Virtual memory primitives for user programs

Memory Management Units (MMUS) are traditionally used by operating systems to implement disk-paged virtual memory. !30me 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?'