Editing Expanded memory (section)

==Background==
[[File:Expanded memory description 1.svg|right|150px|thumb|A section of the lower 1&nbsp;MiB address space provides a "window" into several megabytes of Expanded Memory]]

The [[Intel 8088|8088]] processor of the [[IBM PC]] and [[IBM PC/XT]] can address one [[megabyte]] (MiB, or 2<sup>20</sup> bytes) of memory. It inherited this limit from the 20-bit external address bus (and overall memory addressing architecture) of the [[Intel 8086]]. The designers of the PC allocated the lower 640&nbsp;[[Kilobyte|KiB]] ({{val|655360}} bytes) of address space for read-write program memory (RAM), called ''conventional memory'', and the remaining 384&nbsp;KiB of memory space is reserved for uses such as the system [[BIOS]], video memory, and memory on expansion peripheral boards.

Even though the [[IBM PC AT]], introduced in 1984, uses the [[80286]] chip that can address up to 16&nbsp;MiB of RAM as [[extended memory]], it can only do so in [[protected mode]]. The scarcity of software compatible with protected mode (no standard [[DOS]] applications can run in it) meant that the market was still open for another solution.<ref name="Mendelson_1989"/>

To make more memory accessible, a [[bank switching]] scheme was devised, where only selected parts of the additional memory is accessible at any given time. Originally, a single 64&nbsp;KiB (2<sup>16</sup> bytes) window of memory, called a [[page frame]], was used; later this was made more flexible. Programs are written in a specific way to access expanded memory. The ''window'' between conventional memory and expanded memory can be adjusted to access different locations within the expanded memory.

A first attempt to use a bank switching technique was made by Tall Tree Systems with its JRAM boards,<ref name="TallTree"/> but these did not catch on.<ref name="Mendelson_1989"/> (Tall Tree Systems later made EMS-based boards using the same JRAM brand.)