View source for LIM Expanded Memory Specification V4: Introduction


Because even the maximum amount (640K bytes) of conventional memory isn't always enough for large application programs, Lotus Development Corporation, Intel Corporation, and Microsoft Corporation created the Lotus/Intel/Microsoft (LIM) Expanded Memory Specification.

The LIM Expanded Memory Specification defines the software interface between the Expanded Memory Manager (EMM) -- a device driver that controls and manages expanded memory -- and application programs that use expanded memory.

== WHAT IS EXPANDED MEMORY? ==

Expanded memory is memory beyond DOS's 640K-byte limit.  The LIM specification supports up to 32M bytes of expanded memory.  Because the 8086, 8088, and 80286 (in real mode) microprocessors can physically address only 1M byte of memory, they access expanded memory through a window in their physical address range.  The next section explains how this is done.

== HOW EXPANDED MEMORY WORKS ==

Expanded memory is divided into segments called logical pages.  These pages are typically 16K-bytes of memory.  Your computer accesses logical pages through a physical block of memory called a page frame. The page frame contains multiple physical pages, pages that the microprocessor can address directly.  Physical pages are also typically 16K bytes of memory.

This page frame serves as a window into expanded memory.  Just as your computer screen is a window into a large spreadsheet, so the page frame is a window into expanded memory.

A logical page of expanded memory can be mapped into (made to appear in) any one of the physical pages in the page frame.  Thus, a read or write to the physical page actually becomes a read or write to the associated logical page.  One logical page can be mapped into the page frame for each physical page.

The page frame is located above 640K bytes.  Normally, only video adapters, network cards, and similar devices exist between 640K and 1024K.

This specification also defines methods for operating systems and environments to access expanded memory through physical pages below 640K bytes.  These methods are intended for operating system/environment developers only.

== LIM Expanded Memory Specification V4 Contents ==

*[[LIM Expanded Memory Specification V4 | Title and Contents]]
*Chapter 1: Introduction (this page)
*[[LIM Expanded Memory Specification V4: Writing Programs That Use Expanded Memory | Chapter 2: Writing Programs that use Expanded Memory]]
*[[LIM Expanded Memory Specification V4: Emm Function Reference | Chapter 3: EMM Function Reference]]
*[[LIM Expanded Memory Specification V4: Appendix A | Appendix A]]
*[[LIM Expanded Memory Specification V4: Appendix B | Appendix B]]
*[[LIM Expanded Memory Specification V4: Appendix C | Appendix C]]
*[[LIM Expanded Memory Specification V4: Appendix D | Appendix D]]
*[[LIM Expanded Memory Specification V4: Glossary | Glossary]]

== See Also ==

*[[Lo-tech 2MB EMS Board]]