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 Post subject: hybrid MBR
PostPosted: Sat Jun 12, 2021 12:40 am 
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Posts: 232
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.


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 Post subject: Re: hybrid MBR
PostPosted: Sat Jun 12, 2021 2:24 am 
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Location: Chichester, UK
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.


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 Post subject: Re: hybrid MBR
PostPosted: Sat Jun 12, 2021 4:06 pm 
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kerravon wrote:
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.

iansjack wrote:
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.


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 Post subject: Re: hybrid MBR
PostPosted: Sat Jun 12, 2021 6:37 pm 
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Octocontrabass wrote:
kerravon wrote:
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.


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 Post subject: Re: hybrid MBR
PostPosted: Sat Jun 12, 2021 11:44 pm 
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Location: Chichester, UK
You're right. CSM was only removed in 2015, and the OP's Mac is 2011.


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