Early SunOS versions (SunOS 3 and earlier) were based on the old BSD-style memory system, which was not modularized, and thus it was difficult to move the memory system to different platforms. The virtual memory system was completely redesigned at that time, with the new memory system targeted at SunOS 4.0. The new SunOS 4.0 virtual memory system was built with the following goals in mind:
Use of a new object-oriented memory management framework
Support for shared and private memory (copy-on-write)
Page-based virtual memory management
The VM system that resulted from these design goals provides an open framework that now supports many different memory objects. The most important objects of the memory system are segments, ...