An address space is a collection of regions, each region tied to some file or UIO object. The kernel acts as a cache manager for regions and sends messages to the file or UIO object manager when page faults happen. An ownership protocol and a lock manager at the object's site handle consistency of cached data. All paging is on demand. Device Management Device management is handled by a device server module.
Device drivers are mostly at user level.
Some parts such as protection, interrupt handling, and privleged execution of instructions are handled are the kernel level. When process migration appeared, this was a problem because all processes with the same logical host subfield had to be moved together. The introduction of process groups and multicast led to a better abstraction and simpler code. There was originally an optimization of a process group called a local group. A group was a local group if all processes in the group lived on the same machine.