HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS).
The memory beyond the first 1 MB of address space is required by Windows 9x/Me in order to load; therefore, these versions of Microsoft Windows require HIMEM.SYS to be loaded to be able to run.
It is also plug compatible with MS-DOS HIMEM.SYS, offering additional control parameters.
It can load extended memory services after the system boots into the command prompt.
The hard bug exists in recent versions of HIMEM.SYS from MS-DOS and Windows 9x for handling /a20control:off option causing a hang-up.