Startup key combinations are invoked immediately following the startup chime, and before the grey Apple logo appears in the middle of the screen, when you power-on your Mac.
The object of these key combinations is to perform different functions, many of which are useful in the maintenance of Macs. The key combinations listed in this tutorial are specifically for use with Macs with Intel processors.
Tip: If you have a Windows keyboard, you can usually use the Windows key as the substitute for the Command key on the Mac keyboard.
Tip: If you are having difficulty invoking startup key combinations, ensure that you press and hold the keys immediately after the startup chime. Alternatively, use a wired keyboard where possible, to rule out any issues possible with Bluetooth keyboards.
Available Key Combinations
Boot From External Device
Pressing the C key immediately after the startup chime will enable the option to boot from a range of media such as a bootable CD, bootable DVD or a bootable USB drive. This might include OS X install media (up to OS X 10.7 Lion) or USB install drives that you have created for other versions of OS X.
Apple Hardware Test (AHT)
Pressing the D key immediately after the startup chime will boot your Mac into a suite of diagnostic utilities that enable you to test the hardware of your Mac. This is a useful way to rule out any hardware issues when trying to diagnose a possible problem with your Mac (which is why you may not have heard of it and, no doubt, have never used it!)
Tip: Remember D for Diagnostics.
Reset the PRAM/NVRAM
This one is a bit of a finger-twister and, unless you are more dextrous than I, you’ll need both hands. The PRAM, on PowerPC Macs, or NVRAM, as it is on Intel Macs, is the non-volatile (random access) memory that stores various information about your Mac. This information includes:
- speaker volume
- screen resolution
- startup disk selection
- recent kernel panic information, if any
Resetting the NVRAM, on Intel Macs, may be one way of solving an issue related to the above areas.
When you invoke the Command-Option-P-R keyboard combination, keep the keys held down immediately after the first startup chime and release them upon hearing the second startup chime.
Select a Startup Volume (Drive)
Pressing the Option key immediately after the startup chime will show you the available startup volumes.
A startup volume is a hard drive, USB drive, CD or DVD that contains a usable operating system from which the Mac can be booted.
This is particularly useful if you have your hard drive partitioned with two (or more) operating systems from which you wish to boot your Mac. Or, in the case of maintenance and recovery, when you need to boot from an external drive.
Eject a CD or DVD
Pressing the Eject or F12 keys immediately after the startup chime will eject any removable media, such as an optical disc.
This is a useful option on those occasions when you just don’t seem to be able to get OS X to eject a DVD from the SuperDrive.
That said, the reliance on optical media is diminishing now that modern Macs are supplied without a SuperDrive.
Pressing the N key immediately after the startup chime will allow you to boot the Mac from a compatible network server. This option is most likely to be used by businesses with a network of Macs.
Target Disc Mode
Pressing the T key immediately after the startup chime will allow another Mac with a FireWire port (the target Mac) to be used as an external hard drive connected to another Mac (the host).
Target Disk Mode is useful for accessing the contents of a Mac which cannot be booted from its own hard drive.
Safe Boot Mode
Pressing the Shift key immediately after the startup chime will start up your Mac in a way that performs particular checks and prevents certain software from automatically loading or opening.
This is particularly useful if any maintenance is required on your Mac and can be used to resolve or isolate certain problems that exist on the internal hard drive (startup volume).
Pressing the Command-V keys immediately after the startup chime will start up your Mac in verbose mode. Verbose mode is typically used for troubleshooting; it shows what is happening during system startup.
It is possible to start in verbose mode every time you start your Mac, by opening Terminal and entering the following command:
To disable verbose mode booting, enter:
If you just want to boot into verbose mode on an ad-hoc basis simply hold the Command-V keys.
Pressing the Command-S keys immediately after the startup chime will start up your Mac in single-user mode.
Single-user mode is a mode in which a multiuser OS X operating system boots into a single superuser for the purposes of maintenance.
Default Boot Image From a NetBoot Server
Pressing the Option-N keys immediately after the startup chime will start up your Mac from a NetBoot server using the default boot image. This is of most use to businesses with a network of Macs.