CODEWITHSUNDEEP
coperating-system
Radha Gogia
Radha Gogia

Reputation: 795

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

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

Upvotes: 1

Views: 478

Answers (3)

user3344003
user3344003

Reputation: 21617

Actually, pointers normally hold LOGICAL ADDRESSES.

There are several reasons for using logical addressing, including:

  1. Ease of memory management. The OS never has to allocate contiguous physical page frames to a process.

  2. Security. Each process has access to its own logical address space and cannot mess with another processes address space.

  3. Virtual Memory. Logical address translation is a prerequisite for implementing virtual memory.

  4. 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.

Upvotes: 1

Lanting
Lanting

Reputation: 3068

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

Upvotes: 2

Eknoes
Eknoes

Reputation: 518

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.

Upvotes: 1

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