In legacy MS-DOS memory management, where are Upper Memory Blocks (UMBs) located within the PC's address space?

Introduction / Context:
Understanding the classic IBM PC memory map is essential for configuring DOS-era systems and troubleshooting legacy software. Upper Memory Blocks (UMBs) were a key optimization in MS-DOS to free precious conventional memory for applications by loading drivers and TSRs high. Knowing where UMBs reside helps explain HIMEM.SYS/EMM386.EXE configuration and why some drivers can be loaded with DEVICEHIGH or LOADHIGH.

Given Data / Assumptions:

• Environment: DOS/real-mode memory map.
• Conventional memory spans 0 to 640 KB.
• The address space between 640 KB and 1 MB is often called the Upper Memory Area (UMA).

Concept / Approach:

The PC's first 1 MB address space is split into conventional memory (0–640 KB) and the UMA (640 KB–1 MB). The UMA is largely reserved for BIOS ROMs, video memory, and adapter ROMs. Memory managers (e.g., EMM386) can reclaim unused holes in this reserved region as Upper Memory Blocks, creating allocatable chunks where drivers/TSRs can be loaded “high.” Therefore, UMBs are carved from the reserved region, not from extended (XMS) or expanded (EMS) directly, although managers may map XMS into UMA holes.

Step-by-Step Solution:

Verification / Alternative check:

Examining MEM /C /P output on DOS shows UMB regions located between segment A000 and F000, confirming their placement in the reserved portion above 640 KB.

Why Other Options Are Wrong:

• Conventional Memory: 0–640 KB; UMBs are not here.
• Extended Memory: memory above 1 MB (XMS), not where UMBs physically reside.
• Expanded memory: EMS is a paging scheme via a page frame, conceptually different from UMB placement.
• None of the above: incorrect because “Reserved Memory” is correct.

Common Pitfalls:

• Confusing EMS (expanded) with UMBs because both involve memory managers.
• Assuming UMBs are the same as extended memory; they are mapped within UMA.

Final Answer:

Reserved Memory.