Boot Process: BIOS

Firmware runs before the Linux kernel. On PC-class hardware, the two major interfaces are legacy BIOS and UEFI. They use different boot discovery models, so “the BIOS reads the bootloader” is only a description of one path.

Legacy BIOS Boot

After early platform initialization and boot-device selection, a legacy BIOS commonly reads the first 512-byte sector from the chosen disk and transfers control to its boot code if the sector has the expected signature.

In an MBR layout, that sector contains a small boot-code region, four partition entries, and a signature. The code is too small for a feature-rich loader, so it often locates another stage elsewhere on disk or in a filesystem.

BIOS boot from a GPT disk is possible, but the protective MBR alone does not provide the loader's later stages. GRUB commonly uses a small BIOS Boot Partition on GPT for embedded core code. The exact arrangement belongs to the installed loader.

UEFI Boot

UEFI firmware can understand a defined filesystem on an EFI System Partition, or ESP, and load EFI executable files. Firmware boot entries stored in nonvolatile variables normally identify a disk, partition, and executable path. A standardized fallback path can be used for removable media or recovery scenarios.

The ESP contains boot applications and supporting files, not “all startup information.” Kernel images, initramfs files, and loader configuration can reside there or elsewhere depending on the boot design. GPT is conventional for UEFI systems, though the firmware interface and partition-table scheme remain distinct layers.

Secure Boot and Trust

With Secure Boot enabled, UEFI verifies signatures in the boot chain according to enrolled platform keys and policy. A Linux distribution can use a signed shim, boot loader, kernel, and kernel-module policy to extend this chain.

Secure Boot does not encrypt the disk and does not prove that every user-space program is safe. It helps prevent unauthorized pre-boot code from being accepted under the configured trust policy.

Entering Firmware Setup

Firmware setup keys vary by manufacturer and model, commonly including keys such as Delete, Escape, or a function key during early startup. Consult the device documentation rather than trying random changes. Some UEFI systems also expose an operating-system request to reboot into firmware setup.

Record existing values and recovery keys before changing Secure Boot, storage-controller mode, TPM, virtualization, or boot order. A firmware change can make encrypted volumes or the installed operating system temporarily inaccessible.

Summary

You can now distinguish the legacy BIOS and UEFI boot discovery models.

1. Relate legacy BIOS to first-sector boot code and later loader stages.
2. Relate UEFI boot entries to EFI executables on an ESP.
3. Treat GPT, firmware interface, and bootloader layout as separate choices.
4. Change firmware trust and storage settings only with a recovery path.