The Extended Memory (XMS)

The extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS programs, running in real mode or virtual x86 mode, cannot directly access this memory, but are able to do so through an application programming interface called the eXtended Memory Specification (XMS).
With the exception of the first 65,520 bytes, extended memory is not accessible to a PC when running in real mode. This means that under normal DOS operation, extended memory is not available at all; protected mode must be used to access extended memory.

Note: Extended memory is different from expanded memory (EMS), which uses bank switching and a page frame in the upper memory area to access memory over 1 MB.

There are two ways that extended memory is normally used. A true, full protected mode operating system like Windows NT, can access extended memory directly. However, operating systems or applications that run in real mode, including DOS programs that need access to extended memory, Windows 3.x, and also Windows 95, must coordinate their access to extended memory through the use of an extended memory manager. The most commonly used manager is HIMEM.SYS, which sets up extended memory according to the extended memory specification (XMS). XMS is the standard that PC programs use for accessing extended memory.
The main uses extended memory are:
- RAM-disks : It is a chunk of semiconductor memory that behaves like an ordinary disk but is extremely fast. It also loses its data instantly once power is turned off but is great for temporary files such as index files, extracted data from Lotus to be imported into another application etc.
- Disk caches : It is a program to speed up disk access by storing the most frequently use information in the computer's memory and reading ahead from the disk in anticipation.
- Print spoolers : It utilizes the computer's memory as a high speed buffer so that a fast computer is not slowed down by a slow printer. For example you can print a 100 page database report and then load a spread sheet program, print reports and graphs, then use your word processor while the database report is still printing. Print spoolers that use extended memory usually come with the memory card.
- OS/2 : The operating system OS/2 can make good use of extended memory.
- UNIX : UNIX is another operating system that can use extended memory.