GATE CSE 2001 | Question: 1.8

Question

Which of the following statements is false?

• A. Virtual memory implements the translation of a program's address space into physical memory address space
• B. Virtual memory allows each program to exceed the size of the primary memory
• C. Virtual memory increases the degree of multiprogramming
• D. Virtual memory reduces the context switching overhead

Comments

actually there is no relation between Context switching overhead and virtual memory

Hence D is the wrong answer

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TLBs are flushed on context switches.

Answer 1

(D) should be the answer.

(A) - MMU does this translation but MMU is part of VM (hardware).

(B), (C) - The main advantage of VM is the increased address space for programs, and independence of address space, which allows more degree of multiprogramming as well as option for process security.

(D) - VM requires switching of page tables (this is done very fast via switching of pointers) for the new process and thus it is theoretically slower than without VM. In anyway VM doesn't directly decrease the context switching overhead.

Comments

@Higgs In VM we only load the process partially i.e only the selected number of pages as demanded by the process during execution and never load the entire process in MM and thus we can save some space in MM and load extra process' in that space and increase degree of MP.

 

At any time a process never needs all the pages to be present in MM during it's execution.

 

Say Process P1 has 10 pages P2 = 5 pages and P3 = 7 pages and now P1 is executing then we won't load all 10 pages in MM, but only load the pages required by process P1 at any point of time.

 

Say our MM has 5 frames and now P1 is running and need/demand only 2 pages and thus we load only 2 pages in MM instead of all 10 pages (that's demand paging) and now in remaining 3 frames of MM we can load pages from process P2 and P3 and thus we have more process in MM by only loading the processes partially in MM and hence degree of MP increases! 

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In the context of the implementation of the system without virtual memory, it doesn’t require maintaining page tables explicitly for every process.

The virtual address generated by the CPU itself is the physical address.

In that sense presence of virtual memory increases context switch overhead.

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… and independence of address space, which allows more degree of multiprogramming as well as option for process security.

I can understand how the ‘independence of address spaces’ for the processes can provide increased process security. But, I am can’t seem to understand how the same can directly cause an increase in the degree of multiprograming. IIUC, isn’t the degree of multiprogramming increasing with the use of VM because of the sparsely populated process pages in the virtual address space of the process, and the liberty to swap out currently inactive process pages from page frames when in paucity of page frames, and later swapping them in as per need? Or am I going wrong somewhere?

Answer 2

VM increases the context switching overhead . VM is based on demand paging so more number of swaps == increase context switch , during page fault .

1. Virtual memory implements the translation of a program's address space into physical memory address space

This statement is Correct.

 Virtual memory require both hardwire (main memory and TLB) and OS support ( to control those hardwire) , that actual translation from program's address space to physical address space is done by main memory .  so statement A is correct . That makes option A wrong.

Only statement D is false and it is the answer .

Comments

@Bikram sir pls clear 2nd point wht does it mean ? primary memory refers to physical memory then how it exceeds?

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@ set2018

That 2nd point , is main job of Virtual memory .

The term virtual  in ' virtual memory ' refers to memory address is Virtual.

So when memory address is virtual it can take any amount of memory space ..hence it looks like Virtual memory exceeds the size of physical memory .

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Bikram Sir this may sound silly but still I need to get this clarified.. 

"A computer without Virtual memory support can generate virtual address". Is this True or False?

Recently I have seen some questions where these are being related. But I have read this

Each of those(virtual address space and virtual memory) could exist without the other in an OS. But combined they are most useful and therefore usually implemented in a tightly coupled way by the OS.

Ref: https://stackoverflow.com/questions/17377887/virtual-address-space-vs-virtual-memory

Please help.