Introduction:
BIOS, or Basic Input/Output System, is a firmware that is built-in to the computer's motherboard. It plays a critical role in the functioning of a computer system. BIOS is responsible for initializing and testing the hardware components during the boot process. It also provides a low-level interface for the operating system to communicate with the hardware.

Explanation:
BIOS is primarily used by the operating system, and it serves several important functions in this regard:

1. Booting process:
BIOS is responsible for initiating the boot process when the computer is powered on. It performs a Power-On Self-Test (POST) to check the integrity of the hardware components. It also loads the operating system into the computer's memory from the boot device, such as a hard drive or a USB drive.

2. Hardware initialization:
During the boot process, BIOS initializes and configures the hardware components, such as the CPU, memory, storage devices, and input/output devices. It ensures that the hardware is properly recognized and ready for use by the operating system.

3. Firmware updates:
BIOS can be updated to fix bugs, improve compatibility, or add new features. These updates are typically provided by the computer manufacturer and can be installed through the BIOS interface. The operating system may interact with BIOS to facilitate the installation of firmware updates.

4. Power management:
BIOS provides power management features that allow the operating system to control the computer's power usage. This includes functions such as sleep mode, hibernation, and wake-on LAN. The operating system communicates with BIOS to control these power management features.

5. System configuration:
BIOS provides a user interface, known as the BIOS setup utility, which allows the user to configure various system settings. These settings include boot order, date and time, password protection, and hardware configurations. The operating system may interact with BIOS to read or modify these settings.

Conclusion:
In conclusion, BIOS is used primarily by the operating system to initialize and communicate with the hardware components. It plays a crucial role in the boot process, hardware initialization, firmware updates, power management, and system configuration. Without BIOS, the computer system would not be able to function properly.