expanded memory

   An MS-DOS mechanism by which applications can access more than the 640KB of memory normally available to them. The architecture of the early Intel processors restricted the original IBM PC to accessing 1MB of memory, 640KB of which was available for applications; the remaining 384KB was reserved for system use, the BIOS, and the video system. At that time, 640KB was more than ten times the amount of memory available in other personal computers. However, as both applications and MS-DOS grew, they began to run out of room.

   The Expanded Memory Specification LIM 4.0 let programs running on the Intel family of processors access as much as 32MB of expanded memory. The expanded memory manager (EMM) creates a block of addresses into which data (held in memory above the 1MB limit) is swapped in and out as needed by the program. In other words, a 64KB segment of addressable memory creates a small window through which segments of expanded memory can be seen, but only one segment at a time.