Legacy DOS memory manager: what is the primary function of the device driver HIMEM.SYS?

Introduction / Context:
HIMEM.SYS is a classic MS-DOS and early Windows component that enabled systems to utilize extended memory beyond the 1 MB real-mode boundary. Knowing what it actually manages helps distinguish extended memory (XMS) from expanded memory (EMS) and other device drivers.

Given Data / Assumptions:

• DOS-era PC with CONFIG.SYS loading HIMEM.SYS.
• 386 or later CPU may also use EMM managers for EMS, but that is separate.
• ANSI.SYS and display code pages are unrelated drivers.

Concept / Approach:
HIMEM.SYS implements the XMS (Extended Memory Specification), controls access to memory above 1 MB, and enables use of the High Memory Area (HMA). It is not an ANSI terminal driver, not a code-page driver, and not the EMS page-frame manager used for expanded memory (that role belongs to EMM386.EXE or similar).

Step-by-Step Solution:

Verification / Alternative check:
MS-DOS MEM command shows XMS available once HIMEM.SYS is active; removing the driver eliminates XMS access.

Why Other Options Are Wrong:

• ANSI emulation belongs to ANSI.SYS.
• Expanded memory requires EMM386.EXE; HIMEM.SYS handles extended memory.
• Code page switching is managed by DISPLAY.SYS and related utilities.

Common Pitfalls:
Confusing EMS with XMS. EMS uses paging in a 64 KB page frame; XMS uses linear extended memory and HMA.

Final Answer:
Manages the use of extended memory