MBR activities require a trigger turned on by the BIOS (Basic Input / Output System) when turning on the PC. BIOS is special software, also known as firmware ( firm : firm in this context). It is located on the motherboard of a PC with x86 architecture, where it is embedded in a special chip (for example, an EPROM chip, a flash memory). The BIOS remains a fixed component, even when the computer is turned off.
The BIOS itself does not need to know exactly how a disk is partitioned. It just ensures that the MBR bootloader is loaded into memory and executed. If the primary boot sector has been read and its bootloader is active in working memory, the active (that is, boot) partition of a partitioned hard disk is first determined by the partition table.
When this is found, a chain reaction occurs according to the chain loading principle . The identified boot sector of the addressed partition is operationally integrated, and the partition's bootloader takes control of main memory. Later, more extensive processes and routines are executed that take care of the actual startup of the operating system. Since the partition's bootloader itself performs more complex tasks, it is usually larger than the MBR bootloader.
If the computer has more than one operating system installed , the boot process stops before finishing until the user makes a choice (for example, between Windows 7 and Windows 10). These special bootmanagers are usually timed; If there is no external input, the preferred operating system starts automatically after a specified period of time.