Understanding BIOS, UEFI, MBR, GPT, and GRUB: Key Concepts for Linux Boot

01 Preface

Before learning Linux system boot principles, we first understand several concepts related to OS startup.

02 Concepts Related to OS Startup

Both Windows and Linux run on physical hardware; before the OS starts, the hardware is detected, then the disk boots the OS. Below are key concepts.

2.1 BIOS

BIOS (Basic Input Output System) is a set of programs stored in a ROM chip on the motherboard; it reads/writes system settings from CMOS, provides basic I/O, self‑test, and system startup functions.

In short, BIOS controls all input/output devices, adjusting parameters for keyboard, mouse, interfaces, frequency, power, and disk drives.

2.2 UEFI

UEFI (Unified Extensible Firmware Interface) replaces traditional BIOS, offering easier implementation, stronger fault tolerance, and error correction.

Traditional BIOS mainly supports MBR boot; UEFI adds support for new hardware, including disks larger than 2 TB.

2.3 MBR and GPT

New disks must be partitioned; the two main partition schemes are MBR and GPT .

2.3.1 MBR

MBR is the traditional partition table. During boot, BIOS reads the MBR boot record, which then loads the OS. Its main limitation is lack of support for disks larger than 2 TB.

Example of a disk showing an MBR partition:

Check your own disk partition type using the method below.

1、win+R，输入diskmgmt.msc，回车，打开磁盘管理器。<br/>2、在磁盘上点右键，选属性。<br/>3、在卷这一页，可以看到是GTP还是MBR。

2.3.2 GPT

GPT is a newer partition scheme that overcomes MBR’s limits, supporting up to 18EB disks and relying on UEFI.

All Windows versions support MBR, while GPT requires a 64‑bit system. BIOS only supports MBR; GPT can be used only with UEFI. Modern motherboards often integrate BIOS with UEFI to support both.

2.3.3 Summary

BIOS+MBR : Traditional, universally supported; cannot handle disks >2 TB.

BIOS+GPT : BIOS can use GPT disks as data drives but cannot boot from them; on systems with both <2 TB (MBR) and >2 TB (GPT) drives, the OS must be 64‑bit.

UEFI+MBR : UEFI can operate in Legacy mode to support MBR boot, or create a FAT partition for UEFI files, enabling dual‑mode boot.

UEFI+GPT : Required to boot a system from a >2 TB disk; also requires a 64‑bit OS.

2.4 GRUB

GRUB (GRand Unified Bootloader) is a multi‑OS boot manager that lets users select among several installed operating systems at startup.

GRUB can choose different kernels on a partition and pass boot parameters to them. It is a multi‑system boot manager for Windows, Linux, etc. Common Linux bootloaders include LILO, GRUB, and GRUB2. The diagram below shows the GRUB loading process.

CentOS 7 uses GRUB2 by default; GRUB2 is a more secure, modular evolution of GRUB, used by the latest Linux distributions and eliminates the need for separate stages 1, 1.5, and 2.

After understanding these concepts, we will explore the Linux boot process tomorrow.