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.
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:
Corrupted Boot Blocks
Corrupted System File
Corrupted Finder File
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 Macintoshes.
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. Some computers using systems prior to version 7.5 also require a system enabler. Once the computer finds a blessed System folder, it will b
egin loading the operating system (ignoring any other System files on the disk).
On startup, the mac loads the operating system files contained in the System
folder; ignoring any other system files on the hard drive.
When the computer searches for a bootable system disk; it checks each disk's boot blocks for a "blessed" system folder. When a System folder is "blessed", the folder's identification number (all Macintosh folders are given a unique number when they are cr
eated) is written into the boot blocks. As the Macintosh checks for a System folder, it finds the ID number in the boot blocks and then checks the specified folder for a System file and a Finder. The process of writing the System folder's ID number into
the boot blocks is known as "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.
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.
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 PRAM.
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
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.
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