I hope this is the right place. This is a homework assignment for my Operating Systems course and I have to implement a working virtual memory system in C++ so programming is directly involved.
I've read some sources about Paging and Virtual Memory now (Tanenbaum, What Every Programmer Should Know About Memory, etc) and noticed that everybody starts off with the phrase
Every process gets its own virtual memory.
Then off they go into a detailed analysis about page size, page replacement algorithms, the MMU and TLB, etc. Those things are actually quite fine in my mind, what doesn't make sense is the concept of virtual memory.
If I have four processes running simultaneously and each process uses it's own virtual address space, how is the physical memory protected? The MMU translated the virtual address 0 the same whether it comes from process one, two, three or four.
Let me give an example:
Every process has a 32-bit virtual address space that it can use. Every process starts off the same way
MOVI 10, 0 // r10 = 0 LOAD 11, 10 // r11 = MEM[r10]; value at address 0x00000000
I just don't understand this concept of distinct virtual address space if every process tries to call the same virtual address which will be translated the same way by the MMU. So all four processes will receive the same data?
We have a page table which keeps an index of every mapped and unmapped page to a page frame. If each page entry is unique, then how can it account for the duplicated virtual address space that each process has?
I'm at the point where I have written design drafts for the MMU, TLB and the Page Table, but I can't continue until I properly understand how the heck virtual space is defined and used for each process.