Are you familiar with the Opal OS from UW CS&E during the 90s ? All tasks in a single system-wide address space, with h/w access control for memory regions.

> The Opal project is exploring a new operating system structure, tuned to the needs of complex applications, such as CAD/CAM, where a number of cooperating programs manipulate a large shared persistent database of objects. In Opal, all code and data exists with in a single, huge, shared address space. The single address space enhances sharing and cooperation, because addresses have a unique (for all time) interpretation. Thus, pointer-based data structures can be directly communicated and shared between programs at any time, and can be stored directly on secondary storage without the need for translation. This structure is simplified by the availability of a large address space, such as those provided by the DEC Alpha, MIPS, HP/PA-RISC, IBM RS6000, and future Intel processors.

> Protection in Opal is independent of the single address space; each Opal thread executes within a protection domain that defines which virtual pages it has the right to access. The rights to access a page can be easily transmitted from one process to another. The result is a much more flexible protection structure, permitting different (and dynamically changing) protection options depending on the trust relationship between cooperating parties. We believe that this organization can improve both the structure and performance of complex, cooperating applications.

> An Opal prototype has been built for the DEC Alpha platform on top of the Mach operating system.

https://homes.cs.washington.edu/~levy/opal/opal.html