BIOS (Basic Input/Output System) is the firmware stored on the motherboard that runs first when a computer is switched on, tests and initializes the hardware and then locates and starts a boot loader from a storage device. In the boot process of operating systems, the term names both the classic legacy firmware of IBM PC compatibles and, in everyday speech, the firmware setup screen of any modern board.
Strictly speaking, almost every PC sold since the early 2010s uses UEFI firmware instead of a true BIOS. Many UEFI implementations still offer a Compatibility Support Module (CSM) that imitates BIOS behavior, and vendors still label the setup menu “BIOS”, which explains the persistent use of the name.
What the BIOS does from power-on to boot
When power stabilizes, the processor starts executing at a fixed reset vector near the top of the 1 MB address space, which is mapped to the firmware chip. From there the BIOS runs a fixed sequence of stages until it hands control to the operating system.
| Stage | What happens | Typical evidence of failure |
|---|---|---|
| Reset and early init | CPU starts in 16-bit real mode, chipset is configured | No display, fans spin |
| Memory training | RAM timings are detected and calibrated | Beep codes or memory LED |
| POST | Power-on self-test of CPU, memory, video and keyboard | Beep codes or POST code display |
| Option ROMs | Graphics and storage controllers run their own firmware | Missing drives or no video |
| Boot device search | Reads the first sector of each device in the boot order | “No bootable device” message |
| Handoff | Loads the 512-byte MBR to memory and jumps to it | Boot loader errors |
The Master Boot Record (MBR) is the crucial link: the BIOS knows nothing about file systems, so it loads only that single 512-byte sector and relies on the code inside it to continue loading the operating system.
Where the BIOS lives and what it stores
Modern boards keep the firmware in a SPI NOR flash chip, typically 16 to 32 MB in size, which can be rewritten during an update. Older boards used mask ROM or EEPROM chips that were far smaller and sometimes not rewritable at all.
User settings such as boot order, clock and memory profiles are kept separately in CMOS memory or, on newer boards, in a reserved area of the flash. A coin-cell battery (CR2032) keeps the real-time clock running when the system is unplugged. Clearing CMOS with a jumper or by removing the battery restores default settings without erasing the firmware itself.
The BIOS also offers runtime services through software interrupts, such as INT 13h for disk access and INT 10h for video. Early operating systems such as MS-DOS depended on these services, while modern systems load their own drivers and ignore them after boot.
BIOS compared with UEFI
UEFI was designed to remove the architectural limits of BIOS. The differences matter whenever a system is installed, upgraded or secured.
| Property | Legacy BIOS | UEFI |
|---|---|---|
| Processor mode at boot | 16-bit real mode | 32-bit or 64-bit mode |
| Partition scheme | MBR, disks up to 2 TiB | GPT, disks far beyond 2 TiB |
| Boot mechanism | Executes code in the first sector | Loads EFI applications from a FAT partition |
| Secure boot | Not available | Signature checks with Secure boot |
| Setup interface | Text menus, keyboard only | Graphical menus, mouse support |
| Driver model | Option ROMs in real mode | Modular UEFI drivers |
Because many modern features depend on UEFI, current operating systems increasingly require UEFI with Secure boot and a TPM. Running a board in CSM or legacy mode disables Secure boot and limits the boot disk to MBR partitioning.
Common BIOS settings and why they matter
The firmware setup screen, reached by pressing a key such as Delete or F2 during POST, controls how the hardware behaves before any operating system loads. The following settings cause the most support questions.
| Setting | Effect | Recommendation |
|---|---|---|
| Boot order | Sets which device is tried first | Put the system drive first after installation |
| SATA mode | Chooses AHCI, RAID or legacy IDE | Use AHCI unless RAID is planned |
| Memory profile (XMP or EXPO) | Applies rated RAM speed and timings | Enable after confirming stability |
| Virtualization | Enables VT-x or AMD-V | Enable for virtual machines and containers |
| CSM | Turns legacy BIOS compatibility on or off | Disable on modern systems |
| Fan curves | Map temperature to fan speed | Balance noise against thermal headroom |
Updating the BIOS safely
A firmware update can add support for newer processors, fix memory compatibility, patch security vulnerabilities and correct microcode. It also carries a real risk, because an interrupted write can leave the board unable to start.
- Update only for a reason: a new CPU, a security fix or a documented bug that affects the system.
- Use stable power: a power loss during flashing is the most common cause of a bricked board.
- Use the built-in flash tool from a USB drive rather than an update utility running inside the operating system.
- Check recovery options: many boards offer dual firmware chips or a flashback button that can restore a corrupted image.
- Re-check settings afterward, because updates often reset memory profiles and boot options to defaults.
Common misconceptions about the BIOS
The informal use of the term causes several misunderstandings. Resetting the BIOS clears settings but does not roll back the firmware version. Clearing CMOS does not erase data on drives. A computer that shows the manufacturer logo has passed POST, so a later boot failure usually lies with the boot loader or the storage device, not with the firmware.
Put simply
The BIOS is the small program built into the motherboard that wakes the computer up. Before Windows, Linux or macOS can start, something has to check the processor, memory and drives. Then it has to find the operating system.
It works like a building caretaker who arrives first in the morning. The caretaker switches on the lights, checks the doors and then unlocks the main entrance for everyone else. Most computers now use a more modern caretaker called UEFI. Many people still call it the BIOS.
In practice
A small design studio buys a newer processor for an existing workstation to speed up rendering. After installation the system powers on, the fans spin, but the screen stays black and the POST code display stops at a CPU initialization code. The board’s firmware is two years older than the processor and does not contain its microcode.
The technician reinstalls the old processor, downloads the latest firmware from the board maker, copies it to a USB drive and runs the built-in flash utility on a machine connected to a UPS. After the update the new processor boots normally. The technician then re-enables the memory profile and switches the board from CSM to pure UEFI mode so that Secure boot can be turned on. The upgrade costs a single maintenance window instead of a new motherboard.