PC BIOS/CMOS setup: Which hard-drive related values are stored in CMOS setup for legacy systems (prior to automatic LBA detection)?

Introduction / Context:
Older PCs required manual entry of hard-disk geometry in CMOS setup so the BIOS could address the drive correctly at boot. Understanding which parameters belong in CMOS helps when working with legacy hardware.

Given Data / Assumptions:

• Legacy BIOS with CMOS RAM storing configuration.
• Hard disk uses CHS parameters or later LBA translation.
• Modern SMART/free space tracking is OS-level, not CMOS.

Concept / Approach:

CMOS setup records drive geometry: number of cylinders, heads, sectors per track, and sometimes landing zone and translation mode. These values allow BIOS INT 13h routines to read/write sectors during POST/boot.

Step-by-Step Solution:

Verification / Alternative check:

Incorrect geometry prevents OS boot or causes data corruption. Autodetect routines in later BIOS versions populate CMOS with LBA-translated values automatically.

Why Other Options Are Wrong:

• IRQ: Interrupt lines for controllers are separate settings; not the drive geometry itself.
• Free space: Calculated by OS file systems; not stored in CMOS.
• Virus alert: Optional BIOS feature toggles, unrelated to drive geometry values.
• None of the above: Incorrect since geometry is stored.

Common Pitfalls:

Confusing controller settings with disk geometry; removing CMOS battery resets parameters leading to boot failures on legacy systems.

Final Answer:

size (heads, cylinders, sectors)