Legacy DOS/PC memory map: What is the size of the “reserved” (upper) memory area between 640 KB and 1 MB in the IBM PC-compatible architecture?

Introduction / Context:
Early IBM PC-compatible systems divided the first megabyte of address space into conventional memory and upper (reserved) memory. Understanding these historical boundaries helps explain legacy constraints, device drivers, and the origins of terms like “conventional memory.”

Given Data / Assumptions:

• Total first address space considered: 0 to 1,024 KB (1 MB).
• Conventional memory: 0 to 640 KB for applications and DOS.
• Upper memory area (UMA): 640 KB to 1,024 KB reserved for BIOS, video RAM, and device ROMs.

Concept / Approach:

The classic PC memory map allocates 640 KB to software and reserves the remaining space up to 1 MB for system use. The difference 1,024 KB - 640 KB equals 384 KB. This reserved block hosts BIOS, option ROMs, and memory-mapped I/O such as VGA RAM, hence the term “640 KB barrier.”

Step-by-Step Solution:

Verification / Alternative check:

Legacy documentation and DOS memory utilities show conventional memory ending at 640 KB and the UMA occupying the remainder to 1 MB, confirming a 384 KB reserved region.

Why Other Options Are Wrong:

• 64 kb / 640 kb / 1,024 kb: Do not match the calculated difference between 1 MB and 640 KB.
• None of the above: Incorrect because 384 KB is correct.

Common Pitfalls:

Confusing “reserved memory” with extended memory above 1 MB; assuming all upper memory is unusable (UMA can host Upper Memory Blocks for drivers via memory managers).

Final Answer:

384 kb