CODEWITHSUNDEEP
linuxpagingvirtual-memory
user3139545
user3139545

Reputation: 7374

How does virtual to pyhsical memory mapping work

Im currently trying to understand systems programming for Linux and have a hard time understanding how virtual to physical memory mappings work.

What I understand so far is that two processes P1 and P2 can make references to the same virtual adress for example 0xf11001. Now this memory adress is split up into two parts. 0xf11 is the page number and 0x001 is the offset within that page (assuming 4096 page size is used). To find the physical adress the MMU has hardware registeres that maps the pagenumber to a physical adress lets say 0xfff. The last stage is to combine 0xfff with 0x001 to find the physical 0xfff001 adress.

However this understanding makes no sens, the same virtual adresses would still point to the same physical location??? What step is I missing inorder for my understanding to be correct???

Upvotes: 0

Views: 96

Answers (1)

Nemanja Boric
Nemanja Boric

Reputation: 22157

You're missing one (crucial) step here. In general, MMU doesn't have hardware registers with mappings, but instead one register (page table base pointer) which points to the physical memory address of the page table (with mappings) for the currently running process (which are unique to every process). On context switch, kernel with change this register's value, so for each running process different mapping will be performed.

Here's nice presentation on this topic: http://www.eecs.harvard.edu/~mdw/course/cs161/notes/vm.pdf

Upvotes: 1

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