Apple has released a knowledge base article describing the key commands and what they enable when pressed during start-up:
- Press C during start-up: Start from a bootable CD/DVD, such as the Mac OS X Install disc.
- Press D during start-up: Start the Apple Hardware Test when the Install DVD 1 is inserted in your Mac’s drive.
- Press and hold (Command + Option + P + R) until you hear two beeps: Reset the NVRAM.
- Press Option during start-up: Loads Startup Manager, enabling users to select which volume they would like to boot from. Pressing N will show the first bootable Network volume as well.
- Press the Eject button, press F12, or click and hold the mouse or trackpad button: Ejects any removable media.
- Press N during start-up: Attempt to start from a compatible network server (NetBoot).
- Press T during start-up: When two Macs are connected via FireWire, this will start one in FireWire Target Disk Mode (two Macs with FireWire ports are required).
- Press Shift during start-up: Starts your Mac in Safe Mode, temporarily disabling log-in items.
- Press (Command + V) during start-up: Starts in Verbose Mode.
- Press (Command + S) during start-up: Starts in Single-User Mode.
- Press (Option + N) during start-up: Start from NetBoot server using the default boot image.
Apple recommends pressing the start-up key combinations immediately after hearing the start-up tone.