When you first power up a PC, the machine goes through several internal processes before it's ready for you to use. This is called the boot process, or booting the PC. Boot is short for bootstrap, a reference to the old adage, "Pull yourself up by the bootstraps," which means to start something from the very beginning. The boot process is controlled by the PC's basic input-output system (BIOS).
The BIOS is software stored on a flash memory chip. In a PC, the BIOS is embedded on the motherboard. Occasionally, a PC manufacturer will release an update for the BIOS, and you can carefully follow instructions to "flash the BIOS" with the updated software.
Besides controlling the boot process, the BIOS provides a basic configuration interface for the PC's hardware components. In that interface, you can configure such things as the order to read drives during boot and how fast the processor should be allowed to run. Check your PC's documentation to find out how to enter its BIOS interface. This information is often displayed when you first boot the computer, too, with a message such as, "Press DEL to enter Setup Menu."
The following is a summary of the boot process in a PC:
- The power button activates the power supply in the PC, sending power to the motherboard and other components.
- The PC performs a power-on self-test (POST). The POST is a small computer program within the BIOS that checks for hardware failures. A single beep after the POST signals that everything's okay. Other beep sequences signal a hardware failure, and PC repair specialists compare these sequences with a chart to determine which component has failed.
- The PC displays information on the attached monitor showing details about the boot process. These include the BIOS manufacturer and revision, processor specs, the amount of RAM installed, and the drives detected. Many PCs have replaced displaying this information with a splash screen showing the manufacturer's logo. You can turn off the splash screen in the BIOS settings if you'd rather see the text.
- The BIOS attempts to access the first sector of the drive designated as the boot disk. The first sector is the first kilobytes of the disk in sequence, if the drive is read sequentially starting with the first available storage address. The boot disk is typically the same hard disk or solid-state drive that contains your operating system. You can change the boot disk by configuring the BIOS or interrupting the boot process with a key sequence (often indicated on the boot screens).
- The BIOS confirms there's a bootstrap loader, or boot loader, in that first sector of the boot disk, and it loads that boot loader into memory (RAM). The boot loader is a small program designed to find and launch the PC's operating system.
- Once the boot loader is in memory, the BIOS hands over its work to the boot loader, which in turn begins loading the operating system into memory.
- When the boot loader finishes its task, it turns control of the PC over to the operating system. Then, the OS is ready for user interaction.
Now that we're all powered up, what's next? A great deal of how PCs work depends on the operating system you use. In the next section, let's examine how operating systems work on a PC.