Winows NT 3.1, Windows NT 3.5, Windows NT 3.51, Windows NT 4.0, Windows 2000, Windows XP, Windows Server 2003, Windows Vista and Windows Server 2008 are all part of the Windows NT family of computer operating systems produced by Microsoft. They are all preemptive, reentrant operating systems, which have been designed to work with either uniprocessor or symmetrical multi processor (SMP)-based computers. To process input/output (I/O) requests it uses packet-driven I/O, which utilizes I/O request packets (IRPs) and asynchronous I/O. Starting with Windows XP, Microsoft began making 64-bit versions of Windows available—before this, these operating systems supported only 32-bit processors.