How does a computer boot and how does this differ for Windows, Linux and other OSes?
Answer
What are the several steps the computer does during boot on a BIOS PC?
When you power your computer on, the processor will start the BIOS boot program.
The BIOS phase
If everything goes well; the computer will look for a video card, execute a power-on self test and show all the useful information about this and the next steps on the BIOS startup screen:
You will see the computer perform a memory test, detect any storage devices connected and depending on BIOS you might also see a second screen showing you an inventory of everything the computer has detected through common interfaces like COM/LTP/USB/PnP/...
So, as your computer is now ready it will now follow the boot sequence to determine which drive to boot from, it will look through the master boot records until a boot partition has been found, in which case the computer will start to boot the operating system by executing the boot record.
The Boot Loader phase
Back in these days, we had to boot DOS and Unix which we could boot in a direct way. Nowadays, the boot procedure is subject to limits. So at this point we'll have to let a small boot loader load the larger boot procedure. The small boot loader will set up important stuff the boot procedure needs and will load the essential parts of boot procedure into memory.
This leads on to booting Windows, booting Linux or ...
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