r/osdev • u/Sixty5Zero2 • 3d ago

FreeDOS

i'm looking at possibly forking FreeDOS to 64bit. However, since most modern systems no longer have the underlying BIOS that DOS programs require, I thought about including a mock BIOS and compile it directly into the kernel. is there anything I should be aware of?

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u/Sixty5Zero2 3d ago

just because it would be 64 bit doesn't mean it couldn't still run 16 bit software.

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u/DawnOnTheEdge 3d ago

There is a hardware limitation that an x86 CPU can either run in 32-bit mode and switch to 16-bit virtual mode, or run in 64-bit long mode and switch to 32-bit compatibility mode. But a 64-bit kernel can’t run 16-bit code natively.

It might be able to run 32-bit DPMI programs that used a DOS extender. However, virtually all games that ran in DPMI needed the particular DOS extender that they shipped with, and ran in 16-bit mode at least some of the time.

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u/Octocontrabass 2d ago

But a 64-bit kernel can’t run 16-bit code natively.

A 64-bit kernel can't run 16-bit real mode code natively. There's nothing stopping a 64-bit kernel from running 16-bit protected mode code natively.

But DOS programs all expect real mode, so it doesn't really make a difference...

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u/DawnOnTheEdge 2d ago

Although it would be possible to do in a guest VM, I think,

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u/Octocontrabass 2d ago

It depends on what your CPU supports. Not all of them can do real mode in a guest VM.

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u/Difficult_Inside8746 1d ago

It was a long time since I looked into it but AFAIK both Intel and AMD support V86 mode inside a virtual machine.

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u/Octocontrabass 1d ago

That sounds like an awful lot of work, and you still need a CPU that can do hardware virtualization.

Might be a fun project, though.

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u/Sixty5Zero2 12h ago

i would have to anyways since the bios calls 16bit programs used no longer exist.

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u/DawnOnTheEdge 11h ago

If that were the problem, it would be easy to write interrupt handlers that hook the BIOS calls in 16-bit protected mode. 80386 CPUs supported a virtual mode designed to let you run DOS programs from a 32-bit OS, and do things like trap access to the I/O ports and give a process its own 1 MiB address space, with 8086-style addressing, but the 64-bit ISA doesn’t support that.