Analyzing BIOS Firmware Boot Process from a Beginner's Perspective
Understanding BIOS from a Beginner’s Perspective – A Brief Analysis of BIOS Firmware Boot Process
BIOS firmware and UEFI firmware are the two types of firmware used in modern computers. Older machines use BIOS motherboards, while newer machines use UEFI motherboards.
Although more and more older machines are being phased out, during computer repair work, we may still encounter a machine from over a decade ago running BIOS and feel completely stuck.
Today, I’ll walk through the boot process of BIOS firmware for your reference.
(For the UEFI firmware boot process, please refer to my articles #19, #20, and #21: “Windows Computer Boot Process Analysis Part 1/2” and “21 Windows System Installation Summary”)
What is BIOS?
BIOS stands for “Basic Input Output System.” On IBM PC compatible systems, it is an industry-standard firmware interface. BIOS is the first software loaded when a computer starts up.
(Content above sourced from Baidu Baike)
Similar to UEFI, the primary function of BIOS is to load basic configuration before starting the Windows system, thereby booting the Windows operating system.
BIOS Boot Process
Disk Requirements
As we know, an operating system must typically be installed on a disk.
BIOS reads a series of files and code on the disk to correctly boot the operating system. Therefore, as an industry-standard firmware interface, the disk must meet BIOS’s requirements. Otherwise, BIOS simply won’t “recognize” the disk.
So, what exactly does BIOS require from a disk?
- The disk must use the MBR partition table.
Partition Tables:
Currently, there are two types of disk partition tables: MBR and GPT (also called GUID). MBR is an older partition table type, while GPT is a newer one.
BIOS firmware can only recognize the MBR partition table, while UEFI firmware can recognize both.
These two partition table types can be converted to each other without affecting the existing data on the hard drive (lossless conversion).
Disks using the MBR partition table support a maximum of 4 primary partitions and can only recognize disks up to 2TB. However, more partitions can be created by using logical partitions.
Disks using the GPT partition table support up to 128 partitions and can recognize disks up to 9.4ZB.
Clearly, GPT is a more advanced partition table type, offering more flexible management.
As long as the disk uses the MBR partition table and is intact, it can be recognized by BIOS.
Master Boot Record (MBR)
First, note a naming conflict here:
The Master Boot Record is called MBR. There is also a disk partition table type called MBR. To avoid confusion, we’ll refer to the Master Boot Record simply as “MBR” and the partition table type as “MBR partition table.”
What is MBR?
The Master Boot Record (MBR) is a piece of code stored in the first sector of a hard drive that uses the MBR partition table.
Simply put, MBR is a piece of code located in the first sector of a disk.
What does MBR do?
MBR has two main functions:
It records the entire partition structure of the hard drive. For example, if an MBR partition table disk has 3 partitions, it records which sector each partition starts at, etc. This information is stored in the MBR for BIOS to read.
It records the active partition. If an MBR partition table disk has an operating system installed, the partition containing the OS must be the active partition. If the disk only stores data without an OS, the active partition is optional.
Which partition is the active partition is recorded in the MBR using 1 byte of space.
The active partition can be manually set using tools like DiskGenius.
So, the first step of the BIOS boot process is: by reading the MBR, BIOS knows which partition contains the operating system.
From here on, everything happens within that active partition.
Partition Boot Record (PBR)
The Partition Boot Record (PBR) is similar to the MBR—it is also a piece of code.
It follows the Master Boot Record and is responsible for the next step in booting the system.
If MBR manages the entire disk, PBR manages the partition it belongs to.
For disks using the MBR partition table, each partition has its own PBR. The PBR is located at the very beginning of its partition.
- For a regular partition, the PBR stores parameters related to that partition, such as the file system type and partition size.
- For the active partition, the PBR also stores the location of BOOTMGR.
What is BOOTMGR?
BOOTMGR is a file—the Windows Boot Manager—a program that runs within the BIOS environment.
Its location is fixed: it must reside in the root directory of the active partition, because the PBR expects it there.
One of the functions of the active partition’s PBR is to locate BOOTMGR and then execute it.
Once BOOTMGR runs, the process becomes largely similar to the UEFI boot process.
BOOTMGR’s Job
BOOTMGR is typically located in the root directory of the C: drive and is a protected operating system file. Even if you enable “Show hidden files,” File Explorer won’t display it unless you uncheck “Hide protected operating system files.”
That’s because it’s critical for Windows startup.
BOOTMGR’s role is the same as bootmgfw.efi in UEFI: it reads the BCD file and displays the operating system menu listed within it.
The BCD file is the boot configuration file for the operating system. It contains information about the OS, including how many operating systems are on the disk, the boot mode, etc.
For a detailed understanding of the BCD file, please refer to the relevant content in my 20th article: “20 Windows Computer Boot Process Analysis Part 2.”
In BIOS mode, the BCD file is located in the boot folder of the active partition. Typically, it is at C:\boot\BCD.
After the menu from the BCD file is displayed, the user selects a valid system and presses Enter. Then, BOOTMGR, based on the information in the BCD file, runs winload.exe, located in the \Windows\System32 folder.
After that, winload.exe loads the Windows operating system, and the BIOS boot process ends.
Summary
The BIOS boot process is as follows:
Power on → Read MBR → Find the active partition → Read the active partition’s PBR → PBR locates and runs BOOTMGR → BOOTMGR reads the BCD file → User selects an operating system → Run winload.exe → Boot Windows
BIOS can only recognize disks using the MBR partition table.
MBR records the disk’s partition details and which partition is the active partition.
All partitions have their own PBR, which stores information about their respective partitions.
The active partition’s PBR stores the location of BOOTMGR (in the root directory) and other partition information.
The BCD file stores the boot configuration and boot mode for all operating systems.