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5.7 Case Study: Evolution of Enforced Mo... > 5.7.3 Page-Based Virtual Address Spa... - Pg. 287

5.7 Case Study: Evolution of Enforced Modularity in the Intel x86 287 kernel) to ensure, for example, that user-level programs cannot access kernel-only segments. Although Intel sold 15 million 80286s, it achieved the three goals only partially. First, 24 bits was small compared to the 32 bits of address space offered by compet- ing processors. Second, although it is easy to go from real to protected mode, there was no easy way (other than exploiting an unrelated feature in the design of the processor) to switch from protected mode back to real. This restriction meant that an operating system could not easily switch between old and new programs. Third, it took years after the introduction of the 80286 to develop an operating system, OS/2, that could take advantage of the segmentation provided by the 80286. OS/2 was jointly created by Microsoft and IBM, for the purpose of taking advantage of all the protected-mode features of the 80286. But when Microsoft grew concerned about the project, it disowned OS/2, gave it to IBM, and focused instead on Windows 2.0. Most buyers didn't wait for IBM and Microsoft to get their operating system acts together and instead simply treated the 80286-based PC as a faster 8086 PC that could use more memory. Overlapping with the 80286, Intel invested over 100 person-years in the design of a full-featured segment-based processor architecture known as the i432. This proces-