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EFI to grub to DOS?

From: Jake Thomas
Subject: EFI to grub to DOS?
Date: Sun, 22 Jul 2012 17:33:52 -0700

What's wrong with going in and switching the firmware to BIOS mode when you 
need to boot into DOS, and then switching it back to EFI mode when you're done?

The times when you need DOS to see all your real hardware and/or your real 
machine's firmware should be so few and far between, that those rare switches 
won't be much of an inconvenience.

In cases where you don't need DOS to see your real hardware and/or firmware, 
simply use DOSBox or run DOS in some other virtual machine.

If you need DOS to see a real hard drive, you can probably share the hard drive 
with DOS through a hypervisor such as  QEMU.

Other hardware sharing options include Xen's PCI passthrough. Though at this 
point I'd just switch the firmware mode and boot DOS.

For something like flashing the BIOS, that can be done from within Linux, which 
can be booted from EFI. Certainly DOS games can be ran in DOSBox in Linux 
booted from EFI. Low-level DOS hard drive utilities might be able to work on a 
hard drive if DOS is running in a hypervisor where the hypervisor is giving DOS 
access to the hard drives. The hypervisor itself can be booted from EFI if it's 
its own OS (called a baremetal hypervisor), or you can run a hypervisor in 
Linux booted from EFI. Either way, the hypervisor supplies DOS with a BIOS.

Yes, DOS is one of the few (that I know of) operating systems that never (or at 
most rarely, as far as I know) communicates to hardware. Instead, it asks the 
processor to run a program to communicate to hardware _for_ it. These programs 
are a part of the BIOS specification and are in memory. Either the chip on the 
motherboard containing the BIOS has memory addresses pointing to it to get to 
the programs, or the programs have been copied to RAM (called shadow RAM). 
Therefore DOS really does need a BIOS in its full glory.


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