The (Hardware) Boot Process

Turn on the power

Timer chip begins sending continual reset signals to the CPU to prevent it from booting the PC.

Power Supply performs internal checks. When that's done & power output levels have stabilized, it sends the Power_Good signal to the CPU.

With the arrival of the Power_Good signal, the timer stops sending reset signals to the CPU, thereby permitting it to begin the boot process.

CPU loads the BIOS & then searches for devices that have BIOS extensions to load. Video adapters are the best example of devices that would require BIOS extension.

BIOS checks whether this is cold or warm boot. Cold boot happen when you boot the PC from a powered-off state or when you press the hardward reset button on the front of the system case. Warm boot is a restart.

If this is a cold boot, the BIOS performs the POST procedure.

BIOS tests the core hardware, including itself, the processor, CMOS, the input/output system, and so forth. Any errors detected at this stage are reported as a series of beeps.

BIOS tests the video subsystem. This test includes checking memory dedicated to video operations, checking video processing circuitry, & checking the video configuration (That's it's valid & not corrupted)

BIOS identifies itself, including its version, manufacturer, and date.

BIOS test main system memory. Some BIOS versions display a running count of how much memory has been tested. Others are "silent" unless an error is found.

BIOS reads the CMOS configuration data and configures devices.

Plug and Play devices are detected & configured.

BIOS determines which drive to boot from by checking CMOS configuration data.

BIOS reads the master boot record (MBR) from the boot drive. This record provides information necessary for the boot process to proceed from this point onward. If MBR can't be located, the CMOS displays an error message, such as "Non-system disk or disk error."