Levels of UNIX, an opinion, a question

[gwyn%brl-vld at sri-unix.UUCP]

From:      Doug Gwyn (VLD/VMB) <gwyn at brl-vld>

The early UNIXes for memory-managed (KT11) PDP-11s were available for
the PDP-11/40 as well as the PDP-11/45.  The /40 architecture was
similar to the more familiar /34 in that it had Kernel & User modes
but not separate Instruction & Data spaces.

Two "modes" are both necessary and sufficient for controlled resource
sharing with the typical memory manager architecture.