HIMEMSYS

HIMEM.SYS is a DOS device driver that implements the Extended Memory Specification (XMS) and provides controlled access to memory beyond the 1 MB boundary on x86 systems. It enables applications and system components to allocate and use extended memory, and coordinates with other memory managers to expose memory as XMS memory and, when available, as Upper Memory Blocks.

In practice, HIMEM.SYS is loaded from the system configuration file with a line such as device=HIMEM.SYS. It must be loaded before other memory managers that rely on XMS, and it is responsible for enabling access to the High Memory Area (HMA) and for managing access to extended memory. On many systems running DOS with extended memory, HIMEM.SYS works in concert with other memory managers such as EMM386.EXE; EMM386 can create Upper Memory Blocks from XMS memory, and DOS itself can run in the High Memory Area when available.

HIMEM.SYS supports a set of optional parameters that control its behavior, such as memory testing, relocation, and the amount of memory made available to XMS and to other drivers. The exact options may vary by DOS version and vendor.

Limitations and considerations: enabling extended memory access reduces conventional memory available to DOS. Some BIOSes or hardware configurations may require specific parameters or loading order to avoid conflicts. On modern systems, HIMEM.SYS is largely obsolete but remains for compatibility with legacy software and configurations.

See also: XMS, EMS, EMM386, DOS memory management.