Wow, this is an old subject. HIMEM.SYS is an extention to the memory manager in the DOS operating system. HIMEM.SYS gained access to memory above 640K on Intel 80286 and later based IBM PC and Compatible computers. It allowed the operating system to use memory above 1024 kilobytes as extended memory. It needed to be loaded into the config.sys script used in the boot sequence ahead of the second extention to the memory manager named EMM386.SYS. The EMM386.SYS added LIM 3.0 expanded memory, and access to the upper memory block (RAM between 640k and 1024k) on 80386 class and later PC computers equipped with a BIOS that enabled a process called "Shadow RAM". This program was an issue until Windows XP eliminated the need for DOS to be a shell below windows. COnsider your life enriched that you don't have to deal with memory management on computers like we used to back when DOS ruled.