This mechanism or something very similar to it is used in most multiprogrammed systems. It is interesting to see how there is a continual interplay between hardware designs and the uses to which the hardware is put. For example, if computers had continued to be used by just one person at a time, none of this stuff for protection and relocation would have been needed. In Chapter 13 we will see how another user trend influenced hardware, namely the need to run very large programs or use more memory than is actually available. This is how virtual memory came about.