There have, in fact, been multiple fork() implementation for Windows without kernel support. They weren't very efficient, but IIRC, only in version 1.7 (at abou

There have, in fact, been multiple fork() implementation for Windows without kernel support. They weren't very efficient, but IIRC, only in version 1.7 (at about 15 years of age), did the Cygwin fork() implementation start to use kernel support. And that still does not answer my question: In what way is the "NT kernel subsystem" different from a user mode library? Either the underlying kernel supports the features you need, or it doesn't. From my perspective, there's an NT kernel (with the so-called Native API) and a few libraries that make it look like Win32 or severely limited POSIX or at one point in time OS2.