HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory via the Extended Memory Specification. This device driver is of particular importance because various versions of Microsoft Windows that ran on top of the DOS operating system required HIMEM.SYS to be loaded to be able to run.

HIMEM.Sys provides access to memory beyond the first 1 MB of space and thus is required by Windows 9X / Me to load the graphical part of the operating system.

As in MS-DOS 5.0 1991, HIMEM.SYS was introduced and can be used to load the DOS kernel code into the high memory area HMA is to increase the available memory by specifying DOS high in CONFIG.SYS.

In DR DOS 5.0 6.0 1990 and 1991, the driver was named HIDOS.SYS not HIMEM.SYS as appropriate DCONFIG.SYS or CONFIG.SYS Directive HIDOS=on.

OS FreeDOS, the relevant file is HIMEM.EXE and can be downloaded from the FreeDOS OS configuration file FDCONFIG.SYS or CONFIG.SYS.

In Windows 3.1 and Windows 9X, there is also a command-line downloadable version HIMEM.SYS called XMSMMGR.EXE. It can download additional features of RAM after boot the system on the command line. This allows Windows setup to load, even if HIMEM.SYS not loaded.