Is virtual memory related to virtual address space of a process

pagingvirtual-memory

I have understood that Paging is a memory management technique which allows computer to bring data from secondary storage to main memory for executing process. It gives an impression to process that it has a large contiguous address space available to it (virtual address space).
Page table maps virtual addresses of a process to physical address.
Virtual addresses are the address that CPU uses. When need to access memory, physical address(actual address in RAM) corresponding to virtual address is obtained from page table using translation process.
And
concept of Virtual memory – when CPU needs memory more than the physical memory present, OS uses some part of secondary storage as RAM.

I get confused when I try to relate these two concepts. My question is – is virtual memory related to virtual address space of a process.
Is virtual address space of a process actually present in virtual memory? But how is it possible since virtual memory is actually part of secondary storage. OR virtual address space of process is located in RAM? Does both the Virtual and Physical address space of a process present in RAM?
Kindly clarify.

Best Answer

Virtual memory and paging should not be confused with the page file (also called the swap file).
The two concepts are indeed related, but still different.

Virtual memory refers to the virtual address space of a process: each process "thinks" it has a dedicated 32-bit (or 64-bit) address space, without actually doing so -- it might be that some pages don't actually exist in memory, or might exist at a different physical address, but the process doesn't care.

The method by which this is implemented is called paging: Whenever a process accesses memory, the Memory Management Unit translates the virtual address by looking up the physical page of RAM corresponding to the virtual page of memory. (A page is a larger unit of memory management, often 4KiB big). If no mapping exists, it triggers a page fault, to let the operating system know.

At this point, the operating system decides what happens next. It could terminate the program, it could generate the data randomly (!), or it could store or retrieve it from a page file (or swap file) on another storage device. But at this point, the hardware doesn't care what happens -- the operating system could do whatever it likes when handling the request.

So, virtual memory and the page file are not inherently related, but they just happen to come hand-in-hand in typical situations.

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Virtual memory

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. :-(

Virtual memory vs. Virtual address space

The virtual address space is what an individual program sees when it executes. Depending on how the program has been configured this address space will be as large as the maximum the operating system supports.

The operating system kernel is then responsible for mapping addresses in the vas to physical memory, be that RAM, or system page files.

With this design, the programs themselves remain unaware of resources and real addresses, and can operate as if they had all system memory to themselves, or at least the maximum memory a single process can use.

In a nutshell a program works with VAS, and the operating system handles mapping VAS to real storage so that this is invisible to the running program. The running program sees only its VAS.

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Virtual memory

Virtual memory vs. Virtual address space

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