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How Your Computer Boots

When you press the power button on your computer, power is provided to all the components and the boot process begins. This process happens basically as follows:

As soon as power is supplied to your main system board, a process called the Power On Self Test (POST) begins. This process is controlled by read-only memory (ROM) chips on the main board that test the more important pieces of hardware on the computer.

Control of the testing process is first passed to your graphics adapter so that it can test itself. This is why the first screen you see on starting a computer is usually a blank screen with information on your graphics adapter at the top.

Next, control is passed back to the regular POST routine and the main POST screen appears, similar to the screen shown in Figure 6.1.

Figure 6.1. The main POST screen when starting a computer.

The POST routine tests your processor and displays the processor version on your screen.

Once the processor test is complete, POST hands control of your system over to the Basic Input Output System (BIOS). BIOS is firmware coded on a chip on your main board that controls the behavior of your computer before any software loads and the basic configuration settings for most of the hardware attached to the computer (e.g., hard drives, CD-ROM drives, parallel and serial ports, memory, and so on). If, during setup, you hit the Delete key (or whatever key enters setup on your computer), it is the BIOS settings that will appear for you to configure.

Assuming you do not enter the BIOS setup, BIOS next tests your memory. This step (the memory countdown) is probably the one you are most familiar with. The memory is displayed on the next line after the processor.

BIOS then proceeds to check the connection to your various hard drives, CD drives, and floppy drives. If the connections are not present or are different from what is listed in the BIOS settings, you will get an error message and the boot process won't continue. You will have to enter BIOS setup to fix things.

Assuming all goes well, BIOS next displays a screen that summarizes the state of your system.

BIOS then calls a special code, named the BIOS operating system bootstrap interrupt (Int 19h). This code first finds a bootable disk using the guidelines for which drive should be booted first as set in the BIOS settings.

Once BIOS finds a bootable disk, it loads the program found at the Master Boot Record (MBR) of the disk into your computer's memory, and then passes control of the computer to that program. The startup phase now passes from the booting of your computer to the booting of Windows.



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