Hey PDP-8
IMO, for as far back as I can remember (which is a very long time) we have incorrectly used terms like BIOS to refer to the “Setup utility” or more recently known as “firmware setup utility” where BIOS and UEFI actually refer to the environment for BOOT execution.
As far as Intel are concerned, there is no such thing as BIOS anymore, Intel embraced UEFI with support for UEFI only, UEFI with Compatibility Mode or legacy (support for MBR) modes.
I should back up a little, The old MBR boot environment was limited with size constraints of the drives first sector and 16 bit code. Whereas UEFI has none of these limitations and is additionally cross platform compatible.
Ok so moving on with the various modes.
UEFI only mode expects a Guid Partition Table, an ESP and a boot loader (file) in a specific location within the ESP.
UEFI with CSM (Compatibility mode) adds support for hardware which doesn’t yet support EFI and provides support for a boot loader as per the MBR specification
Legacy mode supports only an MBR partition table and boot loader with part of its code installed on the boot drive’s first sector.
How manufacturers present these modes will vary considerably. When they gave a Windows or other OS option they were actually referring to the type and more importantly the environment and location of the boot loader.
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