Basic Concepts

Each memory management module provides semantics for subsets or variants of these concepts. These semantics and variants are introduced, but are not covered in detail, in the following sections.

Address Spaces

The address space of a processor is split into two subsets: the supervisor address space and the user address space. A separate user address space is associated with each user actor. The address space of an actor is also called the memory context of the actor.

A memory management module supports several different user address spaces, and performs memory context switches when required in thread scheduling.

The supervisor address space is shared by every actor, but is only accessible to threads running with the supervisor privilege level. The microkernel code and data are located in the supervisor address space.

In addition, some privileged actors, that is, supervisor actors, also use the supervisor address space. No user address space is allocated to supervisor actors.

Regions

The address space is divided into non-overlapping regions. A region is a contiguous range of logical memory addresses, to which certain attributes are associated, such as access rights. Regions can be created and destroyed dynamically by threads. Within the limits of the protection rules, a region can be created remotely in an actor other than the thread's home actor.

Protections

Regions can be created with a set of access rights or protections.

The virtual pages that constitute a memory region can be protected against certain types of accesses. Protection modes are machine-dependent, but most architectures provide at least read-write and read-only. Any attempt to violate the protections triggers a page fault. The application can provide its own page fault handler.

Protections can be set independently for sub-regions inside a source region. In this case, the source region is split into several new regions. Similarly, when two contiguous regions get the same protections, they are merged into one region. The programmer is warned that abusing this module could result in consuming too many of the microkernel resources associated with regions.