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Computing (FOLDOC) dictionary
extended memory
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storage Memory above the first megabyte of address space
in an IBM PC with an 80286 or later processor.
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
prescribed method for an MS-DOS program to access extended
memory under a multitasking environment.