Primary-memory or Main-memory is a large array of words or bytes. Each word or byte has its own address. Main-memory provides storage that can be accessed directly by the CPU. That is to say for a program to be executed, it must be in the main memory.
The major activities of an operating system in regard to memory-management are :
1. Keep track of which part of memory are currently being used and by whom.
2. Decide which process are loaded into the memory when memory space becomes available.
3. Allocate and deallocate memory spaces as needed.