Answer:
Unix system memory management based on swapping . swappin is the process of transferring a block of data from secondary sorage to main memory and vice versa. This done to accommodate larger size of the program, to be executed, in a small sized main memory of the computer. Movement between memory and disk was handled by the upper level of the two-level scheduler, known as the swapper. Swapping from memory to disk was initiated when the kernel ran out of free memory on account of one of the following events :
1. a fork system call needed memory for a child process.
2. A brk system call needed to expand a data segment.
3. A stack became larger and ran out of the space allocated to it.