The PDP-1x is a Digital Equipment Corporation PDP-1 computer modified to support both ones and twos complement arithmetic, and to include a memory management unit. It ran a locally developed operating system with specific facilities to represent virtualized devices and inter-process communications. The top-level element was called a "Sphere". It contained resources (called capabilities) and execution elements. I believe the execution elements were called "processes".
There was a manual for the OS named "INSTR5 ALL". I would like to read more about the OS to see if I was just a youngster amazed by something I was unfamiliar with, or if this operating system was unique and worthy of re-examination today.
Where could I find documentation of the PDP-1x Operating System?