The ability to create a reminiscence device can withstand a notable deal of benefits.
• A small I / O wide variety might be required to load or alter each user application in memory.
• The system will not be confined by the quantity of reminiscence to be had.

Each consumer program can seize a small quantity of bodily memory, multiple applications can be used concurrently, and a corresponding growth in CPU and tool utilization.

Modern microprocessors are meant for general use, a memory control unit, or MMU, built into the hardware. The function of MMU is to translate visible addresses into nearby addresses. A fundamental example is given below –

Implementation of Virtual Memory

Virtual Memory is frequently used for demand paging. It also can be used in the page segmentation. Demand segmentation can also be used to provide virtual memory.

Demand Paging

It is exactly the same as paging in which procedures remain in second memory and pages are loaded most effective on demand, no longer in advance. When a content change occurs, the operating system does now not replica any old application pages to disk or other new application pages in the essential memory Instead, it just begins creating a new software after loading the primary web page and downloading that. Program pages as indicated.

While acting a program, if this system refers to a web page that isn’t handy to the primary reminiscence because it became modified long in the past, the processor treats this invalid memory reference as a page mistakes and transfers manipulate from the device to the software to the application. Need the page again to memory.

Pros

The following are the advantages of Demand Paging –

• Great visual memory.
• Effective use of reminiscence.
• There isn’t any restrict to the extent of multiprogramming.

Cons

The range of tables and the variety of processors that handle page interruptions are larger than the quantity of easy page control strategies.

Algorithms

Page Replacement Algorithm

Page algorithms strategies utilized by the Operating System that decide which memory pages have to be replaced, write to disk where the memory web page needs to be assigned. Packing occurs each time a web page blunders happens and the unfastened page can’t be used to allocate funds in order that the pages are not available or the variety of loose pages is decrease than the specified pages.

If the web page is chosen and changed, and redirected to it, it must be read from disk, and this requires the final touch of I / O. This procedure determines the first-rate of the web page switch algorithm: the shorter the waiting time for pages to go into, the higher the set of rules.

The page replacement algorithm seems at constrained facts approximately get access to given pages of hardware, and attempts to decide which pages must be changed to reduce the overall web page loss rate, even as additionally measuring primary garage costs and set of rules processing time. There are many distinct algorithms for converting page. We take a look at the set of rules through the usage of it in a selected reminiscence reference collection and compiling some of page fault,

Reference String

The alphabetical unit of reminiscence is referred to as the reference string. Reference units are generated by means of simulation or with the aid of monitoring a given device and recording the cope with of every reminiscence reference. The latter choice produces a massive wide variety of data, from which we observe two matters.

• For a selected page size, we want to take into account most effective the page quantity, now not the whole cope with.
• If we’ve got connection with web page p, any instantaneous commands on page p will never cause a web page mistakes. Page p might be in memory after the primary reference; the following immediate warning signs will not be a mistake.
• For example, bear in mind the following cope with collection – 123,215,600,1234,76,96
• If the web page length is 100, it manner that the reference unit is 1,2,6,12,0

Page Buffering Algorithm

To get the procedure began quickly, keep a variety of loose frames.

• At the lowest of the web page, choose the page to change.
• Write a new web page in the free pool frame, mark the web page table and restart the process.
• Now write the dirty page off the disk and place the frame with the modified page inside the free page pool.

Least Frequently Used (LFU) set of rules

The page with the smallest variety is the one in an effort to be decided on for conversion.

This algorithm suffers from a scenario wherein the web page is heavily used at some stage in the first phase of the technique, however may not be reused.

The Most Frequently used (MFU) set of rules

This algorithm is based totally at the argument that a low-fee page is possibly to be these days submitted and may be used.

Conclusion

This assignment deals with the description of Virtual Memory in detail. The assignment begins with the introduction of Virtual memory followed with Implementation techniques of Virtual memory along with Algorithms.

References

Operating System – Virtual Memory – Tutorialspoint. (2019). Tutorialspoint.com. https://www.tutorialspoint.com/operating_system/os_virtual_memory.htm

OPERATING SYSTEMS VIRTUAL MEMORY. (n.d.). https://web.cs.wpi.edu/~cs3013/c07/lectures/Section09-Virtual_Memory.pdf

Operating Systems: Virtual Memory. (2019). Uic.edu. https://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/9_VirtualMemory.html

Page Replacement Algorithms in Operating Systems. (2015, July 16). GeeksforGeeks. https://www.geeksforgeeks.org/page-replacement-algorithms-in-operating-systems/