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The boot process occurs whenever you turn your computer on. It is hard to see because it happens quickly. After pressing the power button, in a few minutes, Windows XP, or Windows Vista, or whatever Operating System you use gets loaded.
In this blog, you will find out what really happens when you press the power button and turn on the computer. The process is called the boot process. A list of what typically happens in a PC is as follows, but it will differ depending on your BIOS and manufacturer.
1. Pressing the power button turns the computer power supply on. A ‘Power Good’ signal is sent to the motherboard after which the CPU looks for the ROM BIOS.
2. The first instruction by the ROM BIOS to the CPU is to run the POST (Power-On-Self-Test).
3. The POST then checks the BIOS and then tests the CMOS RAM. If there are no problems with this, then the POST continues to check the CPU and hardware devices such as the Video Card and the secondary storage devices such as the Hard Drive, Floppy Drives, Zip Drive or CD/DVD Drives.
4. If errors are found, then an error message can be visible on screen or a number of beeps are heard (POST beep codes). If the video card hasn’t been initiated or has an error, the computer uses these beep codes instead of displaying an error message.
5. After this, the BIOS finds the video card and runs the video card’s BIOS . Usually, this is the first thing that modern machines display on the screen. After that the computer looks at the other devices and runs their BIOS if they have one.
6. The BIOS now displays the system configuration.
7. More tests are conducted during the display, including the test that shows your computer testing the memory. If problems are found from now on they will be displayed in a text message on the screen.
8. Next, the BIOS searches for something that it can boot from. Under the boot sequence, this can be set in the CMOS. This can be set to the A: Drive (Floppy) C: (Hard Drive, Primary Partition) D: (CD/DVD Drive) or others such as the USB drive or network card (depending on the bios).
9. Once the target boot device has been selected, the BIOS will search for the Master Boot Record (MBR). While searching for a hard drive it looks at cylinder 0, head 0, sector 1.
10. On finding a valid volume boot sector, the BIOS has done its job and hands over control to the Operating System that completes the booting process. A few hardware tests will also be conducted.
11. The computer will display an error if a valid boot record is not found. This is the “Non-System Disk or Disk Error”. Press any key and replace when ready.
12. If a valid boot record is found but the Master Boot Record cannot be read by it, then the computer will display a message, “Disk Boot Failure, Insert System Disk and Press Enter”