The Macintosh Boot Sequence

Phase 1: Power on

When a Mac is powered on, it plays the startup chime, then undergoes
a thorough hardware diagnostic, including a basic memory check.

Possible problems:

Phase 2: ROM Initialization

During the next step of booting, the Mac looks for it's ROM. The Mac
ROM contains hardware-specific code so the MacOS doesn't need
special drivers for different models or motherboards. Until the release
of the
iMac, these ROMs were actual chips on the motherboard.

Possible Problems

Phase 3: Finding the System Software

Once the Mac initializes the ROM, it scans the local busses for drives. It
looks for a floppy first, then SCSI or IDE devices. Once it finds devices,
it looks for a "blessed" system folder. If there is only one system folder
on a drive, partition or disk, that folder is automatically "blessed". If there
are more than one copy, the last folder that was accessed last is the "blessed"
system folder. This is what the Mac will boot from. To force the Mac to boot
from a specific device hold down these keys right after power-up:

Key Combo
Boot from CD-ROM
Disable internal hard disk drivers
Boot from a specific SCSI ID #
Zap the PRAM

The PRAM is non-volatile memory Macs use to store system-specific
settings. Among these are:

Zapping the PRAM sets it back to the factory defaults. This may help
a non-booting Mac to finally boot. Remember to reset the Appletalk
and printer settings after zapping the PRAM.

Once a Mac finds the system folder, it will flash the Happy Mac icon.

Phase 4: Loading the OS

The Mac now begins to load the operating system. The system folder must
contain at least the following items for the Mac to recognize it:

Some Macs may also require system enablers, which enable the OS to run
on certain machines. If you try to boot a machine with an OS that needs enablers,
you will see an error message that says "This startup disk cannot be used with this

Once the Mac finds the System and Finder, it begins to load the OS into ram. The
Mac starts by loading the Extensions and Control Panels into memory. It does this
alphabetically, which makes it easier to troubleshoot. There are a few key
combos that can help troubleshooting at this point. (These should be held down right
after the Smiling Mac icon appears.

Key Combo
Boot with Extensions Off
Open the Extension Manager
Boot without virtual memory enabled

Once the MacOS startup screen appears, the extensions and control panels
will appear on the bottom of the screen from left to right. Not all the of the
control panels and extensions will show up, but most will, in alphabetical order.

If an extension has a red X through it, it didn't properly load.

Phase 5: Starting the Finder

The Finder is the desktop of the Macintosh. It is a program that is automatically
loaded at the end of the booting process. Here is where all other volumes are
mounted and the GUI desktop loads.

Possible Problems

When the Finder starts up, it looks for the following files:

Any of these could cause the Finder to choke when starting up, so they can all
be deleted. The system will recreate them when it is next booted.

Another common solution to a Mac that won't finish booting is a corrupt
DesktopDB file. This is known as the desktop database, which stores
custom icon information, icon placement and file comments for each file on the
volume. To fix this problem, let the Finder rebuild the desktop.

To rebuild the desktop:

  • Hold down the Apple and Option keys when the extensions are loading.
  • When it asks if you want to rebuild the desktop on a certain volume, click YES.
  • Click here to return to the main page.