Octocontrabass wrote:
most BIOSes only allow AH=42h if the first sector is a "valid" MBR
Where did you get such a nonsense? Let's make this clear: AH=42h reads a sector from disk to memory, nothing more, nothing less.
Octocontrabass wrote:
Most only check for a partition table, but some also check that the partitions don't overlap, or that one of them is marked bootable.
Wrong again. No BIOS checks for partition table of any kind, it's the job of MBR code. As the last step of POST, BIOS checks whether the first sector has a magic, and if yes, transfers control to it. That's all, and it has nothing to do with sector loading nor with partition tables. Writing a VBR in place of MBR would boot just fine, give it a try.
Octocontrabass wrote:
One BIOS checks for a FAT BPB!
Wrong again. No BIOS checks for BPB, it's the job of VBR code. Besides, it would mean BIOS can only boot from FAT, which is simply not true.
Octocontrabass wrote:
If you like to boot Linux ISOs from a flash drive (using isohybrid), you've probably seen a computer boot without using AH=42h, since the isohybrid boot sector is not "valid" enough for many BIOSes.
I don't care linux boot hacks. I was talking about BIOS Boot Specification.
Let's make this to an end. You are not aware of basic terms, and how an IBM compatible PC boots up, and this thread is about using UEFI anyway.
Have a nice day and don't learn too hard.