Although MacBooks aren’t technically outfitted with BIOS, they are supported by a similar boot firmware used by Sun and Apple called Open Firmware. Open Firmware is stored is the first executed program on your MacBook and acts as the platform for Mac OS X. Like the BIOS on PC machines, Open Firmware is accessed on startup and provides you with an interface for technical diagnostics and debugging your computer. Be careful when submitting commands in Open Firmware — simple mistakes can permanently damage your MacBook.
To access your MacBook’s Open Firmware, you must first shut off your computer. Then turn it back on, holding down the “Command,” “Option,” “0” and “F” keys simultaneously as the machine boots to access the Open Firmware interface. Like BIOS on PC machines, the Open Firmware command terminal is two-tone and doesn’t provide extensive prompts. The first prompt you receive is the word “ok”, which signifies that your MacBook is functioning properly and that Open Firmware is ready for you to begin submitting commands.
To boot your Intel-based Mac into any of the macOS built-in startup modes, you must press and hold one or more keys detailed below after turning on the computer. Press the keys immediately after h
earing the startup sound, unless otherwise specified.
Manually restart: Power (?)
Boot from CD/DVD: C
Startup Manager: Option (?)
Single-User Mode: Command (?)—S
Target Disk Mode: T
Verbose Mode: Command (?)—V
Safe Mode: Shift (?)
Recovery OS: Command (?)—R
Internet Recovery OS: Command (?)—Option (?)—R
Apple Hardware Test: D
Apple Hardware Test over the Internet: Option (?)—D
Boot from NetBoot server: N
Boot from NetBoot server using the default image: Option (?)—N
Eject removable media: Media Eject (), F12, mouse button or trackpad button
macOS Startup Volume: X will start up your Mac from an macOS startup volume when it’d otherwise boot from a non-macOS volume, like a Windows partition
Prevent Login Items from opening: To override your current settings and prevent Login Items and Finder windows from your last session from opening when you log in, hold down the Shift (?) key after clicking the Log In button in the macOS login window until the desktop appears.
Prevent automatic login: If you enabled the option to automatically log a specific macOS account every time your Mac starts up, you can override this setting by holding the left Shift (?) key after the macOS boot progress indicator appears.
Reset PRAM/NVRAM: Command (?)—Option (?)—P—R, then release the keys after you hear the startup sound for a second time
Reset SMC: Shut down your Mac, and then press and hold the left-side Shift (?)—Control (?)—Option keys along with the Power (?) button, all at the same time, until the computer turns on. Now release the keys and then turn your Mac back on using the power button.
Other keyboard shortcuts are available after your Mac has started up.