MS-DOS 6.22 EMS Emulation from XMS Memory Driver

In MS-DOS 6.22, running software that requires Expanded Memory Specification (EMS) on modern or extended-memory-equipped systems requires an expanded memory manager. This article identifies the specific driver responsible for emulating EMS out of Extended Memory (XMS), details the prerequisite drivers, and explains the exact CONFIG.SYS configuration necessary to enable and control this behavior.

The EMS Emulation Driver: EMM386.EXE

The driver that enables virtual EMS emulation from XMS memory in MS-DOS 6.22 is EMM386.EXE.

Operating on Intel 80386 and later processors, EMM386.EXE utilizes the CPU's Virtual 8086 mode to map pages of extended memory (XMS) into the Upper Memory Area (UMA)—the region between 640 KB and 1024 KB. This process establishes a 64 KB EMS page frame (typically at segment address E000 or D000), divided into four 16 KB pages, providing full LIM EMS 4.0 compatibility to DOS programs without requiring dedicated hardware expanded memory boards.

Prerequisites: HIMEM.SYS

EMM386.EXE cannot function independently because it relies on existing XMS memory to create the virtual EMS pool. Therefore, HIMEM.SYS, the default MS-DOS extended memory manager, must be loaded prior to EMM386.EXE.

Configuring CONFIG.SYS for EMS Emulation

To activate EMS emulation, both drivers must be added to your CONFIG.SYS file in the proper sequence.

DEVICE=C:\DOS\HIMEM.SYS
DEVICE=C:\DOS\EMM386.EXE RAM

Key Parameters:

  • RAM: Instructs EMM386.EXE to provide both EMS emulation and Upper Memory Blocks (UMBs). UMBs allow you to load drivers and TSR programs into high memory using the DEVICEHIGH and LOADHIGH commands.
  • Memory Size (Optional): By default, EMM386.EXE dynamically allocates as much EMS as requested by running applications. To set a specific allocation limit in kilobytes, specify the number before the parameters: 
    DEVICE=C:\DOS\EMM386.EXE 4096 RAM
    This restricts EMS allocation to 4,096 KB (4 MB).
  • NOEMS (Contrast): If your system only requires UMBs and does not require expanded memory, specifying NOEMS disables the 64 KB page frame entirely, freeing up that address space for loading high drivers while disabling EMS emulation.

Verifying the Configuration

After rebooting the system with the configured CONFIG.SYS, verify that EMS emulation is active by running the following command at the DOS prompt:

MEM

The output will display the total, used, and free expanded memory available, confirming that EMM386.EXE is successfully emulating EMS from XMS.