single user v multiuser boot

Hi,

I was wondering what gives the kernel the ability to boot in multiuser
mode, and whether it is some code in the kernel or whether it is the
init process and associated tools?

As a follow-up question, if the kernel for a new architecture can
already boot in single user mode, would it be just a matter of
compiling init and those tools for that particular architecture to get
the kernel to run in multiuser mode?

The reason I ask these questions is that I would like to continue work
on the FreeBSD/390 port. I have a mini mainframe (P/390) which I could
use to test the code on real hardware, and I'd much rather use
FreeBSD/390 than Linux/390 on the little box! Since I cant write
kernel code yet, I was going to try compiling init and the shared
libraries as 390 code and try to get the system booting further (it
would have to be a memory based filesystem mounted as root, I dont
think a driver for mainframe CKD or FBA disk exists yet).

I was thinking eventually of porting linux 390 drivers to FreeBSD, but
im not sure if licensing issues come into that.

Apologies if i have posted to the wrong mailing list, i am not sure
what this comes under.

Thanks for your patience, Alex J Burke.