When you press the power button, there is no operating system. There is not even anything in RAM — it is volatile, so it woke up full of noise. Something has to take a machine that knows nothing and get an operating system loaded and running, and that something is firmware: code stored in non-volatile memory on the motherboard itself.
What the firmware does
- Power good — the PSU tests its own rails and sends a Power Good signal. Until it arrives, the CPU is held in reset.
- POST (Power-On Self-Test) — the firmware checks that the CPU, RAM and video are present and responding. A failure here is reported by beeps or diagnostic codes, because there may be no working display to report on.
- Hardware inventory — the firmware detects drives, USB controllers and expansion cards, and reads its saved settings.
- Boot device selection — it walks the configured boot order looking for a device with a bootloader.
- Hand-off — it loads the bootloader, which loads the OS kernel into RAM and transfers control. Firmware's job is now finished.
- Kernel initialization — the kernel starts, loads drivers, mounts the file system, and starts user-space services until a login appears.
Why beep codes exist
POST tests memory and video before it can use the screen. If RAM or the graphics adapter fails, there is no way to display an error — so the firmware uses the one output device guaranteed to work: a small speaker. Repeating beeps almost always mean RAM or video. Modern boards use diagnostic LEDs or a two-digit POST code display for the same reason.
BIOS and UEFI
ALegacy BIOS
- 16-bit code, text-only interface
- MBR partitioning: maximum 2 TiB drives, 4 primary partitions
- Boots by running code in the first sector of the drive
- No built-in verification of what it loads
BUEFI
- 32/64-bit, can have a graphical interface and mouse support
- GPT partitioning: enormous drives, 128 partitions
- Boots a proper .efi executable from a dedicated EFI System Partition
- Secure Boot can verify a bootloader's digital signature
Every current machine uses UEFI, though most offer a compatibility mode (CSM) for booting older systems. The practical consequence you will meet is that a Windows installer prepared for UEFI will not boot a machine set to legacy mode, and vice versa — one of the most common reasons an installation USB is "not detected".
Settings worth knowing
| Setting | What it does | When you change it |
|---|---|---|
| Boot order | Which device the firmware tries first | Installing an OS from USB |
| Secure Boot | Only loads digitally signed bootloaders | Turn off to install some Linux distributions |
| SATA mode (AHCI/RAID) | How the chipset presents drives | Changing after installing Windows causes a boot failure |
| XMP / DOCP | Runs RAM at its advertised speed | New build — without it, fast RAM runs at a slow default |
| Virtualization (VT-x / AMD-V) | Allows hardware-accelerated VMs | Running virtual machines or Android emulators |
| CSM / Legacy boot | Emulates old BIOS behaviour | Booting an older OS or an MBR-formatted installer |
The CMOS battery
Firmware settings and the real-time clock are kept in low-power memory maintained by a CR2032 coin cell. When that battery dies, the machine still works but forgets its settings and resets its clock at every power-off. A computer that boots with the date set to 2010 and the boot order reset almost always needs a $5 battery, not a new motherboard.
Firmware updates
A firmware update can add support for a newer CPU or fix a bug, but an interrupted update can leave the board unbootable. Never update firmware unless you have a specific reason, never during a thunderstorm or on an unprotected desktop, and always use the exact file for your board revision.