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, readonly. 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, �M7hat virtual-memory primitives should the operating system provide to user processes, and how well do today�s operating systems provide them?”.