The way BIOS boot works is it loads the MBR, which loads the code on the active partition. In fact, if you take a look at the ISO, you will notice a folder called EFI on root and if you look inside, you will find /EFI/Boot/Boot圆4.efi in it.įor BIOS booting, I will only talk about Windows and normal USB booting (as Grub and El Torito boot are far more complicated and DVDs/CDs are rarely used for booting installation media anyways). From there, it looks for a file /EFI/Boot/Boot.efi So, for x86-64 computers (basically all computers that are 64-bit), it is boot圆4.efi. The way it works is that there must be a filesystem that UEFI understands (with FAT32 usually used since UEFI standards specify that FAT file system must be supported at the very minimum). That said, there are some firmware that do things like switch to CSM (Legacy Boot) if they see the MBR partitions so they do not adhere to the spec and GPT may be more reliable). As far as UEFI is concerned, it can boot from MBR or GPT partitions (It is only Windows which mandates that UEFI booting requires GPT partition. I will go with the newer UEFI booting first, since it is far simpler. The way it works is actually interesting, but it diverges depending on what type of computer you have: the older BIOS-MBR booting or the newer UEFI booting.
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