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What is virtual memory?

What is virtual memory?

I was double checking my notes for 'Virtual Memory' and the definition in my text book is:

Asked by: Guest | Views: 97
Total answers/comments: 3
Guest [Entry]

"Note that ""virtual memory"" is more than just ""using disk space to extend physical memory size""

Virtual memory is a layer of abstraction provided to each process. The computer has, say, 2GB of physical RAM, addressed from 0 to 2G. A process might see an address space of 4GB, which it has entirely to itself. The mapping from virtual addresses to physical addresses is handled by a memory management unit, which is managed by the operating system. Typically this is done in 4KB ""pages"".

This gives several features:

A process can not see memory in other processes (unless the OS wants it to!)
Memory at a given virtual address may not be located at the same physical address
Memory at a virtual address can be ""paged out"" to disk, and then ""paged in"" when it is accessed again.

Your textbook defines virtual memory (incorrectly) as just #3.

Even without any swapping, you particularly need to be aware of virtual memory if you write a device driver for a device which does DMA (direct memory access). Your driver code runs on the CPU, which means its memory accesses are through the MMU (virtual). The device probably does not go through the MMU, so it sees raw physical addresses. So as a driver writer you need to ensure:

Any raw memory addresses you pass to the hardware are physical, not virtual.
Any large (multi page) blocks of memory you send are physically contiguous. An 8K array might be virtually contiguous (through the MMU) but two physically separate pages. If you tell the device to write 8K of data to the physical address corresponding to the start of that array, it will write the first 4K where you expect, but the second 4K will corrupt some memory somewhere. :-("
Guest [Entry]

"I know it’s too late....but thought still it useful.

All are correct based on different viewpoints.
Virtual memory is a memory management technique whereas swap memory was area on the disk drive. Swap memory is generally called as swap space. Swap space refers to the portion of the virtual memory which is reserved as a temporary storage location. Swap space is utilized when available RAM is not able to meet the requirement of the system’s memory
You can refer to the below link for more details"
Guest [Entry]

"Virtual Memory is a block of your hard Drive that the system uses as a paging file in addition to the physical RAM.

It gets tricky, and sometimes slow, because Windows does NOT defrag this part of your hard drive.

Best 2 tips I can offer:
1) Virt Mem should be set both min and max at approx 1.5X your physical Memory. ex. 2GB RAM = 3070MB Virt.
2) When defragging, turn off your paging file. Defrag 2x, and reset back to the original number. This gives a clean slice of drive and will increase the speed of the paging file."