HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory via the Extended Memory Specification (XMS). 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 the memory beyond the first 1 MB of space and thus is required by Windows 9x/Me in order to load the graphical portions of the operating systems.
As of MS-DOS 5.0 (1991), HIMEM.SYS was introduced and could be used to load the DOS kernel code into the High Memory Area (HMA) to increase the amount of available conventional memory by specifying DOS=HIGH in CONFIG.SYS.
In DR DOS 5.0 (1990) and 6.0 (1991), the driver was named HIDOS.SYS rather than HIMEM.SYS, like the corresponding DCONFIG.SYS or CONFIG.SYS directive HIDOS=ON.
In FreeDOS, the matching file is named HIMEM.EXE and can be loaded from the FreeDOS configuration file named FDCONFIG.SYS or CONFIG.SYS.
In Windows 3.1 and Windows 9x, there is also a command-line loadable version of HIMEM.SYS called XMSMMGR.EXE. It can load extended memory services after the system boots into the command prompt. This allows Windows Setup to load even if HIMEM.SYS is not loaded.