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)