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Extended memory is not directly available in real mode, only through EMS, UMB, XMS, or HMA; only applications executing in protected mode can use extended memory directly. In this case, the extended memory is provided by a supervising protected-mode operating system such as Microsoft Windows. The processor makes this memory available through a system of global descriptor tables and local descriptor tables . The memory is "protected" in the sense that memory assigned a local descriptor cannot be accessed by another program without causing a hardware trap. This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Windows can also run real-mode programs and provide expanded memory to them. DOS Protected Mode Interface is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitaskingMultitasking is the apparent simultaneous performance of two or more tasks by a computer's Central Processing Unit. See also: Computer multitasking Time-sharing Pre-emptive multitasking Co-operative multitasking Multithreading 2. Derived from the first se environment.

Having extended memory does not necessarily mean that you have more than one megabyte of memory since the reserved memory area may be partially empty. In fact, if your 386 or higher uses extended memory as expanded memory then that part is not in excess of 1Mb.

See also: conventional memoryConventional 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 b.

This article was originally based on material from the Free On-line Dictionary of ComputingThe Free On-line Dictionary of Computing FOLDOC is an on-line, searchable encyclopedic dictionary of computing subjects. It was founded in 1985 by Denis Howe and is hosted by Imperial College, London. Denis has served as the Editor-in-Chief since the dict, used with permission . Update as needed.