What happens when we start our computers?

When you power up your computer, the first software to run is either BIOS or the UEFI.These firmware systems reside on the motherboard and serve a crucial role in starting the computer.

BIOS or UEFI performs some POST checks to verify the health of the system and hardware components like the processor, memory, and storage devices. Once POST is successful, the firmware follows a specified boot order to locate a bootable device containing an operating system.

For BIOS, this involves reading the Master Boot Record (MBR). This is a small section at the beginning of a storage device(first 512 bytes) like a hard drive or SSD. The MBR contains a bootloader program, responsible for locating and starting the operating system's core (the kernel).

UEFI departs from the MBR approach. Instead, it utilises a special partition called the EFI System Partition (ESP). This can store more sophisticated bootloader programs. This gives UEFI greater flexibility and support for advanced features on modern hardware.

In summary, both BIOS and UEFI are essential for starting your computer. BIOS represents the older standard, while UEFI provides a more modern and versatile approach to initialising hardware and loading the operating system.

A simple bootloader can be found here