EMM386EXE

EMM386.EXE is a DOS device driver that provides Expanded Memory (EMS) support on x86 PCs. It functions as a memory manager in tandem with HIMEM.SYS, allowing programs that require EMS to run on systems with limited conventional memory. EMM386 creates a virtual EMS environment by paging portions of extended memory into an EMS address space, enabling software to access EMS memory without occupying conventional memory.

In addition to EMS support, EMM386 can help optimize conventional memory usage by creating upper memory blocks (UMBs) in the upper memory area. This allows users to load drivers and memory-resident programs into high memory, freeing conventional memory for applications. EMM386 can be configured to enable or disable EMS features, manage UMBs, and control how memory is allocated, with the exact switches varying by DOS version and OEM distribution. It is typically loaded from CONFIG.SYS and works in concert with HIMEM.SYS.

Usage of EMM386 was common in the late 1980s and 1990s, particularly for games and software that demanded EMS memory on systems with limited conventional memory. As DOS-era memory management evolved and Windows-based environments became more prevalent, the reliance on EMS and on EMM386 declined, though the utility remains a notable part of the DOS memory-management ecosystem.

See also: Expanded Memory Specification (EMS), XMS (eXtended Memory Specification), HIMEM.SYS.