The Lotus/Intel/Microsoft Expanded Memory Specification is a functional definition of a bank-switched memory-expansion subsystem. It consists of hardware expansion modules and a resident driver program specific to those modules. In EMS versions 3.0 and 3.2, the expanded memory is made available to application software as 16 KB pages mapped into a contiguous 64 KB area called the page frame, somewhere above the main memory area used by MS-DOS/PC-DOS (0—640 KB). The exact location of the page frame is user configurable, so it need not conflict with other hardware options. In EMS version 4.0, the pages may be mapped anywhere in memory and can have sizes other than 16 KB.
The EMS provides a uniform means for applications to access as much as 8 megabytes of memory (32 megabytes in EMS 4.0). The supporting software, which is called the Expanded Memory Manager (EMM), provides a hardware-independent interface between application software and the expanded memory board(s). The EMM is supplied in the form of an installable device driver that you link into the MS-DOS/PC-DOS system by adding a line to the CONFIG.SYS file on the system boot disk.
Internally, the Expanded Memory Manager consists of two major portions, which may be referred to as the driver and the manager. The driver portion mimics some of the actions of a genuine installable device driver, in that it includes initialization and output status functions and a valid device header. The second, and major, portion of the EMM is the true interface between application software and the expanded-memory hardware. Several classes of services are provided:
Verification of functionality of hardware and software modules
Allocation of expanded-memory pages
Mapping of logical pages into the physical page frame
Deallocation of expanded-memory pages
Support for multitasking operating systems
Application programs communicate with the EMM directly, by means of software Int 67H. MS-DOS versions 3.3 and earlier take no part in (and in fact are completely oblivious to) any expanded-memory manipulations that may occur. MS-DOS version 4.0 and Microsoft Windows, on the other hand, are "EMS-aware" and can use the EMS memory when it is available.
Expanded memory should not be confused with extended memory. Extended memory is the term used by IBM to refer to the memory at physical addresses above 1 megabyte that can be accessed by an 80286 or 80386 CPU in protected mode. Current versions of MS-DOS run the 80286 and 80386 in real mode (8086-emulation mode), and extended memory is therefore not directly accessible.