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In DOS memory management, expanded memory is a system of bank switching that provided additional memory to DOS programs beyond the limit of conventional memory (640 KB). ''Expanded memory'' is an umbrella term for several incompatible technology variants. The most widely used variant was the Expanded Memory Specification (EMS), which was developed jointly by Lotus Software, Intel, and Microsoft, so that this specification was sometimes referred to as "LIM EMS." LIM EMS had several versions. The first widely implemented version was EMS 3.2, which supported up to 8 MB of expanded memory and uses parts of the address space normally dedicated to communication with peripherals (upper memory) to map portions of the expanded memory. EEMS, an expanded memory management standard competing with LIM EMS 3.x, was developed by AST Research, Quadram and Ashton-Tate; it could map any area of the lower 1 MB. EEMS ultimately was incorporated in LIM EMS 4.0, which supported up to 32 MB of expanded memory and provided some support for DOS multitasking as well. IBM however created its own expanded memory standard called XMA. The use of expanded memory became common with games and business programs in the late 1980s through the mid-1990s, but its use declined as users switched from DOS to protected mode operating systems such as Linux, IBM OS/2 and Microsoft Windows. == Background == The 8088 processor of the IBM PC and IBM PC/XT could address one Megabyte (MB or 220 bytes) of memory. It inherited this limit from the 20-bit external address bus of the Intel 8086. The designers of the PC allocated the lower 640 kB (655,360 bytes) of address space for read-write program memory (RAM), called "conventional memory", and the remaining 384 kB of memory space was 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, used the 80286 chip that could address up to 16 MB of RAM as extended memory, it could only do so in protected mode. The scarcity of software compatible with the 286 protected mode, (none of the MS-DOS applications could run in it), meant that the market was still open for another solution. To fit potentially much more memory than the 384 kB of free address space would allow, a bank switching scheme was devised, where only selected parts of the additional memory would be accessible at the same time. Originally, a single 64 kB (216 bytes) window of memory, called a page frame, was possible; later this was made more flexible. Programs had to be written in a specific way to access expanded memory. The 'window' between lower RAM and expanded RAM could be moved around to different locations within the Expanded RAM. A first attempt to use a bank switching technique was made by Tall Tree Systems with their JRAM boards, but these did not catch on.〔 (Tall Tree Systems later made EMS-based boards using the same JRAM brand.) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Expanded memory」の詳細全文を読む スポンサード リンク
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