How a Computer Boots
Introduction
When a computer is switched on, it must prepare its hardware and load an operating system before it is ready for use. This startup process is called booting. The stages shown in this note describe a traditional BIOS-based boot process, including POST, CMOS settings, the Master Boot Record (MBR), and the bootstrap loader.
1. BIOS
BIOS stands for Basic Input/Output System. In a traditional BIOS-based computer, it is firmware that runs when the computer is switched on. It begins the startup process and checks or initialises essential hardware so that the computer can continue booting.
The BIOS is stored in non-volatile memory on the motherboard, so its instructions remain available when the computer is turned off.
Typical tasks include:
- Starting the initial hardware checks.
- Reading configuration settings used during startup.
- Finding a bootable device and beginning the process of loading the operating system.
Many modern computers use UEFI firmware instead of legacy BIOS. The image's MBR-based sequence is the traditional BIOS-style explanation of booting.
2. POST (Power-On Self-Test)
POST stands for Power-On Self-Test. It is a set of checks carried out during startup to see whether essential hardware is present and functioning well enough for the computer to continue.
Depending on the computer, checks may involve the CPU, memory (RAM), keyboard, storage devices, and other hardware.
If a serious problem is detected, the computer may display an error message, produce beep codes, or stop the startup process. The exact behaviour depends on the system and the fault.
3. CMOS and Startup Settings
The note uses CMOS to describe the storage of BIOS settings such as the date and time, boot order, and hardware configuration. Traditionally, these settings were maintained by a small battery-backed CMOS memory.
The small motherboard battery helps preserve the real-time clock and relevant settings when the computer is unplugged or switched off. On modern systems, the settings may be stored in flash memory while the battery primarily maintains the clock; the exact design varies by motherboard.
The boot order determines which devices the computer tries when searching for a bootable operating system—for example, an internal drive or a USB drive.
4. MBR (Master Boot Record)
In a traditional BIOS boot process using an MBR-partitioned disk, the Master Boot Record (MBR) is the first sector of the disk. It contains a small amount of boot code and partition information.
The BIOS uses the selected boot device to begin the next stage of startup. With this traditional arrangement, the MBR's boot code helps direct the computer to the bootloader or next boot stage.
The MBR approach is not used by every computer. Modern UEFI systems commonly boot using an EFI System Partition rather than relying on the traditional MBR boot-code sequence.
5. Bootstrap Loader
A bootstrap loader, commonly called a bootloader, is a small program that starts or continues the process of loading the operating system.
In the traditional sequence shown in the note, the bootloader is reached after the MBR stage. It locates the operating system's required files and loads the appropriate parts into memory so that the OS can start.
The bootloader is a bridge between the initial firmware startup and the operating system.
6. Loading the Operating System into RAM
RAM (Random Access Memory) holds instructions and data that the processor needs while programs are running. The bootloader loads the necessary operating-system components from storage into RAM.
The operating system can then initialise its own components, detect and manage devices, and prepare services needed for normal use.
RAM is volatile memory: its contents are generally lost when the computer is switched off. The operating system files remain on storage and can be loaded again the next time the computer starts.
7. Booting Is Completed
After the operating system has loaded and initialised the required components, it presents a login screen, welcome screen, or desktop. The computer is then ready for the user to interact with applications.
Boot Process: Step by Step
The traditional BIOS/MBR process shown in the short note can be summarised as follows:
Computer is switched on
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BIOS firmware starts
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POST checks essential hardware
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Startup settings and boot order are read
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BIOS selects a bootable device
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MBR boot code starts the next boot stage
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Bootloader locates and loads the operating system
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Operating system components are loaded into RAM
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The operating system starts and prepares the computer
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Login screen or desktop appearsThe exact order and implementation depend on the computer's firmware and boot configuration. The MBR stages above describe a traditional BIOS-based system, not every modern computer.
Example: Starting a desktop computer- You press the power button.
- Firmware begins startup and performs initial checks.
- The computer selects a bootable drive according to its boot configuration.
- The boot code and bootloader begin loading the operating system.
- Operating-system components are loaded into memory and initialised.
- A login screen or desktop appears, allowing you to use the computer.
If a required component fails—for example, if no bootable device can be found—the computer may show an error instead of reaching the desktop.
Important Terms
| Term | Meaning |
|---|---|
| BIOS | Basic Input/Output System; traditional firmware that begins startup and helps select a boot device |
| POST | Power-On Self-Test; startup checks of essential hardware |
| CMOS settings | Traditional term for saved firmware configuration such as date, time, and boot order |
| MBR | Master Boot Record; first sector of a disk in the traditional MBR layout, containing boot code and partition information |
| Bootloader | Program that begins or continues loading the operating system |
| RAM | Volatile memory used to hold active instructions and data |
| Booting | The process of starting a computer and loading its operating system |
Exam-Focused Points
Key Idea: POST is the hardware check performed during startup. It is not the operating system itself.
Key Idea: In the traditional BIOS/MBR sequence, the firmware begins startup, the MBR boot code leads to the bootloader, and the bootloader loads the operating system.
Key Idea: The operating system's required components are loaded into RAM so that the processor can execute them during normal operation.
Quick revision
- BIOS: Begins the traditional firmware startup process.
- POST: Checks essential hardware.
- CMOS/settings: Preserve or provide startup configuration, including boot order.
- MBR: Contains boot code and partition information on a traditional MBR disk.
- Bootloader: Locates and loads the operating system.
- RAM: Holds the operating system components and data needed while the computer runs.
- Boot complete: The OS has started and presents a login screen, welcome screen, or desktop.
Summary
Booting is the sequence that takes a computer from power-on to a usable operating system. In the traditional BIOS-based process shown here, BIOS firmware begins startup, POST checks essential hardware, saved settings guide the choice of boot device, and MBR boot code leads to a bootloader. The bootloader loads the operating system into RAM, and the OS initialises the computer so that the user can begin working.
Modern UEFI-based systems may use a different boot path, so the BIOS/MBR sequence should be understood as the traditional method illustrated in this note.