Memory Management
OS keeps tracks of primary memory i.e. what part of it is in use by whom, what part is not in use etc. and allocates the memory when a process or program requests it. Programs to be executed by the computer systems first must be loaded into the main memory. A set of processes, together with the
data they access, reside in the memory.

A process must be in memory to be executed. OS manages memory in a way that many processes reside in memory simultaneously.