Sometimes, a PTE prohibits access to a virtual page, perhaps because no physical random access memory has been allocated to that virtual page. In this case, the MMU signals a page fault to the CPU. The operating system (OS) then handles the situation, perhaps by trying to find a spare frame of RAM and set up a new PTE to map it to the requested virtual address. If no RAM is free, it may be necessary to choose an existing page (known as a "victim"), using some replacement algorithm, and save it to disk (a process called "paging"). With some MMUs, there can also be a shortage of PTEs, in which case the OS will have to free one for the new mapping.