Книга: Distributed operating systems

9.3.1. Regions and Segments

9.3.1. Regions and Segments

The main concepts behind memory management in Chorus are regions and segments. A region is a contiguous range of virtual address, for example, 1024 to 6143. In theory, a region can begin at any virtual address and end at any virtual address, but to do anything useful, a region should be page aligned and have a length equal to some whole number of pages. All bytes in a region must have the same protection characteristics (e.g., read-only). Regions are a property of processes, and all the threads in a process see the same regions. Two regions in the same process may not overlap.

A segment is a contiguous collection of bytes named and protected by a capability. Files and swap areas on disk are the most common kinds of segments. Segments can be read and written using system calls that provide the segment's capability, the offset, the number of bytes, the buffer, and the transfer direction. These calls are used for doing traditional I/O operations on files.

However, another possibility is mapping segments onto regions, as shown in Fig. 9-4. It is not necessary that a segment be exactly the size of its region. If the segment is larger than the region, only a portion of the segment will be visible in the address space, although which portion is visible can be changed by remapping it. If the segment is smaller than the region, the result of reading an unmapped address is up to the mapper. For example, it can raise an exception, return 0, or extend the segment, as it wishes.

Mapped segments are usually demand paged (unless this feature is disabled, for example, for real-time programs). When a process first references a newly mapped segment, a page fault occurs and the segment page corresponding to the address referenced is brought in and the faulting instruction restarted. In this way, ordinary virtual memory can be implemented, and in addition, a process can make one or more files visible in its virtual address space, so it can access them directly instead of having to read or write them using system calls.

The kernel supports special I/O segments for accessing the machine's I/O registers on machines with memory-mapped device registers. Using these segments, kernel threads can perform I/O by reading and writing memory directly.

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