System initialization — From where do operating systems obtain the system’s total memory information during boot on legacy PCs?

Introduction / Context:
During early boot, the operating system queries the platform for hardware configuration, including usable memory. On legacy BIOS-based PCs, firmware provides standardized interfaces to report memory maps and sizes.

Given Data / Assumptions:

• Classic BIOS environment (not pure UEFI with ACPI tables only).
• OS relies on firmware calls and/or SMBIOS/ACPI data.
• Question asks which component initializes and reports memory totals.

Concept / Approach:

The BIOS performs POST, detects installed memory, and exposes the results via INT 15h services, E820 memory maps, and SMBIOS/ACPI tables. The OS consumes these to understand total and reserved regions of RAM before establishing its own memory manager.

Step-by-Step Solution:

Verification / Alternative check:

Boot logs of many OSes show calls to firmware memory map services. UEFI successors use similar concepts via UEFI Boot Services and ACPI tables.

Why Other Options Are Wrong:

The CPU does not inventory RAM; RAM cannot self-report; “ROM” is too generic—BIOS is the specific firmware entity responsible.

Common Pitfalls:

Confusing CMOS with BIOS roles; assuming the OS magically knows memory without firmware or probing.

Final Answer:

BIOS (firmware reports memory/SMBIOS/INT 15h services).