Running multiple executables linked to 0x400000

Question

I'm interested in operating systems topic and I have a dummy question. Standard PE executable files are linked to 0x400000. My question is how can operating system load multiply executables with same image base, when virtual memory just maps virtual addresses to physical. Is it storing PDE and PTE index of thread somewhere? Is there some addition to each address before execution starts? How does it work?

Answer 1

Each process gets its own virtual address space, and hence there's no conflict. All virtual address spaces that exist in any one time in the system get mapped into the physical address space. Virtual memory that can't or currently isn't mapped onto a particular physical memory is held in the swap file (swap partition, or alike) — this is called paging.

During thread switches, when the CPU is about to execute a thread from a different process than it was executing so far, the operating system's scheduler informs the CPU (sets the respective registers) about the new virtual address translation table to use. Thus the CPU thinks there's just one virtual address space at the given time, while the operating system can manage many more, one for each process.

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Disclaimer: My answer may be a thought of as a bit superficial or imprecise as opposed to the reality. This for the sake of simplicity in respect to the nature of the OPs question. Also, these mechanisms are CPU-dependent and operating system-dependent.