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)