Main Memory                  

The main working  principle of digital computer is Von-Neumann stored program principle. First of all we have to keep all the information in some storage, mainly known as main memory, and CPU interacts with the main memory only. Therefore, memory management is an important issue while designing a computer system.
On the otherhand, everything cannot be implemented in hardware, otherwise the cost of system will be very high. Therefore some of the tasks are performed by software program. Collection of such software programs are basically known as operating systems. So operating system is viewed as extended machine. Many more functions or instructions are implemented through software routine. The operating system is mainly memory resistant, i.e., the operating system is loaded into main memory.

Due to that, the main memory of a computer is divided into two parts. One part is reserved for operating system. The other part is for user program. The program currently being executed by the CPU is loaded into the user part of the memory. The two parts of the main memory are shown in the figure 3.17. In a uni-programming system, the program currently being executed is loaded into the user part of the memory. In a multiprogramming system, the user part of memory is subdivided to accomodate multiple process. The task of subdivision is carried out dynamically by opearting system and is known as memory management.

Figure 3.17 : Partition of main memory.