Virtual memory (MEM_VIRTUAL)

The virtual memory module, MEM_VIRTUAL, is suitable for systems with page-based memory management units and where the application programs need a high-level virtual memory management system to handle memory requirements greater than the amount of physical memory available. It supports full virtual memory, including the ability to swap memory in and out on secondary devices such as video RAM and memory-mapped I/O. The main functionalities are:

• Support of multiple, protected address spaces.

• On systems with secondary storage (the usual case), applications can use much more virtual memory than the memory physically available. This module supports full virtual memory with swapping in and out on secondary devices. This module is designed specifically to implement distributed UNIX subsystems on top of the microkernel.

• Pages are automatically swapped in and out when appropriate.

Segments

The segment is the unit of representation of information in the system.

Segments are usually located in secondary storage. The segment can be persistent (for example, files), or temporary, with a lifetime tied to that of an actor or a thread (for example, swap objects).

The microkernel itself implements special forms of segment, such as the memory objects that are allocated along with the regions.

Like actors, segments are designated by capabilities.

Regions

An actor region maps a portion of a segment to a given virtual address with the associated access rights.

The memory management provides the mapping between regions inside an actor and segments (for example, files, swap objects, and shared memory).

The segments and the 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 requesting actor.

Often, regions can define portions of segments that do or do not overlap. Different actors can share a segment. Segments can thus be shared across the network.

The microkernel also implements optimized region copying (copy -on-write).

Protections

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

The virtual pages constituting a memory region can be protected against certain types of access. An 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 combined into one region.

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Note -

Abusing the MEM_VIRTUAL module could result in consuming too many of the microkernel resources associated with regions.

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Explicit access to a segment

Memory management also allows explicit access to segments (namely, copying) without mapping them into an address space. Object consistency is thus guaranteed during concurrent accesses on a given site. The same cache management mechanism is used for segments representing program text and data, and files accessed by conventional read/write instructions.