DOS Memory Management

In IBM PC compatible computing, DOS memory management refers to software and techniques employed to give applications access to more than 640 kibibytes (KiB) of "conventional memory". The 640KiB limit was specific to the IBM PC and close compatibles; other machines running MS-DOS had different limits, for example the Apricot PC could have up to 768KiB and the Sirius Victor 9000, 896KiB. Memory management on the IBM family was made complex by the need to maintain backward compatibility to the original PC design and real-mode PC DOS (MS-DOS), while allowing computer users to take advantage of large amounts of low-cost memory and new generations of processors. Since MS-DOS has given way to Microsoft Windows and other 32-bit operating systems not restricted by the original arbitrary 640KiB limit of the IBM PC, managing the memory of a personal computer no longer requires the user to manually manipulate internal settings and parameters of the system.

The 640KiB limit imposed great complexity on hardware and software intended to circumvent it; the physical memory in a machine could be organised as a combination of base or conventional memory, upper memory, high memory (not the same as upper memory), extended memory, and expanded memory, all handled in different ways.