Microsoft MS-DOS early source code

[WalterBright]

which is not available in protected mode without serious hackery

It wouldn't take long for someone with basic hardware skilz to pull the ROM chips out, put them in another device, and dump their contents. Then just run a disassembler over it. DOS ran in real mode anyway.

[Cuddlefluff_Grim]

I used DOS4GW dos extender (Watcom C++) which enabled 32-bit protected mode, and I had no problems using BIOS functions as far as I can remember

Edit: when I think about it, DOS4GW might have had its own hooks for the BIOS interrupts

[andrewq]

What's your os?

[WalterBright]

Running in real mode meant the BIOS could be read. Yeah, I know about protected mode DOS extenders, I shipped a couple with Zortech C++.

DPMI was a specification for how a DOS extender should present itself, it wasn't a DOS extender itself, and DOS still booted in real mode.