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.
