Help Desk.How It Works.Page Six

Help: How it Works: 6 of 9

The Startup Sequence

Pressing the POWER key on your keyboard begins a hectic chain of events that we call the "Startup Sequence." If your Macintosh should ever have a problem starting up, understanding the normal startup sequence might enable you to determine exactly WHEN the problem occurs and WHAT may be causing it.

It all starts with the Macintosh ROM (Read Only Memory). The ROM is a 2-4 MB (depending on the computer model) memory chip that PERMANENTLY stores the basic instructions that define how your Mac works. These instructions are known as the Macintosh Tool Box.

The moment you press the power button, the ROM comes to life. It's first task is to send a request to all other motherboard components, asking them to check in and get ready for action. If all components respond positively, the ROM tells the CPU to play the normal "startup sound." That's your indication that all logic board components are OK.

(NOTE: Assuming you haven't turned off your computer's sound or disconnected its speaker(s), the lack of a normal startup sound is a sign of serious logic board problems. Other problems associated with RAM or processor failures might also result in the Mac playing its "crash" sound. On older Macs, this was a series of four distinctive musical notes that were called "the chimes of death." Power Macs play the sound of a high-speed car crash.)

Once the startup chime has played, the ROM loads its critical computer operating instructions into RAM. This is when the monitor comes to life and the "Happy Mac" icon is displayed on screen. (NOTE: If the initial logic board tests fail, the computer may display a "Sad Mac.") This is also when communications are established with the keyboard, mouse, any installed expansion cards, the hard drive, diskette drive, CD-ROM drive or any other external devices. (NOTE: Problems with the SCSI bus or other peripherals may cause the computer to freeze at this point.)

After the ROM's operating instructions have been loaded into RAM, your computer starts looking for a "Boot Disk" that contains valid system software. It always looks at the diskette drive first, then it checks your Startup Drive Preferences. If neither option is successful, it checks to see if any connected storage device contains usable system software. (NOTE: Pressing the "C" key during startup forces the computer to skip its normal search order and look at the internal CD-ROM drive first.) What happens if the computer can't find usable system software? It displays the "flashing floppy disk" symbol, which could indicate that your hard disk(s) has a problem.

If your computer finds a valid system software folder, it loads the system files and "resources" into RAM. At this point, you see the "Welcome to Macintosh" message on screen.

(NOTE: Contrary to popular belief, the files in the system folder don't actually "run" the computer. They merely enhance the Tool Box instructions and commands contained in ROM, or "patch" certain instructions to make older ROM chips work with newer versions of system software.)

(NOTE 2: On newer PCI Macs, the screen may "flash" or "blink" just after the "Welcome to Macintosh" message is displayed. This is NOT a problem! It occurs when the simple commands built into ROM hand off monitor control to the much more powerful instructions contained within the system software folder.)

Assuming that all goes well up to this point, the ROM next instructs the CPU to contact the Extensions Manager Control Panel and identify any necessary extensions, control panels and fonts located in the system folder. These are then loaded into RAM, too. You'll recognize this process as the little "parade of icons" appearing across the bottom of your screen.

(NOTE: A bomb or freeze during this part of the Startup Sequence usually indicates an extension conflict or corruption.)

(NOTE 2: Not all extensions and control panels are loaded into RAM during startup. Some are only loaded later, if needed. Also, if you use a Font Management Utility like Adobe Type Manager, only those fonts located inside the system's Fonts Folder are loaded immediately. All other Fonts are loaded when the Font Management extension or control panel is loaded - usually near the end of the Startup Sequence, or when an application is launched.)

OK, we're almost done now. Only two more steps remain.

First, the ROM instructs the CPU to load and launch the Finder application. It's the Finder's job is to display the background image, icons, files, folders and drives on screen, and allow you to manipulate their appearance and behavior. During startup, the most important thing the Finder does is search the various hard drives, CD-ROM drives, diskette drives and removable drive devices, load their software "drivers" and "mount" the volumes on the desktop so that you can interact with them. (NOTE: A bomb or freeze at this point in the startup sequence may indicate a corrupted device driver or volume directory file.)

Finally, as the last step in the Startup Sequence, the ROM instructs the CPU to search the Startup Items Folder inside the System Folder and activate any applications or aliases stored there. The Launcher, Control Strip and "Stickies" applications are also launched at this time.

And there you have it! Your computer is now ready for work.

