extended memory

extended memory

The name given to memory in an Intel PC above 1MB. Starting with the Intel 286, it was used directly by Windows and OS/2 as well as DOS applications that ran with DOS extenders. It was also used under DOS for RAM disks and disk caches. Contrast with expanded memory (EMS), which was specialized memory above 1MB. Today, the term is rarely heard, because the 1MB barrier was broken long ago. See EMS, XMS and DOS extender.

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(storage)

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.