Virtual memory primitives for user programs

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, read-only, read-write) of pages, and allow user programs to handle protection violations, but these mechanisms are not always robust, efficient, or well-matched to the needs of applications. We survey several user-level algorithms that make use of page-protection techniques, and analyze their common characteristics, in an attempt to answer the question, 'What virtual-memory primitives should the operating system provide to user processes, and how well do today's operating systems provide them?'