The boot sequence for a machine typically starts with the BMC (baseboard management controller) or PCH (platform controller hub). In the case of an Intel CPU, the Intel Management Engine runs in the PCH and starts before the CPU. After configuring the machine's hardware, the BMC (or PCH, depending on the system) allows the CPU to come out of reset. The CPU then loads the boot (unified extensible firmware interface, UEFI) firmware from the SPI (serial peripheral interface) flash. The boot firmware then accesses the boot sector on the machine's persistent storage and loads the bootloader into the system memory. The boot firmware then passes execution control to the bootloader, which loads the initial OS image from storage into system memory and passes execution control to the operating system. For example, in popular Linux distros, GRUB (derived from Grand Unified Bootloader) acts as the bootloader and loads the operating system image for the machine.
This is much like a relay race where one team member passes a baton to another to win the race. In a relay race, you hopefully know the members of your team and trust them to do their part for the team to get to the finish line. With machines, this chain of trust is a bit more complex. How can we verify that each step in the boot sequence is running software we know is secure? If our hardware or software has been compromised at any point in the boot sequence then the attacker has the most privilege on our system and likely can do anything they want.
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