In order to prevent fragmentation of memory, every time a program terminated, the FP-6000's operating system, known as "Executive", would temporarily stop the other programs and recopy them to the lowest available location in core. This way the available memory was always at the top. Although this technique eliminated the need for storing a list of memory blocks, it was at the cost of expensive copies every time a program ended. This would make the system unsuitable for running an operating system such as Unix, which is made up of a series of tiny programs that are frequently started and stopped, but Unix did not exist at the time, and the model for most operating systems was a sort of "extended batch mode", running long-lived programs that paused when not being actively fed data.