Understanding the Startup Process

So what happens when you first turn on your computer? What complex chain of events is initiated when you flip your power switch to the "on" position?

The first thing that happens is that power is sent through the PC's power supply to the boards in the system unit. This initial power surge (called a power-on reset) resets your system's memory, microprocessor, and other electronics. Then your system, using instructions stored in its basic input/output system (BIOS), does a power-on self test (POST). During this test, you see some messages scroll down your screen, letting you know how much RAM is available and that the system checks out as expected.

After the self-test, your computer tries to access drive A, looking for a bootable diskette. A bootable diskette contains important system files necessary for your system to operate files that are normally "hidden," and not displayed in a normal directory listing.

If a bootable diskette is in drive A, your system proceeds based on the startup files on this diskette; if not, your system proceeds to the next bootable drive, which is probably your CD-ROM drive. (This is certainly the case in all newer systems, from about Windows 98 on; older systems typically can't boot from the CD-ROM drive.) If there's no bootable disc in your CD-ROM drive, your system (finally!) proceeds to drive C to look for the system files.

When your computer accesses drive C, it reads into memory the contents of the disk's boot sector, which tells the system how to load the operating system.

If you're running Windows XP, NT, or 2000, your system now loads the Windows operating system. If you're running an older version of Windows, however, your system doesn't load Windows just yet; instead, it loads a copy of the older DOS operating system. This is because all versions of Windows up to and including Windows 98 were based on DOS, and actually sat on top of the older operating system. In any case, this all happens in the background you don't actually see DOS on your screen.

Your system then begins to load Windows and reads important settings from the Windows Registry, a database that contains all of your system configuration settings. As Windows loads, it also loads a variety of device drivers, as specified by the Windows Registry. Device drivers are small files that enable various pieces of hardware to operate; they're loaded into your system's memory, where they're immediately available for use.

note

Older versions of Windows pre-Windows 95 don't have a Registry, and instead rely on the SYSTEM.INI, WIN.INI, CONFIG.SYS, and AUTOEXEC.BAT files to hold these settings. To maintain compatibility with older software programs, these "legacy" files are still loaded in newer versions of Windows, and can cause a variety of problems on boot-up.

Finally, after all the settings have been checked and all the drivers loaded, Windows itself is launched. You see a "loading" screen (which replaces the text-based screen you've seen up till now), hear "The Microsoft Sound," and then the Windows interface begins to build, element by element, until you can see the entire desktop and can use your system.

All this activity is accompanied by beeps and whirrs and clicks and flashing lights, which just indicate that your system is doing its job. If you don't hear all the beeps and whirrs and clicks and see the flashing lights, and if your system doesn't load the boot program, the system files, and Windows, you have a problem.

THE ABSOLUTE MINIMUM

Here are the key points to remember from this chapter:

• Your computer system is composed of various pieces of hardware, almost all of which plug into that big beige (or gray or black) box called the system unit.

• The system unit is the brains and engine of your computer system; it contains the microprocessor, memory, disk drives, and all the connections for your other system components.

• Data is temporarily stored in your system's memory; you store data permanently on some type of disk drive either a hard disk, floppy disk, or CD-ROM.

• You interface with your computer hardware via a piece of software called an operating system. The operating system on your new computer is probably some version of Microsoft Windows.

• When you turn on your computer, it goes through a series of self-tests, loads basic system settings, and then starts the Windows operating system.