extended memory


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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.

extended memory

The name given to memory (RAM) in an Intel PC above 1MB (one megabyte). Starting with the Intel 286, extended memory 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, most people never heard of extended or expanded memory, because the 1MB barrier was broken long ago, and thousands of megabytes (MBs) of memory are commonly used. See EMS, XMS and DOS extender.


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SYS is the special program th,at makes it possible to address extended memory.
The advantage of using extended memory is that it can be addressed directly and thus is much faster than expanded memory.
To add to user confusion, the first 64k of exTEnded memory (RAM) just above the first megabyte) is known as the High Memory Area (HMA).
The standard mode is designed for 80286 processors and will allow your computer to access the entire 16Mb of extended memory.
The 80486 can address more than 4000 megabytes (4 gigabytes) of extended memory, a long way from the original PC's 1 megabyte.
Up to 15 megabytes (megs) of extended memory can be added to the older microcomputers.
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95, you get a program capable of defining the 384K of extended memory (or any portion of it) as expanded memory, which means that it can be accessed by Lotus-Intel-Microsoft Expanded Memory System standard.
If above-board memory is removed from a computer, it may go undetected until someone attempts to access the extended memory.
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