[Solved] virtual memory and its architecture in operating

Systems Essay, Research Paper


Virtual memory refers to an fanciful set of locations, or references, where you can hive away informations. It is fanciful in the sense that the memory country is non the same as the existent physical memory composed of transistors or capacitances. The efficient usage of memory is a job that has remained of import since the development of the stored program computing machine. At a clip when computing machine memory was still expensive and had a little electrical capacity the coders were encouraged to develop plans, that used less than expected memory. This job led to one of the most clever thoughts. In 1961 a group of people at Manchester, England proposed a method for executing the sheathing procedure automatically which, in bend, led to practical memory.

The construct of practical memory is to greatly increase the evident sum of physical memory available to plans. The free infinite on the computing machine? s difficult thrust is treated as an extension of RAM ( random entree memory ) . When necessary, pages are swapped out of existent memory to a location on the difficult thrust. The swapped files can turn and shrivel as thew memory demand fluctuates.

This construct is the key, which allows a user to run an operating system on a machine with 4, or 8 MB of installed RAM. On such systems, the operating system must trade parts of its ego from the physical RAM to the difficult thrust? s barter files. This can besides be done as it loads. In add-on, the practical memory besides utilizes the processor? s ability to halt put to deathing instructions when a page interlingual rendition fails. In this paper the Unix, Apple, and the NT runing systems will be discussed.

There are three basic elements of practical memory. Virtual reference infinite, the chief memory, and the subsidiary memory ( Carr, 1984 ) .

The reference infinite is the set infinite, which is reserved for the usage of memory barter. It creates a distinguishable location for informations to be held. This procedure enormously speeds up the retrieval clip of practical informations. To the undertaking, the reference infinite is merely a uninterrupted array of words or bytes, each with alone practical reference. To the practical memory hardware, the reference infinite is partitioned equal size frame or blocks ( Carr, 1984 ) . The subsidiary memory has a much longer entree clip but a batch less expensive than chief memory. Conceptually, if one thinks of the plan in the secondary memory as the original transcript and the pieces brought in to the chief memory as transcripts, instead the other manner around ( Tanenbaum, 1976 ) .

Keeping the original plan updated must non be over looked. When a plan needs a page out of practical memory, the operating system must happen where that page is located. This undertaking is simplified by partitioning the thrust to let practical memory to be stored in a specific portion of the difficult thrust.

The pages that are placed in frames inside practical memory are known as the undertaking? s occupant set. The occupant set pages and the frames in which they reside is identified by a information construction called the Page tabular array ( Carr, 1984 ) . The User can take one or many pages out of the occupant set. As a plan executes, it accesses sequence practical references in the address infinite. If the page incorporating a practical reference is resident in chief memory, the information at the practical reference is decently obtained or updated.

When a plan references a page and so can non happen it in chief memory, this creates what is called a? Page Mistake? . After a page mistake has occurred it is required for the operating system to read in the needed pages from the secondary, or practical memory. Then the operating system must come in its new physical memory location in the page tabular array, and so reiterate the instructions ( developer, web ) . If the operating system can non finish the undertaking the system will crash. This is besides known as? Fatal Page Fault? .

Address function device is the indispensable hardware component in practical memory. It is located between the processor and the chief memory. This device uses as page tabular array to interpret each practical reference in the reference infinite, to a existent reference in chief memory ( Carr, 1984 ) . The address-mapping device would disrupt executing if the page were losing. It would so signal the operating system for a page mistake. Two activity indexs exist in the chief memory frame, a use-bit, and a dirty-bit. The spots are in a separate memory and have one brace for each chief memory frame. The used-bit indicates that the page is active and, most likely, should non be replaced. The dirty-bit indicates that the page must be moved to an subsidiary memory before it can be replaced ( Carr, 1984 ) . When a missing page is referenced the operating system creates the page tabular array for each undertaking. The system, so, selects a chief memory frame to keep the missing page and performs an I/O operation to travel the page from its slot to the frame. The frames that have non been allocated can be chosen. All the frames are physically indistinguishable except for its existent reference. This brings us to the page replacing algorithm. There are many page replacing algorithm policies, such as ; LFU ( Least Frequently Used ) , LRU ( Least Recently Used ) , and Random. The most effectual page replacing algorithm policy is the LRU. It will replace the memory slot being used the least sum of clip.

A job that can be created by practical memory is when the overall supply of memory is running low so trading may happen. If the memory is highly low, the system may pass all its clip trading memory in and out of disc. This would go forth small clip to carry through purposeful work. This job is normally referred to as disc? threshing? ( developer, web ) . The undermentioned paragraphs will discourse assorted runing systems and how they manage practical memory.

The practical memory trough of Apple computing machines operates invisibly to the applications and its user. Most applications do no demand to cognize if practical memory has been installed unless they have critical timing demands, execute codification at interrupt clip, or execute debugging operations ( Apple, web ) . A twosome of demands still exist for Apple systems to utilize practical memory. First, the computing machine needs to be running system package 7.0 or higher. Second, the computing machine must be equipped with an MMU ( chief memory unit ) or PMMU co-processor. Apples 68040 and 68030 based machines have an MMU built into the cardinal processing unit and are able to run practical memory with no extra hardware. In Apple computing machines the user can besides put the practical memory through the control dad

nel. This can be compared to the system in Windows where the user is allowed to alter the size or disable practical memory. When the practical memory is activated the logical reference infinite is greater than the physical memory. Many factors can make fluctuations in the size of the logical reference infinite these include:

The turn toing manner presently used by the Memory Manager

the sum of infinite available on a secondary storage device for usage by the backing-store file

if 24-bit addressing is in operation, the figure of NuBus enlargement cards, if any, installed in the computing machine ( Apple, web )

24-bit addressing allows the memory director to spread out the size of practical memory to half of the existent RAM available to the system.

