The DEC Alpha specification requires minimum of 43 bits of virtual memory address space (8 TiB) to be supported, and hardware need to check and trap if the remaining unsupported bits are zero (to support compatibility on future processors). Alpha 21064 supported 43 bits of virtual memory address space (8 TiB) and 34 bits of physical memory address space (16 GiB). Alpha 21164 supported 43 bits of virtual memory address space (8TiB) and 40 bits of physical memory address space (1TiB). Alpha 21264 supported user-configurable 43 or 48 bits of virtual memory address space (8TiB or 256TiB) and 44 bits of physical memory address space (16TiB).