虚拟内存
Modern operating systems provide virtual memory, in which processes are given a range of memory, but where the memory does not directly correspond to actual physical RAM. Virtual memory can be backed by physical RAM, a disk file via mmap (on Unix-derivatives) or MapViewOfFile (on Windows), or swap space, and the operating system can move virtual memory pages around as it needs. Because virtual memory does not need to be backed by physical memory, exhaustion of it is rare, and usually there are other limits imposed by the operating system on resource consumption.