The Mac has completed its self-test routines, and now prepares to load the EFI firmware or ‘boot loader’. Where it finds it, what it needs, and how Secure Boot changes this.
ROM
What happens in your Mac when you press the Power button? Before it loads the EFI firmware, it goes through self-testing, and initialising key chips.
A guide to different tones sounded at startup, how to cope with EFI ROM update problems, and MacBook Pro 2016 issues.
The Eclectic Light Company 