Macintosh Startup Key Combinations

This document is loosely based on Charles Poynton's startup key combinations document which has since disappeared from his web site.

A Mac does several checks when it is first powered on; one of the first things it does is scan the keyboard bus (ADB) for any signals. Certain keys will cause various actions to be taken. Note: command (cmd) is the same as the Apple key; option (opt) is the same as the alt key.

The following should be held down when the computer is first powered up to take effect:

• cmd-opt-P-R: Zap PRAM. Causes computer to reboot. Date, time, and AppleTalk settings will be lost.
• cmd-opt-X-O: Boot from ROM (Mac Classic only). Boots from copy of System 6.0.3 in the ROMs.
• cmd-opt-A-V: Reset AV monitor and force it to be recognised as such.
• cmd-opt-T-V: Force 660AV or 840AV to use TV connected to the AV jacks as the monitor.
• cmd-opt-shift-delete-(#): Boot from a SCSI device on the SCSI bus other than the disk selected in the Startup Disk control panel. The optional number allows for booting from a specific SCSI ID.
• R: Force PowerBook to reset screen.
• C: Boot from Apple-brand CD-ROM drive. Works with a few third-party drives too. For drives that can't boot using the C key, try cmd-opt-shift-delete as explained above.
• hold mouse button: Eject bootable floppy and boot from the SCSI bus.

The following should be held down just before the happy Mac first appears to take effect:

• Command: Boot with virtual memory off (System 7 - Mac OS 9.x only).
• shift: Boot with Extensions, Control Panels, and Startup Items disabled (including VM, if on).
• space: Open Extensions Manager (System 7.5.x and higher only) on boot.
• others: Check documentation for third-party extensions and control panels; there are often keys to disable these individually.

The following should be held down immediately after the "icon parade" finishes to take effect:

• cmd-opt: Rebuild desktop database file.
• shift: Disable Startup Items only.
• option: Start Finder with no open windows.

For shortcuts to use in the Finder once the Mac is booted, check out the Finder Shortcuts file in the Help menu.

The following are some other useful shortcuts/undocumented combos:

• cmd-opt while opening Memory control panel: allow startup RAM tests to be turned off (Mac OS 8.5 - 9.x only).
• cmd-power key: Generate non-maskable interrupt, which will drop you into the built-in debugger or Macsbug (if installed). Type 'g' and press return to continue.
• cmd-opt while opening Chooser: rebuild Chooser's cache.

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