you are right. Though with the partition table in there so u can support a 'modern' AHCI controller and SATA it will shrink your bootloader further and require some optimizations.... - you don't have 510 bytes for the loader in this case but a bunch less. if you want to populate the table with valid entries then it becomes even more tricky (can't use any bytes inside of the table...)
If you want to use an actual modern harddisk, you might want to look at GPT rather than MBR, as it won't overflow partition table stuff and allow for very large disks (2TB+?) (uefi gets rid of all of that and allows u to use a proper disk layout without any additional difficulty!)
there is no need for protected mode if you want to launch into 64-bit mode. I would say though, DO NOT USE BIOS. It's a piece of rubbish which will just make things more tedious.
Using UEFI via EDK2 or GnuEFI is the way to go, and both methods are really easy and a blessin to implement. It's a bit hard to get around the initial idea of UEFI, but if you view some other people's example projects on github you can find easily how it works. EDK is a bit shitty with .dec and .inf files etc, and GnuEFI is just reading headerfiles to discover what is there, but it's inifnitely better than the unspecified bios interface. You can litterally not even assume the int 0x10, int 0x15 etc. are there properly if you start running on real hardware. On UEFI systems, you can assume a stable minimal basis, and easily enumerate other things (hardware/platform capabilities etc.) in a sane way.
Also UEFI sets up the platform a long way already, so you don't need to do any initialization for your os-loader component (stage2 or 3). You can simply start loading your os right away. or drivers or whatever kind of design your kernel is. get memory map, get some access to efi file system, start pulling in and loading stuff.
ACHI (which is how SATA is exposed) doesn't change any of this. The only thing that is affected is how the loaded OS has to talk to the disk controller. The real thing that loses space is the BPB (a 60 bytes or so, iirc) because some BIOSes are broken and require it and the MBR (but that's only a couple bytes). At least [bootelf] manages to fit in (without a BPB or an MBR) with 128 bytes to spare, enough for a dummy BPB that makes all BIOSes happy.
Additionally, UEFI's reliability is.. sketchy, as far as I know (using the classic logic of "if Windows doesn't use it does it really matter?"). And GNU-EFI suffers from build portability troubles, AFAIK.
If you chose AHCI on QEMU it will require a partition table to be present on the disk, so it recognizes it as a bootable disk. in addition to the magic value. If you do not add the partition table.
Thanks for this comment. It makes me realize this is likely not an AHCI thing ,but how the seaBIOS(? qemu's flavor?) handles enumerating disks via AHCI rather than IDE.
If it uses the IDE controller then it will recognize the boot disk. If i pick AHCI, i need to add the partition table.
UEFI reliability is sketch, but really BIOS is incredibly crap, so much more than UEFI.
Windows DOES use EFI/UEFI, how else will it boot on a system that has EFI/UEFI firmware inside of it? It can let you do secureboot, edit efi variables... - where do you get this classic logic from? (maybe i am totally misisng something, but they interface with it, and should thus use the spec? even tho they might not use gnuefi or EDK2 ofc :P (edk2 is still likley...))
If you want to use an actual modern harddisk, you might want to look at GPT rather than MBR, as it won't overflow partition table stuff and allow for very large disks (2TB+?) (uefi gets rid of all of that and allows u to use a proper disk layout without any additional difficulty!)
there is no need for protected mode if you want to launch into 64-bit mode. I would say though, DO NOT USE BIOS. It's a piece of rubbish which will just make things more tedious.
Using UEFI via EDK2 or GnuEFI is the way to go, and both methods are really easy and a blessin to implement. It's a bit hard to get around the initial idea of UEFI, but if you view some other people's example projects on github you can find easily how it works. EDK is a bit shitty with .dec and .inf files etc, and GnuEFI is just reading headerfiles to discover what is there, but it's inifnitely better than the unspecified bios interface. You can litterally not even assume the int 0x10, int 0x15 etc. are there properly if you start running on real hardware. On UEFI systems, you can assume a stable minimal basis, and easily enumerate other things (hardware/platform capabilities etc.) in a sane way.
Also UEFI sets up the platform a long way already, so you don't need to do any initialization for your os-loader component (stage2 or 3). You can simply start loading your os right away. or drivers or whatever kind of design your kernel is. get memory map, get some access to efi file system, start pulling in and loading stuff.