I boot systems multiple times a day. Sometimes, I have to pay close attention to how they are booting or why they are not booting up. Being familiar with the boot process is rather important. Here it is in a nutshell.
- Power supply sends signal to the motherboard and other system components
- Processor is hard-coded to know where to find BIOS (in system BIOS ROM – normally at location FFFF0h, right at the end of system memory)
- BIOS runs tests, called POST (Power On Self Test).
- After the test, results of POST are compared with data in CMOS chip. Typically one beep means “all is well” while other sequences of beeps indicate “all is not well”. The sequences of beeps are used for troubleshooting and are typically specific to the motherboard in question. Check with the manufacturer for POST beep codes.
- Next, the video card’s BIOS is loaded and run. Other BIOSes are found and executed. The order depends on timing, mostly?
- BIOS displays its startup screen and performs additional tests such as memory count-up tests and an inventory of hardware. Settings for memory and hard drives are set at this time. Messages are typically displayed to the screen for logical devices it finds as well as Plug and Play devices.
- BIOS looks for boot devices according to boot order and a master boot record (at cylinder 0, head 0, sector 1)
- The master boot record points to the boot sector where the kernel can be found
- Entry point for kernel is located and the kernel is loaded into RAM. BIOS hands off to the kernel at this point.
- Operating system continues to load according to instructions and parameters passed to kernel
Thanks to flint.cs.yale.edu for their wonderful write-up with links to more in-depth articles.