A key feature of the Orion was what was then known as "multiprogramming", the ability to run multiple programs at once, rapidly switching between them to make it appear as if they were all running at the same time. The key problem in supporting multiprogramming was the need for programs to be loaded into different locations in memory. Without multiprogramming support, a program was normally loaded into the "base" of memory, its notional location zero. To provide the illusion of being at zero, in a multiprogramming machine each program was assigned a fixed amount of the core memory, its base location is known as the "datum" and the last location as the "limit". Every store operation by the CPU automatically offset the effective address by the datum for that program.