PC's Boot Process

Boot Process

Firstly let me state this is my "adapted" version of the boot process for a Windows system.

The boot process is divided into 3 stages:

1. Powering on the PC
2. BIOS Power On and Self Test (Post)
3. Bootstrapping the Operating System

ATX V1.0 standard

Powering on the computer

The power supply in a PC conforms to the ATX standard. For the purposes of this discussion what does matter is the functionality of the POK pin on the ATX connector.

POK or “Power Good” pin is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the power supply is turned on.

When the POK pin had power, a power good signal which is received by the motherboard timer. When the timer receives this signal, it stops forcing a reset signal to the CPU and the CPU begins processing instructions.

When the CPU has reset it passes control to the software in the BIOS ROM.

BIOS ROM

BIOS Chip

In IBM PC compatible computers, the Basic Input/Output System (BIOS), also known as System BIOS, ROM BIOS or PC BIOS, is a de facto standard defining a firmware interface. Nowadays the BIOS is stored in Flash memory so the BIOS is upgradeable but traditionally the BIOS was "burned" into non volatile read-only memory (ROM).

The BIOS contains a sequence of instructions to start up the computer system. The boot sequence otherwise known as the power on and self test procedure is now executed.

BIOS Power On and Self Test (Post)

When the CPU is reset and control is passed to the BIOS at 0xF0000 to 0xFFFFF. Each manufacturers BIOS is unique but the aim of the BIOS is to initialise the hardware functionally, test the motherboard and the RAM on board and initialise each hardware component  plugged into the system bus.

An irregular POST could generate no beeps at all or a combination of different beeps to indicate the cause of the failure. 

Below is the BIOS Sequence for American Megatrends BIOS & Phoenix Technologies BIOS

	American Megatrends	Phoenix Technologies
1	Disable the NMI	Check the CPU
2	Power-on Delay	Test CMOS RAM
3	Initialize chipsets	BIOS ROM checksum
4	Reset determination	Test chipset(s)
5	BIOS ROM checksum	Test PIT
6	Keyboard test	Test DMA
7	CMOS shutdown check	Test base 64KB memory
8	Controller disable	Check serial and parallel ports
9	Disable video	Test PIC
10	Detect memory	Check the Keyboard Controller (KBC)
11	PIT test	Verify CMOS data
12	Check memory refresh	Verify video system
13	Check low address lines	Test RTC
14	Check low 64KB RAM	Test CPU in protected mode
15	Initialize support IC's	Verify PIC 2
16	Load INT vector table	Check NMI
17	Check the Keyboard Controller (KBC)	Check the keyboard
18	Video tests	Check the mouse
19	Load the BDA	Check system RAM
20	Test memory	Test disk controller
21	Check DMA registers	Set shadow RAM areas
22	Check the keyboard	Checkextended ROMs
23	Perform high-level tests	Test cache controller
24	Load the Operating System (OS)	Test CPU cache
25		Check hardware adapters
26		Load the Operating System (OS)

Note about BIOS CMOS

Battery for BIOS CMOS memory

The PC maintains system values set up in the BIOS Setup Screen in the battery backed up CMOS memory. This memory contains information such as the system time and date and information, the boot disc sequence and hardware configuration information required to get the computer running. If the battery fails this may be enough to stop the boot sequence.

Booting the Operating System

After the computer has passed the POST the last act in the boot sequence is to "bootstrap" the operating system. The bootstrap process gets quite mirky but essentially the BIOS loads the Master Boot Record (MBR) sector of the selected boot device and hands control over to the bootstrap loader.

The bootstrap loader further instructs the computer where to find the remaining code on the hard drive. Next, Windows loads the ntdetect.com file, which displays the Windows splash screen and loads the Windows registry. After loading the registry, Windows begins to load dozens of low-level programs that make up the operating system into memory. Many of the initially loaded programs are what allow Windows to communicate with the essential hardware and other programs running on the computer.

After the registry has loaded the initial basic hardware devices, it begins to load Plug and Play devices, PCI, and ISA devices. After loading all these devices, Windows then moves to loading full support of the hard drive, partitions, and any other disk drives and then moves to all other drivers that have been installed.

Finally, after successfully completing the above steps any additional required services are loaded and Windows starts.