This is a 2015 paper that I just noticed.
Booting an Intel System Architecture:
Taking many little steps walking a path is a good analogy for understanding the Intel Architecture boot flow. The bare minimum firmware requirements for making an Intel architecture platform operational and for booting an OS are presented in this article in a particular order. For most of the system designs, the order of these steps are sufficient for a full or cold boot, from a state where the power is off to the handoff to the operating system. Depending on the architecture of the BIOS, there may be multiple software phases to jump through with different sets of rules, but the sequence for actually engaging the hardware is very much the same.
https://www.researchgate.net/publication/295010710_Booting_an_Intel_System_Architecture
Firmware Security 