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Comment by userbinator | Hacker News Reader

Very easy indeed, especially if you're not concerned with 100% compatibility with existing MS-DOS API. It's closer to a library of routines that serve as a thin layer on top of the BIOS that just provides a filesystem and program loader. No (preemptive) multitasking (although TSRs are still possible), dynamic linking, virtual memory, etc.

If you want nothing more than a filesystem and program loader, you can fit everything in 512 bytes: https://news.ycombinator.com/item?id=20569438

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It's easy now because plenty of documentation exists of how to do it and there are plenty of open source examples, and there widely available, inexpensive or free, easy-to-use development environments and emulators and debuggers.

The original DOS boot sector was also a "filesystem and program loader", albeit one that wasn't able to do much other than find IO.SYS, load it, and run it.

There's also the fact that a modern emulator (assuming you're running as an emulator, instead of real/raw) is that you aren't constrained by memory the way real hardware implementations were either.

I keep thinking it would be cool to emulate enough of DOS and earlier x86 to just run BBS Doors directly and connections over ws(s) to make authentication and program selection a bit easier. Even if no classic BBS terminals (currently) support connecting over WebSockets, which are easy enough to bridge.

What I wanted to do is make a "DOS" that's actually just a bootloader for a UNIX-like OS, but it's actually structured and works like DOS, just for launching kernels and minimal file management.

I think the real hard thing would be writing a full BIOS for an 8086 from scratch, capable of running a real DOS. I also wanted to do that, but...

There were loadlin and grub4dos back in the day. A more recent development is doslinux (https://github.com/lpsantil/doslinux) but it is far less complete. But more hackable if you just want to see something work.

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