[mintsuki]

That is a very niche use case, and also won't really work on most real BIOS machines as they actually check for a proper MBR and/or BPB (and no GPT), which is also why GPT on BIOS support is very spotty, since a lot of BIOSes will detect the GPT and refuse to boot, or refuse to boot in legacy (CSM) mode.

In any case that is far beyond the original scope of booting old PC OSes, which MBR support alone serves really well (99.9% of the way there, really), which is why I assumed by default you were thinking of MBR, not some other weird scheme.

[superkuh]

GPT is not UEFI. It's just the format of the storage device. I have many computers with GPT formatted disks with actual master boot records at the start of the disk that boot MBR on both original BIOS machines and CSM BIOS machines.

MBR is two things with the same name. It is both the format of the storage device and describes a booting mode using a master boot record and up to 4 partitions.

GPT is one thing. It is a format for a storage device. It is the alternative to the MBR format for storage devices. It has nothing to do with (U)EFI.

BIOS or BIOS/csm are not types of formats for storage devices. They are types of boot processes. (U)EFI is another type of boot process.

You can easily mix and match boot types (BIOS/csm vs UEFI) and storage device format types (GPT vs MBR). As others have said, there may be some slight incompatibilities on some rare hardware/software configs, but mostly it just works.

[mintsuki]

I know every word of what I said in the message you're replying to. While what you said is mostly accurate, as in MBR != BIOS and GPT != UEFI, never in my original message have I stated that that is the case. And the rest of what you said is incorrect, as per my original message.

I invite you to read what I said again.

[superkuh]

Oof. On re-read I see now this sub-thread's topic was "non-GPT, non-MBR". So my "response" was ... off topic and useless. Sorry.

I am not sure what I said that was incorrect though. As I'm always looking to learn, which bit? I am currently assuming the 'rest' about GPT on true BIOS machines. I suppose I have never booted off GPT formatted disks in true BIOS machines. I always boot off MBR formatted disks and just used the GPT as storage/etc.

But I'm unclear on how this would break things since you can still have the MBR in the first sector on a GPT formatted disk. And how could the true BIOS motherboards detect GPT format on the disks when they don't know about it?

[mintsuki]

Because they do know about them. At least some of them do, of course.

Ideally, BIOS/CSM systems should not care about what type of partition table format a disk uses, and just boot by loading the boot sector (the 1st sector), and passing control to it.

In the real world, some machines will check for whether an MBR exists (by MBR meaning the partition table contained in the first sector, also called "msdos" partition table, naming varies depending on who you ask), some of them will check whether there is any partition set as active in said MBR, some of them will, indeed, check if there is a GPT (past any protective MBR), and ignore such disks - possibly because firmware writers thought that no GPT disk should be bootable on BIOS systems (Windows doesn't support it, so...), to avoid passing control to an empty protective MBR and possibly crash or hang as a result.