Levels of UNIX, an opinion, a question

[G.TOMASEVICH]

In the paging-in-somewhere discussion someone asked if UNIX has any levels
besides kernel and user.  I believe not; it originated on a PDP-11, probably
one without supervisor level (I'm not sure).  Now the only use of supervisor
mode on the PDP-11 is to accommodate a kernel that outgrew the kernel-mode
address space.  At that point, UNIX already started becoming a pig, and
now look where it is!  You need a VAX, huge disks, etc. to run everything.
The shell was outside the kernel because of the address space limit.
The question: where should one draw the line on putting shell, terminal
features, editor, etc when the machine has a huge address space?  This leads
to Rob Pike's discussion.