Details
An expanded memory board, being a hardware peripheral, needed a software device driver, which exported its services. Such a device driver was called expanded memory manager. Its name was variable; the previously mentioned boards used REMM.SYS (AST), PS2EMM.SYS (IBM), AEMM.SYS (AT&T) and EMM.SYS (Intel) respectively. Later, the expression became associated with software-only solutions requiring the Intel 80386 processor, for example Quarterdeck's QEMM, Qualitas' 386MAX or the default EMM386 in MS-DOS, PC DOS and DR-DOS.
Expanded memory was a common term for several incompatible technology variants. EEMS, an expanded memory management standard competing with LIM EMS, was developed by AST Research, Quadram and Ashton-Tate. It allowed to also remap some or all of the lower 1024 KiB of memory not associated with interrupts or dedicated I/O memory (such as network or video cards), so that entire programs could be switched in and out of the extra RAM. This feature was used by early DOS multitasker software such as DESQview. The two standards were eventually combined as LIM EMS 4.0.
Expanded Memory usage declined in the 1990s. The IBM AT Intel 80286 supported 24 bits of address space (16 MiB) in protected mode and the 386 supported 32 bit addresses, or 4 gibibytes (GiB) of RAM - roughly four thousand times the addressable space of the original 8086. DOS itself did not directly support protected mode, but Microsoft eventually developed DPMI and several DOS extenders were published based on it. DOS programs like DOOM could use extenders like DOS/4G to run in protected mode while still using the DOS API. In the early 1990s new operating systems like Linux, Microsoft Windows NT, OS/2, and BSD/OS supported protected mode 'out of the box'. These and similar developments rendered Expanded Memory an obsolete concept.
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