Virtual memory!

^^^ more about it.

What is it?

Virtual memory combines your computer’s RAM with temporary space on your hard disk. When RAM runs low, virtual memory moves data from RAM to a space called a paging file. Moving data to and from the paging file frees up RAM so your computer can complete its work.

History and uses

Before computers had cache, they had virtual memory.Personal computers in the 1980s did not use virtual memory. Initially, virtual memory meant the idea of using disk to extend RAM.Programs wouldn't have to care whether the memory was "real" memory (i.e., RAM) or disk. The operating system and hardware would figure that out. Later on, virtual memory was used as a means of memory protection. Every program uses a range of addressed called the address space. Virtual memory can help prevent programs from interfering with other programs.