Conventional memory - Definition

Conventional Memory is the first 640 kilobytes of an IBM PC's memory. Prior to extended memory such as EMS, XMS, and HMA, real mode applications could use only this part of the memory. Even when extended memory is present, real-mode applications have to be specially programmed to use it using a complex system of overlays; consequently, many applications continued to use only conventional memory.

With modern operating systems and 32-bit machines the distinction between conventional and extended memory is rarely relevant, since all memory can be accessed uniformly in protected mode, but for older real mode OSes such as MS-DOS it was often worthwhile to tune the system to load drivers and other system components into extended memory in order to leave as much conventional memory as possible free for running large applications.