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Help: How it Works: 6 of 9
The Startup Sequence Pressing the POWER key on your keyboard begins a hectic chain of events that we call the "Startup Sequence." If your Macintosh should ever have a problem starting up, understanding the normal startup sequence might enable you to determine exactly WHEN the problem occurs and WHAT may be causing it. It all starts with the Macintosh ROM (Read Only Memory). The ROM is a 2-4 MB (depending on the computer model) memory chip that PERMANENTLY stores the basic instructions that define how your Mac works. These instructions are known as the Macintosh Tool Box. The moment you press the power button, the ROM comes to life. It's first task is to send a request to all other motherboard components, asking them to check in and get ready for action. If all components respond positively, the ROM tells the CPU to play the normal "startup sound." That's your indication that all logic board components are OK. (NOTE: Assuming you haven't turned off your computer's sound or disconnected its speaker(s), the lack of a normal startup sound is a sign of serious logic board problems. Other problems associated with RAM or processor failures might also result in the Mac playing its "crash" sound. On older Macs, this was a series of four distinctive musical notes that were called "the chimes of death." Power Macs play the sound of a high-speed car crash.) Once the startup chime has played, the ROM loads its critical computer operating instructions into RAM. This is when the monitor comes to life and the "Happy Mac" icon is displayed on screen. (NOTE: If the initial logic board tests fail, the computer may display a "Sad Mac.") This is also when communications are established with the keyboard, mouse, any installed expansion cards, the hard drive, diskette drive, CD-ROM drive or any other external devices. (NOTE: Problems with the SCSI bus or other peripherals may cause the computer to freeze at this point.) After the ROM's operating instructions have been loaded into RAM, your computer starts looking for a "Boot Disk" that contains valid system software. It always looks at the diskette drive first, then it checks your Startup Drive Preferences. If neither option is successful, it checks to see if any connected storage device contains usable system software. (NOTE: Pressing the "C" key during startup forces the computer to skip its normal search order and look at the internal CD-ROM drive first.) What happens if the computer can't find usable system software? It displays the "flashing floppy disk" symbol, which could indicate that your hard disk(s) has a problem. If your computer finds a valid system software folder, it loads the system files and "resources" into RAM. At this point, you see the "Welcome to Macintosh" message on screen. (NOTE: Contrary to popular belief, the files in the system folder don't actually "run" the computer. They merely enhance the Tool Box instructions and commands contained in ROM, or "patch" certain instructions to make older ROM chips work with newer versions of system software.) (NOTE 2: On newer PCI Macs, the screen may "flash" or "blink" just after the "Welcome to Macintosh" message is displayed. This is NOT a problem! It occurs when the simple commands built into ROM hand off monitor control to the much more powerful instructions contained within the system software folder.) Assuming that all goes well up to this point, the ROM next instructs the CPU to contact the Extensions Manager Control Panel and identify any necessary extensions, control panels and fonts located in the system folder. These are then loaded into RAM, too. You'll recognize this process as the little "parade of icons" appearing across the bottom of your screen. (NOTE: A bomb or freeze during this part of the Startup Sequence usually indicates an extension conflict or corruption.) (NOTE 2: Not all extensions and control panels are loaded into RAM during startup. Some are only loaded later, if needed. Also, if you use a Font Management Utility like Adobe Type Manager, only those fonts located inside the system's Fonts Folder are loaded immediately. All other Fonts are loaded when the Font Management extension or control panel is loaded - usually near the end of the Startup Sequence, or when an application is launched.) OK, we're almost done now. Only two more steps remain. First, the ROM instructs the CPU to load and launch the Finder application. It's the Finder's job is to display the background image, icons, files, folders and drives on screen, and allow you to manipulate their appearance and behavior. During startup, the most important thing the Finder does is search the various hard drives, CD-ROM drives, diskette drives and removable drive devices, load their software "drivers" and "mount" the volumes on the desktop so that you can interact with them. (NOTE: A bomb or freeze at this point in the startup sequence may indicate a corrupted device driver or volume directory file.) Finally, as the last step in the Startup Sequence, the ROM instructs the CPU to search the Startup Items Folder inside the System Folder and activate any applications or aliases stored there. The Launcher, Control Strip and "Stickies" applications are also launched at this time. And there you have it! Your computer is now ready for work.
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HOME . SERVICES . HELP DESK . RESOURCES . WHY MAC . CREDENTIALS
Copyright 1998, 5-Minute Mac Consulting, Wampum, Pennsylvania, USA