The Undergraduate Computer Society (UCS) made a deal with the EE 
department that they could run UNIX on the departments 11/45 if they 
could get the BASIC PLUS that was running on the existing RSTS system 
running.    Turns out that wasn't too difficult.    UNIX uses trap as a 
system call, RSTS (like most DEC OS's for some odd reason) uses EMT.    
It only took a couple of calls that needed emulation in UNIX as well as 
an option to disable UNIX's automatic statck management (the "nostatck" 
system call).

BASIC PLUS was at the core of the largest freshman EE class:   Models 
and Simulation.   MNS students had a disk quota of a whopping 8 blocks 
(4KB).   It was encouraged that you buy a DECtape (something around 
500Kb) for long term storage though the system had a papertape 
reader/punch (how else to load the MAINDEC software).   I thought I was 
in fat city when me and my roommate chipped in and bought an RK05 pack 
(4872 blocks).   At the time the system ran on three "always mounted" 
RK05's:  The root, /sys1, and /sys2 (the latter being the user home 
directories).    The system swapped to an RF-11 fixed head disk (1 MB).  
   There were two extra RK05's shared between various users and were also 
dual ported to an 11/40 that ran MiniUNIX from time to time (until the 
guys upstairs bought an 11/23 that I moved UNIX to as well).

By the time I left, the system had picked up an 80MB removable drive, a 
bulk core box (emulated another RF-11), and a tape drive.