HIMEM.SYS is a device driver that allows DOS programs and drivers, data in extended memory area of the Extended Memory Specification (XMS ) to save. With the HIMEM.SYS memory range above 1 MB can be managed so that various programs and device drivers do not conflict with the use of Erweiterunsspeichers.

It also controls access to the upper memory area ( High Memory Area), which is used to load the DOS kernel or another program into the upper memory area.

The amount of usable disk space is above the 1 MB limit depends on the processor type. She is at 80286 processors maximum of 15 MB and at 80386 and 80486 processors, up to 1023 MB MS- DOS 6.x. In such systems, the drivers during the installation of MS- DOS is installed automatically.

HIMEM.SYS can be configured and customized by settings in CONFIG.SYS. To run the Windows versions 3.0 HIMEM.SYS is absolutely necessary; does not exist, the driver, Windows will not start. All other Windows versions of MS -DOS line from Windows 3.0 need the program but not the Windows NT-based versions from Windows XP. For DOS programs can run on them make the NT-based versions of Windows but a HIMEM.SYS available.