virtual memory: how does it provide protection

[user4592357]

it's said that virtual memory solves the problems of raw physical memory, namely lack of memory, fragmentation, and protection. but does it provide a guarantee that two processes don't map to the same physical memory? how does, when "allocating" a space for a new process, virtual memory manager know that "this physical memory is allocated so i can't touch it, but that one isn't so i'll give it to the new process"? is there some kind of mapping stored?

 

[drescherjm]

The OS will have mapping tables.

 

[whateverer]

Yeah, the operating system uses the Memory Management Unit on the processor to translate virtual addresses.

https://en.wikipedia.org/wiki/Memory_management_unit

It keeps track of what is allocated to each task with a Page Table. This is an actual physical table maintained in ram by the operating system.

https://en.wikipedia.org/wiki/Page_table

Each time you create a new task, a space is set-aside in the Table that's not currently used by any other task.

And then the OS adds more memory Pages as needed. This is called a Page Fault.

https://en.wikipedia.org/wiki/Page_fault

They can also be deallocated by the OS when not needed any longer. This is how you avoid fragmentation: pages can be used in any order, and can be managed individually. As-opposed to flat-mapped systems, where you need one continuous block of memory when it's requested.