进程如何访问内存映射的IO地址不属于它的虚拟地址空间？

I was reading the OS theory in which I found that a process have it's virtual address space (loosely using technique base and limit register for sake of simplicity) But it can't access any other addresses. But in embedded systems we can use placement new or reinterpret_cast (C++) to access memory mapped IO address'es, but how that can be possible if we can't access any other address except process's virtual address space?I was reading the OS theory in which I found th