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 them.
DOS Protected Mode Interface is
Microsoft's prescribed method for an
MS-DOS program to access extended memory under a
multitasking 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 memory.
(1996-01-10)