Logical Memory and Virtual Memory
Logical memory enables the user to use large amount of memory to store data. It defines way to organize the physical memory such as RAM and cache. This enables the Operating System to arrange memory into a logical manner such as assigning a logical address. Logical address is a memory location and it is accessed by an application program. The system maps the logical address to real physical storage address. During the execution of the program, same logical address can be mapped to many different physical addresses.
Logical Memory Organization
The computer's main memory is divided into different categories such as conventional, upper and extended memory based on the motherboard, CPU and the Operating System.
It is essential to understand how the computers use these memories efficiently.
Size of the conventional memory is 640 KB and it is used by the Disk Operating System (DOS) and other programs such as WordStar, Lotus etc. This memory is also called DOS Memory or BASE Memory. DOS cannot use more than 640 KB for most of its work.
Upper Memory-of High DOS Memory lies between 640 KB and 1 MB area. There are many empty memory locations in this area that are not used by a PC. 386 or higher processor can map these empty locations to some real memory area and use them for storing small driver or memory resident programs.
The memory area lies above 1 MB limit is called Extended Memory. Extended Memory is available in 286 and later processor based computers. 286 processors can support extended memory up to 16 MB and 386 processors can support up to 4 GB. The extended memory is not much useful for the DOS users as DOS doesn't know how to use this memory area. But, for Windows and OS/2 user this memory is very useful as these operating systems can use extended memory. The user can view the memory details by typing "mem" command in the command prompt.