Who Assigns the Process Address

Question

I understand that every process in a computer has an address space, which contains addresses for all the instructions of the process. I also understand that this address is a virtual one - meaning it maps to the physical RAM addresses, and doesn't actually exist.

When a process is to be executed, the CPU sends the MMU the virtual address of the instruction (the page I guess?) that it needs, and then MMU can provide the physical address of it in RAM, which is what the CPU fetches to execute the instruction.

What I don't get is who assigns that virtual address to the process in the first place - how does the CPU know the virtual address? I get that these virtual addresses are generic (I read: Difference between logical addresses, and physical addresses?, and it was very helpful in that aspect), but I don't get who assigns that to begin with. Where even are the page tables stored, and why is the swap space needed?

I've been trying to understand memory mapping for a few days now, mostly through youtube videos, but if anyone knows any good sources I'd really appreciate it!

Answer 1

On older time with virtual address, you have a fixed layout: some data at beginning, the program, the program data, libraries, and than stack. Sometime Kernel had the last bytes reserved. Note: ometime with different layout, but usually same structure on the same OS/version. We had also the possibility to prelink libraries, so you can have quick loading time. Virtual address allow this.

Now it is the kernel that randomize the addresses (to reduce the success rate of buffer overflow attack). So you may have libraries and programs in random places. It is not the MMU, the kernel choose the virtual addresses randomly.

So now you have the virtual address. Now the kernel should assign it to something. If you clone, the virtual address point to the parent process, or when you execute the program, your virtual address point to disk (where the binary reside), and some (the rest, and maybe some guard) are just unassigned, so that program will get a segmentation fault (really a protection exception of CPU). This "physical pointer" could change. Because of swapping the physical address could change, or the pointer could point to different part of disk, but also you may point to old process memory, and then to your modified memory, or to different part of disk.

So virtual addresses are defined by kernel and kernel will handle it (either having a physical address, or an exception, which kernel may check a different source [other process memory, disk]). If the page (virtual address block) has linked a physical address, it is the MMU which do the works (and updating also access count of the page), else it is the kernel (and so much slower).