The original version of the Macintosh runing systems was non a user of practical memory. A peculiar location in RAM could be accessed by its physical reference. Therefore, there was no difference between the practical and existent reference infinite. Since both the hardware and package have been revolutionized so significantly, this forced the Apple operating system to divide the logical reference infinite from the physical reference infinite. This was accomplished by utilizing the MMU coprocessor to map the logical references to their corresponding physical references ( Apple, web ) .

In Macintosh IIc: with 8MB of physical RAM the physical memory appears to the CPU and to the NuBus enlargement as two single 4MB scopes. In add-on, the operating system uses the MMU coprocessor to find the physical reference matching to the logical reference ( Apple, web ) .

In Windows NT, although the procedures are used in general when reconstructing references in a procedure, no pages of physical memory are committed, and possibly more significantly, no infinite is reserved in the page file for endorsing memory. Besides reserving a scope of references is no warrant that at a ulterior clip there will be no physical memory available. Alternatively, it is merely salvaging a specific free reference scope until needed ( NT, web ) .

The Virtual Allow map can be invoked to reserve a scope of references utilizing the undermentioned codification:

/*Reserve a 10 MB scope of references */

1pBase = VirtualAlloc ( NULL,




Reserved references can merely be used when memory is foremost committed to the references. Memory can be committed every bit small as one page at a clip. The maximal sum of memory that can be committed is based entirely on the maximal scope of immediate free or reserved references ( NT, web ) . In the Windows NT practical memory system, page tabular arraies are used to entree physical pages of memory. Each page tabular array is itself a page of memory ( NT, web ) . To free practical memory references have to be set as either reserved or committed and by utilizing practical free the operating system ca liberate the sum of infinite on the difficult thrust. Processes in Windows NT have a minimum set of pages called a working set that in order for the procedure to run decently, must be present in memory when running ( NT, web ) . Windows NT assigns a default figure of pages to a procedure to a procedure at start up and bit by bit turns that figure to accomplish a balanced/ optimal public presentation. It besides keeps a cheque if the procedure has the needed pages in physical memory. In Windows NT the practical memory direction maps offer direct control over the practical memory. It splits 2GB-user reference infinite into parts of memory that are reserved, committed, or free practical references. It besides allows for applications to change the province of pages in the practical reference infinite.

Virtual memory made its visual aspect in to Unix with the debut of the VAX-11/780 in 1978, with its 32-bit architecture, 4 Gs address infinite, and hardware support for demand paging ( Vahalia 96 ) . The demand paging is responsible for dividing both memory and the procedure reference infinite in to repair size pages. Besides Unix system is intelligent plenty to expect the following transportation or prevenient paging. The plan size is free to turn and shrivel as the demand rises or falls it can maximise its size up to 4 G for a 32-bit machine ( Vahalia 96 ) . But it must be noted since the size of the plan can be existent big, increased transportation between the practical memory and the physical memory can decelerate down the system dramatically. The address infinite used by each Unix procedure is divided into a figure of parts. Each part is a meat construction used to stand for a immediate country of non-overlapping practical references and is governed by its ain set of properties specifying the part type and extra belongingss such as whether the part can be read from, written to or executed. The following are data constructions that define the practical memory country in Unix:

? Struct vm_area_struct {

/* parametric quantities for practical memory area*/

struct task_struct * vm_task ;

unsigned long vm_start ;

unsigned long vm_end ;

pgprot_t vm_page_prot ;

unsigned short vm_flags ;

/*AVLtree for practical memory country of procedure, sorted by references */ ? ( Beck 96 )

the codification vm_task is the arrow indicating to the entry of the procedure tabular array to which the country of memory is allocated. Vm_start and vm_end find the beginning and the terminal of references in practical memory. vm_page_prot precautions feature of pages in the practical memory. The system handiness such as read entree or read-write entree is stored in the vm_flags.

Virtual memory for the most portion has the same construct in all the operating system, what makes them different is the manner they manage it. Operating system takes advantage of this phenomenal characteristic, which has changed the manner computing machines are used everlastingly.


1.About the Virtual Memory Management hypertext transfer protocol: //developer.apple.com/techpubs/mac/memory/memory-152.html.

2.Managing practical memory in win 32 hypertext transfer protocol: //www.microsoft.com/win32dev/base/virtmm.htm.

3.Carr, Richard W. practical memory direction UMI research imperativeness, 1984.

4. Tanenbaum Andrew S. Structured computing machine organisation Englewood drop NJ: Prentice hall 1976.

5. Vahalia, Uresh UNIX Internal The New Frontier Upper saddle river New jersey: Prentice hall, 1996.

6. Beck, M. , Bohme H. , Dziadzka M. , Kunitz U. , Magnus R. , and Verworner D. LINUX Kernel Internals Harlow, England: Addison Wesley Longman limited 1996.

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