Macintosh Guide: Inside Macintosh: The Startup Process

Turning on the Macintosh
Startup Diagnostic
Loading the PRAM
Finding the Startup Disk
Loading Fonts
Loading Control Panels
Turning on a Macintosh
Different Macintosh models are turned on (and off) in different ways; but all Macintoshes have a power switch (or button) on the front or rear of the case. Some Macintoshes support "soft-power"; they can be turned on directly from the keyboard by pressing the key. These models turn themselves off after the "Shutdown" command is selected from the Finder's "Special" menu. Choosing "Shutdown" on Macintoshes that do not support soft-power will diplay a dialog box indicating that it is now safe to turn off the mac (by pressing the power switch).
It is important to properly Shutdown a Macintosh before turning it off. The computer keeps several files "open" (active) when it is turned on, including the Finder and System files. If these files are not closed before the Macintosh is turned off, the open files may be corrupted. Also, any changes to open files that are still cached will not be saved.

Startup Diagnostic
As soon as the Mac is turned on, it runs a diagnostic test stored in ROM (Read Only Memory). Part of the Macintosh Operating System is in ROM as well.
The startup hardware diagnostic checks the memory, boot blocks, and other componants of the Macintosh. For example, the diagnostic checks a specific location in the boot sector that is written when the system is properly shut down. If the computer was not shut down correctly the last time it was on, the diagnostic will see this and perform a quick test of the file structure of the hard drive. If any of the diagnostic tests fail, the Macintosh will halt the startup sequence and display the "Sad Mac":
If you see this icon (on a black background), there is either a hardware or software problem with the computer. The 10 most common causes are:
- Problematic SCSI Chain
- Faulty or incorrectly installed memory chips
- Faulty or incorrectly installed expansion cards
- Physically damaged hard drive
- Physically damaged motherboard
- Physically damaged power supply
If a Macintosh boots and gives a "Sad Mac" immediately, the problem is probably caused by either faulty hardware or corrupted boot blocks. If the "Sad Mac" occurs later in the startup sequence, there may be other problems. Use the TroubleShooting Flowchart to diagnose problematic Macin toshes.

Loading the PRAM
Startup Disk
After the diagnostic test, the computer looks for a startup disk. Any disk can be a startup disk (hard disk, floppy disk, CD-ROM, etc) as long as it contains a "blessed" system folder. If there is a diskette in the floppy drive, the computer will check it first for a blessed system folder. If it doesn't find one, the Macintosh will eject the floppy and scan the SCSI chain for a blessed system folder, starting with the internal hard drive (SCSI ID #0).
A startup disk's System folder must contain both a System File and a Blessing" the System folder. If the computer does not find a blessed System folder, it will display the "flashing disk" icon:
After the computer finds the Startup disk, it searches for any additional devices attatched to the computer. This is similar to how the Finder "mounts" volumes when it is launched. Without specialized disk drivers or utilities such as SCSI Probe, any devi ce not recognised at this time cannot be addressed by the computer. This is why all periferal devices should be turned on before the computer itself; so they can be recognised by the initial startup sequence.

Loading the System File
Once the computer has found a startup device, it begins to load the operating system into memory, beginning with the System file. If the Macintosh model is one that requires a system enabler, it opens the enabler soon after it begins loading the System file. If the enabler is n ot present, a dialog will appear stating that the operating system is not recent enough to run on the Macintosh. As the Macintosh is loading the System file (and enabler file), it will display a "Welcome to Macintosh" dialog box. The text of the startup dialog is stored in the DSAT resource in the system file. Under System 7.5.1, this dialogue box is replaced with a "MacOS" screen with a progress bar. The MacOS image is stored in t he PICT resource in the "System 7.5 Update" file. The System file contains patches to the Macintosh ROMs, the Macintosh operating system, and system and graphical interface resources shared by the Finder and Macintosh applications.

Loading Fonts
Until the release of System 7.1, fonts used to be stored as recources within the System file. Current versions of the System software store fonts inside a "Fonts" folder within the System folder. During the startup process, the fonts are loaded alphabetic ally from the "Fonts" folder at the same time that they used to be loaded from the System file. System 7 can only address 128 font files or suitcases, if there are more than that installed, only the first 128 can be used. This limitation can be bypassed by storing multiple fonts in each font suitcase.

Loading Extensions
Once the mac has loaded the system, enabler, and fonts, it begins to load the Extensions contained in the Extensions folder within the System Folder. Extensions load alphabetically. Extensions modify or add to the code already loaded. For example, when the system draws a button, it uses a specific set of instructions stored in the system file. An extension may be loaded that "patches" this code, so that when the mac wants to draw a buton, it uses a different set of instructions stored inthe resource. Extensions can be very useful, but because they modify the system code, thaey can cause problems is they are not written properly. They can also "conflict". If two extens patch the same code, they may conflict with each other and cause a crash, freeze, or bomb. This may occur as the extensions are loading, or after the mac has booted and calls the modified code for the first time. Extensions can also conflict with the system itself if they are not written correctly.

Loading Control Panels
After the Extensions load, the Control Panels are next. The Control Panels usually function exactly like extensions; they modify or add to the system software. They can also cause crashes, freezes, or bombs. The only difference is that they typically have an interface to control their function. Double-clicking a control panel opens this interface and allows you to customize the control panel. Control Panels do not always act like extensions, however. Some control Panels are used to change what is stored in the Monitor control panel, for example, is used to set the monitor size, position, and color depth, as well as the location of the menu bar and startup dialog box. The Date and Time control panel is used to set the system clock. Finally, some control panels modify preference files. The Views control panel modifies the "Finder Preferences" file and controls how the finder displays files.

Extensions and Control Panels can often cause problems; because of this they can be disabled easily during startup. After the "Happy Mac" appears, holding down the key will prevent the system from loading the control panels and extensions. The startup dialog will indicate this by saying "Extensions Disabled".

Once the Control Panels have been loaded, the system loads the Finder. The Finder is an application like any other Mac application, but it is central to the operating system. The Finder is the application with which you interact with the operating system; you use it to find, open, copy, move, and delete files. It is also the application that you use to open Control Panels and Desk Accessories, restart and shut down the computer, and unmount storage devices. The first thing the finder does is mount any storage devices that are connected to the computer and check their desktop files. If the desktop files are outdated or if the and

Immediately after the Finder loads, the mac opens any files that are in the "Startup Items" folder. Applications, files, folders, and aliases can be kept here if the user wants them to be opened every time the mac is turned on. If the user does not want the startup items to be loaded, they can be bypassed by holding down the shift key immediatley after the Finder loads. Some applications (like "Stickies") load and then switch themselves into the background, leaving the Finder as the active application. Other aplications will not. Once all the Startup Items have loaded, the mac has finished the startup process.

Turning on a Macintosh

Startup Diagnostic

Startup Disk

Loading the System File

Loading Fonts

Loading Extensions

Loading Control Panels