Mac OS X for Unix Geeks

nvram

The nvram utility modifies Open Firmware variables, which control the boot-time behavior of your Macintosh. To list all Open Firmware variables, use nvram -p. The Apple Open Firmware page is http://bananajr6000.apple.com/.

To change a variable, you must run nvram as root or as the superuser. To set a variable, use variable= value. For example, to configure Mac OS X to boot verbosely, use nvram boot-args=-v. (Booting into Mac OS 9 or earlier will reset this.) Table 8-7 lists Open Firmware variables. Some variables use the Open Firmware Device Tree notation (see the technotes available at the Apple Open Firmware page).

Warning

Be careful changing the nvram utility, since incorrect settings can turn a G4 iMac into a $2000 doorstop. If you render your computer unbootable, you can reset Open Firmware by zapping the PRAM. To zap the PRAM, hold down Option-⌘-P-R as you start the computer, and then release the keys when you hear a second startup chime. (If your two hands are busy holding down the other buttons and you have trouble reaching the power button, remember that you can press it with your nose.)

Table 8-7. nvram variables

Variable	Description
auto-boot?	The automatic boot settings. If true (the default), Open Firmware will automatically boot an operating system. If false, the process will stop at the Open Firmware prompt. Be careful using this with Old World (unsupported) machines and third-party graphics adapters, since the display and keyboard may not be initialized ...