GATE CSE 1999 | Question: 2.11

Question

Which of the following is/are advantage(s) of virtual memory?

• A. Faster access to memory on an average.
• B. Processes can be given protected address spaces.
• C. Linker can assign addresses independent of where the program will be loaded in physical memory.
• D. Program larger than the physical memory size can be run.

Comments

The use of Memory Management Unit (MMU) and virtual memory address makes position independent code almost obsolete for executables. Shared libraries though, have to be written as position independent code because they can be mapped to any position in the address space of the executables.

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Those who have doubt: "Why Option C is incorrect?"

The Core Distinction: Logical Memory vs. Virtual Memory

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My Explanation related to Option C (True)

Note : First of all QUestion is not about what virtual address can do ? i think it about advantages ?

@Arjun plz check sir and correct me ....Option C should be correct because without virtual memory, compilers and linkers might need to generate position-independent code . But virtual memory, the linker can confidently assign starting virtual addresses for every program. The program operates entirely under the illusion that it resides at these specific addresses, while the underlying MMU dynamically maps these virtual addresses to wherever free physical frames happen to be available in RAM.

References : A Programmer's Perspective by Bryant and O'Hallaron

References : Galvin 

Answer 1

Virtual memory provides an interface through which processes access the physical memory. So,

• A. Is false as direct access can never be slower.
   
• B. Is true as without virtual memory it is difficult to give protected address space to processes as they will be accessing physical memory directly. No protection mechanism can be done inside the physical memory as processes are dynamic and number of processes changes from time to time.
   
• C. Position independent can be produced even without virtual memory support.
   
• D. This is one primary use of virtual memory. Virtual memory allows a process to run using a virtual address space and as and when memory space is required, pages are swapped in/out from the disk if physical memory gets full.

So, answer is (B) and (D).

Comments

@gatecse Sir, I am just telling the purpose of “permission bits of PT” and MMU and just trying to tell that though we have MMU why do we need “permission bits of PT.”

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okay 👍

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VM and Logical Memory are diff. concepts right? If that so, Option B shoudn't be correct since we can achieve "protected address spaces" using Logical memory system also meaning without VM.

Answer 2

it was asked in 2018 nptel weekly quiz...answer is only D)..prog.larger than m.m. can be run...

only D)

Comments

now I’m a bit confused, only D or B,D?

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nptel quiz ans are not totally reliable. many of them are given as incorrect

both b,d are correct

https://www.cs.princeton.edu/courses/archive/fall19/cos318/lectures/3.ProtectionVirtualMemory.pdf

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Thanks man, answer shoukd be D

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It's not about the answer given somewhere else, valid points are needed to say anything.

But i appreciate your work.

Answer 3

There are several advantages of virtual memory:

1. Increased effective memory capacity: Virtual memory allows the computer to use more memory than physically available. It uses a combination of physical RAM and disk space to create a larger addressable memory space. This helps in running large applications and multitasking efficiently.

2. Improved performance: Virtual memory enables the operating system to prioritize and manage memory resources effectively. It can transfer data between RAM and disk as needed, ensuring that the most frequently used data is kept in RAM for faster access. This helps in reducing the overall response time and improving system performance.

3. Memory protection: Virtual memory provides a layer of protection by isolating each process's memory space from others. This prevents one program from accessing or modifying the memory of another program, enhancing system stability and security.

4. Simplified programming: Virtual memory simplifies the programming process by providing a uniform address space for all processes. Developers can write programs assuming they have access to a large contiguous memory space, without worrying about the actual physical memory limitations.

5. Efficient memory utilization: Virtual memory allows for efficient utilization of physical memory resources. It enables the operating system to allocate and deallocate memory dynamically based on the needs of different processes, optimizing the usage of available resources.

6. Flexibility: Virtual memory provides flexibility by allowing the operating system to allocate different amounts of memory to different processes based on their requirements. It can also allocate additional memory to handle peak loads or temporary spikes in memory usage.

Overall, virtual memory plays a crucial role in improving system performance, enabling multitasking, and providing a more efficient and secure computing environment.

answer B and D is correct

Answer 4

should be B and D

Answer 5

I believe the answer to be B, C and D

Arjun sir has given good explanations for B and D. 

But the question asked about advantages, and not something unique to virtual memory.

Option C is also one of the advantages of Virtual Memory. 

Reference :

 

https://www.cs.fsu.edu/~lacher/courses/COP4610/lectures_9e/ch09.pdf

Answer 6

• Position Independent Code (PIC) can be generated without virtual memory

• Relocation can be done by:

  • Loader

  • Base + offset registers

  • Static relocation

 So strictly speaking, VM is not necessary for linker independence.

so options   B,D are true