Memory Management Units (MMUs) are traditionally used by operating systems to implement disk-paged virtual memory. Some operating systems allow user programs to specify the protection level (inaccessible, readonly. read-write) of pages, and allow user programs t.o handle protection violations. bur. these mechanisms are not. always robust, efficient, or well-mat. ched to the needs of applications.

Dehn surgery is perhaps the most common way of constructing 3-manifolds, and yet there remain some profound mysteries about its behaviour. For example, it is still not known whether there exists a 3-manifold which can be obtained from S 3 by surgery along an infinite number of distinct knots. 1 (See Problem 3.6 (D)

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