To use any of these key combinations, press and hold the keys immediately after pressing the power button to turn on your Mac, or immediately after your Mac begins to restart. Keep holding until the described behavior occurs.
Zap the P-RAM
Command-Option-P-R
https://support.apple.com/en-us/HT204063
Safe Boot Mode
Shift (⇧)
https://support.apple.com/HT201262
Reset the SMC
https://support.apple.com/en-us/HT201295
Target Disk Mode
https://support.apple.com/en-us/HT201462
Recovery Mode
Command-R
Diagnostic Mode
D or Option-D (depending on your Mac model)
https://support.apple.com/en-us/HT202731
https://support.apple.com/en-us/HT201257
Specify Boot Drive (and more)
https://support.apple.com/en-us/HT201255
Mac keyboard shortcuts
https://support.apple.com/en-us/HT201236
Keyboard Symbols
https://support.apple.com/kb/PH18802?locale=en_US
