hybrid MBR

[kerravon]

I have a 2011 Macbook Pro and I spent a lot of time trying
to get it to boot Freedos as a precursor to booting PDOS,
but it refuses to recognize the USB stick (hard disk image)
as bootable, despite the fact that it is supposed to
support legacy boot/BIOS to some degree.

I would like to know what the Macbook is looking for. I
don't care (at this stage) if PDOS doesn't work, I just want
it to be recognized as bootable.

I would like to know what the minimal changes are required
to my disk image to see some action. My disk image is here:

http://pdos.org

I'm wondering whether zapping 80 to EE and maybe zapping
a legacy boot flag will get some action. I don't care if my
disk is trashed etc. I continually reburn the USB stick anyway.

Any idea what the situation is?

I'm not interested in running any tools/Bootcamp etc. I only
want to run my own "zap" program to change on byte at a
time on my image.

Note that after a lot of experimentation and research I
stumbled across the suggestion that the Mac uses a "hybrid
MBR" which I had never heard of. But that doesn't answer
my specific question of which bytes exactly the Macbook
is looking for. Not x'aa55' signatures presumably.

Thanks. Paul.

[iansjack]

Macs don't work with BIOS calls or MBR boot loaders - they don't have a PC-type BIOS. At the very least you need a UEFI boot loader.

To run DOS-type programs you need to use an emulator/virtual machine.

[Octocontrabass]

it is supposed to support legacy boot/BIOS to some degree.

But does it support legacy boot from USB? Some Macs only support legacy boot from the internal hard disk or from DVD.

Macs don't work with BIOS calls or MBR boot loaders - they don't have a PC-type BIOS.

Yes they do. Apple added a CSM to the Intel Mac firmware shortly after the release of the first Intel Macs.

[kerravon]

it is supposed to support legacy boot/BIOS to some degree.

But does it support legacy boot from USB? Some Macs only support legacy boot from the internal hard disk or from DVD.

I don't know that. It's late 2011. It's certainly strange that they have CSM, and they support USB booting of UEFI - but refuse to allow those to be combined.

I guess what I want is a definitive .img file that works on Macs that support USB legacy boot. And if that doesn't work, I know I must give up trying to make PDOS work.

[iansjack]

You're right. CSM was only removed in 2015, and the OP's Mac is 2011.