BIOS Boot Sequence - A Closer Look

1. Internal power supply turns on, initializes and then takes few moments to generate reliable power for rest of the system. If power received is not within expectd parameters (by chipset and subsequently processor), then the chipset will generate a reset signal to the processor in the same fashion as you were to touch the reset button. This will continue until motherboard receives "Power Good" signal from power supply
2. Since processor has no idea on what to do next as there is nothing to execute in memory, it has been designed to look at the same place in the system, the BIOS ROM, for the small bit of startup code to begin the boot process (CS = 0xFFFF and EIP = zero => 0xFFFF0). Since there are 16 bytes (total is FFFFF), this location contains just a "jump" instruction telling the processor where to go to find the real BIOS startup program (see the diagram).
3. BIOS then performs POST (Power-On Self Test). If there are any fatal errors, the boot process stops. If the POST is successful, the BIOS calls INT 19 (Interrupt 19) and then proceeds to look for devices attached to the motherboard
4. BIOS enables the video: The BIOS code begins searching by looking for a video card, and video card's built in BIOS program (normally at 0xC000 in memory) and if found, it runs it.
5. Once video is enabled, the BIOS begins searching for other devices that may have their own ROM and whether ROM has its own BIOS code. Normally, the floppy driver is located at 0000:7c00 and the IDE/ATA HDD BIOS will be found at C8000h. If any of these is found, their codes are executed. If during this INT 19 process, any other device BIOS's are found, they are executed as well
6. BIOS displays its startup screen, which provides some key information about the BIOS as well as other system information
7. BIOS then performs some kind of inventory of the hardware installed in the system and then communicates or interrogates it to ensure that the hardware is functioning (at this stage, BIOS will populate BIOS table, can be ACPI table, if the system is ACPI compliant. This table can be used by several purposes including power management, by the OS applications)
8. During the final phase of POST and BIOS boot process, the BIOS will display a summary screen with your systems' configuration.
9. NOW, BIOS finishes what it needs to do, it begins searching for a drive to boot an OS. If its searching a hard disk, it looks for a MBR at cylinder 0, head 0 and sector 1, the first sector on the disk. If its searching for Floppy disk, it looks at the same address on the floppy disk for a volume boot sector
10. Once the boot sector is found, BIOS starts the process of booting OS by using the info in boot sector. If its floppy, the boot sector is also read into memory at 0000:7C00 and then MBR program itself jumps to memory location 0000:7c00 (NOTE: Each OS has its own boot sector format. The next step involves the small program in the boot sector locating the first part of OS's kernel loader proram or in some cases the kernel itself or perhaps a boot manager program, and read then that into memory. In windows, this kernel loader referred to as NTLDR.
11. If no boot device of any type can be found, the system will display an error message and stop.

NOTE: This entire process is called as "cold boot" (since the machine was off, or cold, when it started). A "warm boot" also known as "soft boot" is the same thing except it occurs when the machine is rebooted using Ctrl+Alt+Del keys. In this case, POST is skipped and boot process continues at roughly step 8 above. As a side note, INT19 is also called when the CTRL+ALT+DEL keys are used.