extended memory
<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)