PC boot sequence — Which firmware/logic runs first during a normal power-on boot of a classic PC?

Introduction / Context:
Understanding the boot sequence helps diagnose startup failures, beep codes, and “no video” conditions. The process begins in immutable firmware and proceeds toward loading an operating system from storage.

Given Data / Assumptions:

• A legacy PC architecture with system firmware stored in ROM/flash.
• Standard POST (Power-On Self-Test) followed by bootstrap loader.
• Stable CMOS configuration accessible by the firmware.

Concept / Approach:

On power-on, the CPU fetches its first instruction from a predefined reset vector mapped to system firmware. The ROM BIOS (or UEFI in newer systems) initializes hardware, performs POST, reads CMOS settings, and then locates bootable media to execute the OS loader.

Step-by-Step Solution:

Verification / Alternative check:

Motherboard manuals and firmware design references document this flow. Beep codes and splash screens originate from the ROM BIOS stage.

Why Other Options Are Wrong:

“RAM BIOS” is not a standard component; CMOS holds settings but does not “run.” The hard disk boot sector code executes only after firmware hands off control. “None of the above” is invalid.

Common Pitfalls:

Confusing CMOS data with the executing BIOS code; overlooking that modern UEFI replaces legacy BIOS but serves the same initial role.

Final Answer:

ROM BIOS (firmware executes POST and bootstrap).