what is the benefit of storing virtual address in a pointer rather than physical address?

Question

I have gone through below link and it says that on most Operating Systems , pointers store the virtual address rather than physical address but I am unable to get the benefit of storing virtual address in a pointer .

when at the end we can modify the contents of a particular memory location directly through a pointer so what is the issue whether it is a virtual address or a physical address ? Also during the time the code executes , most of the time the data segment will also remain in memory ,so we are dealing with physical memory location only so how the virtual address is useful ?

C pointers and the physical address

It'd be pretty bad if the chrome, outlook and MSVC on my desktop at the moment all had access to each others physical memory. — StoryTeller - Unslander Monica, Mar 21 '17 at 09:50
Because that's how virtual memory *works* in user-space applications on operating systems with process separation? And what does it matter? — Some programmer dude, Mar 21 '17 at 09:50
"most of the time the data segment will also remain in memory" - except when it *doesn't* ? Didn't your "most" observation at-least-partly answer your own question? Wouldn't the case where the exception were true be a case where virtmem mgmt is *beneficial*? — WhozCraig, Mar 21 '17 at 10:07

score 2 · Answer 1 · edited May 23 '17 at 11:46

Security issues (as noted before) aside, there is another big advantage:

Globals and functions (and your stack) can always be found at fixed addresses (so the assembler can hardcode them), independently of where the instance of your program is loaded.

If you'd really like your code to run from any address, you have to make it position independent (with gcc you'd use the -fPIC argument). This question might be an interresting read wrt the -fPIC and virtual addressing: GCC -fPIC option

score 1 · Answer 2 · answered Mar 21 '17 at 09:56

The same virtual address can point to different physical addresses in different moments.

If your physical memory is full, your data is swapped out from the memory to your HDD. When your program wants to access this data again - it is currently not stored in memory - the data is swapped in back to memory, but it often will be a different location as before. The Page Table, which stores the assignment of virtual to physical addresses, is updated with the new physical address. So your virtual address remains the same, while the physical address may change.

score 1 · Accepted Answer · answered Mar 21 '17 at 16:51

Actually, pointers normally hold LOGICAL ADDRESSES.

There are several reasons for using logical addressing, including:

Ease of memory management. The OS never has to allocate contiguous physical page frames to a process.
Security. Each process has access to its own logical address space and cannot mess with another processes address space.
Virtual Memory. Logical address translation is a prerequisite for implementing virtual memory.
Page protection. Aids security by limiting access to system pages to higher processor modes. Helps error (and virus) trapping by limiting the types of access to pages (e.g., not allowing writes to or executes of data pages).

This is not a complete list.

what is the benefit of storing virtual address in a pointer rather than physical address?

3 Answers3