In 4.2BSD, only buffer management and data-link layer processing happens at "device IPL." The device driver then places the packet on a queue, and generates a software interrupt to cause further processing of the packet.
At the time that 4.2BSD was developed, in the early 1980s, the rationale for this was that network adapters had little buffer memory, and so if the system failed to move a received packet promptly into main memory, a subsequent packet might be lost.
ULTRIX, derived from 4.2BSD, closely follows the network design of that system.
Following the first approach, one could modify the 4.2BSD design (see Section 4.1) by eliminating the software interrupt, polling interfaces for events, and processing received packets to completion at device IPL.
Most of these versions are derived from some form of 4.2BSD
or 4.3BSD Berkeley Unix.
Sequent, for example, used UNIX 4.2BSD as the basis of its DYNIX operating system for the Balance line of multiprocessor systems .
The scheduler can operate in either a time-shared or fixed priorityy mode, building on features of both System V and 4.2BSD UNIX.
An enhanced version of UNIX 4.2BSD runs on Alliant's (Littleton, Massachusetts) tightly coupled environment of up to 12 interactive processors and up to 8 computational elements .
For example, the message below is generated by 4.2BSD
UNIX while formatting files on the terminal (see  for a more detailed discussion of this message):
The netlib server runs the UNIX operating system (the eight edition at Bell Labs and 4.2BSD
at Argonne), and consists of a few shell scripts and C programs.
On the distribution tape of 4.2BSD (a version of Berkeley UNIX), it happens that the directory named "/usr/spool/at" is universally writeable by all users; it also happens that it is fairly easy to trick the operating system into executing privileged commands by storing them in that directory.
This was caused in part because the people who prepared the 4.2BSD release tape did not anticipate the diversity of potential customers for their software.