what is meant by page fault in operating system
Page fault A page fault (sometimes called #PF, PF or hard fault[a]) is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process. Logically, the page may be accessible to the process, but requires a mapping to be added to the process page tables, and may additionally require the actual page contents to be loaded from a backing store such as a disk. The processor's MMU detects the page fault, while the exception handling software that handles page faults is generally a part of the operating system kernel. When handling a page fault, the operating system generally tries to make the required page accessible at the location in physical memory, or terminates the program in case of an illegal memory access.
Denial-of-service attack A wide array of programs are used to launch DoS-attacks.
Single user mode Unix-like operating systems provide single user mode functionality either through the System V-style runlevels, BSD-style boot-loader options, or other boot-time options.
Page table There is normally one hash table, contiguous in physical memory, shared by all processes. A per-process identifier is used to disambiguate the pages of different processes from each other. It is somewhat slow to remove the page table entries of a given process; the OS may avoid reusing per-process identifier values to delay facing this. Alternatively, per-process hash tables may be used, but they are impractical because of memory fragmentation, which requires the tables to be pre-allocated.