Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
NVRAM
Listing of known NVRAM variables and their usage/meaning, together with notes on how to change them.
Important info about resetting the NVRAM with a firmware password, and new procedures for resetting the SMC in Macs with T2 chips.
All about firmware, the SMC, NVRAM, and the new T2 chips which come in some models of Mac. When and how to reset, and more.
When should you reset your SMC and NVRAM, repair permissions, install a Combo update, or re-install the whole of macOS?
How to control whether your 2016 MacBook Pro sounds a startup-chime or start up when you open its case, why you can’t unmute a 2017 iMac, and how High Sierra changes your NVRAM.
How to update your firmware, reset the SMC, and reset the NVRAM (or zap the PRAM), and what they do. Updated for High Sierra.
Additional info about how you can install and upgrade to High Sierra, its new Content Caching feature, how it changes firmware updates, changes to NVRAM, and more.
Safe mode, or Recovery? Which type of Recovery? How to engage all those useful modes at startup, and when shutting down.
How to enable or mute your Mac’s startup chime, and other neat tricks of the NVRAM.
The Eclectic Light Company 