Before we talk about protection, relocation and virtual memory in detail, we must review basic operating systems concepts which drive the need for more sophisticated memories.

Multiprogramming is the technique of keeping more than one partially completed user program in memory and switching rapidly from one to the other in order to give the illusion that the computer is working on all of them simultaneously. The operating system, abbreviated as the OS, is just another program whose code and data always occupy a portion of main memory. The OS is usually but not always at the section beginning at address 0. Some OS's start at high memory. Other OS's occupy several separate sections of memory.