Source : Free On-Line Dictionary of Computing

extended memory
     
         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)