On Sat, 5 Aug 1995, Greg Troxel wrote:

> Back in the days of PDP-11s, Version 6 and 7 and BSD 2.8, I believe
> these were meant to correspond to processor traps.
> There are two reasons (at least) on an 11 that a memory reference can
> fail:

Add two more reasons for the PDP-11:  
   16-bit word operand or instruction reference on an odd address,

   Byte reference to an I/O space address where the device doesn't 
   support byte references.

(Not that many people still give a darn about this ancient workhorse!)