When a program issues a memory load or store operation, the virtual addresses (VAs) used in those operations have to be translated into "real" physical memory addresses (PAs). This translation is themain task of theMMU (MemoryManagement Unit). TheMMUmaintains a page table (a big hash table) that maps virtual pages to physical pages. Since memory accesses are happening all the time, the MMU needs to be extremely fast and implemented correctly. For these reasons, MMUs are almost always implemented in hardware. Note that we can't map every single byte of virtual memory to a physical address; that would require a huge page table. Instead, the MMU maps virtual pages to physical pages. Also, since we want to isolate each program's address space from the address spaces of other applications, the MMU must keep a separate page table for each process; this implies that different processes could use the same virtual address to refer to different data, and that would not be a problem since these virtual addresses would be mapped into different physical addresses. The page table also marks all virtual pages that are allocated (and therefore are being used), so that it is possible to know which virtual pages have valid mappings into physical pages. All virtual pages that are not being mapped to a physical page are marked as invalid; segfaults occur when a program tries to reference or access a virtual address that is not valid. In addition to valid bits, page table entries (PTEs) also store a lot of other information, such as "read" and "write" bits to indicate which pages can be read/written.