In computing, a plug and play (PnP) device or computer bus, is one with a specification that facilitates the discovery of a hardware component in a system without the need for physical device configuration or user intervention in resolving resource conflicts. The term "plug and play" has since been expanded to a wide variety of applications to which the same lack of user setup applies.Expansion devices are controlled and exchange data with the host system through defined memory or I/O space port addresses, direct memory access channels, interrupt request lines and other mechanisms, which must be uniquely associated with a particular device to operate. Some computers provided unique combinations of these resources to each slot of a motherboard or backplane. Other designs provided all resources to all slots, and each peripheral device had its own address decoding for the registers or memory blocks it needed to communicate with the host system. Since fixed assignments made expansion of a system difficult, devices used several manual methods for assigning addresses and other resources, such as hard-wired jumpers, pins that could be connected with wire or removable straps, or switches that could be set for particular addresses. As microprocessors made mass-market computers affordable, software configuration of I/O devices was advantageous to allow installation by non-specialist users. Early systems for software configuration of devices included the MSX standard, NuBus, Amiga Autoconfig, and IBM Microchannel. Initially all expansion cards for the IBM PC required physical selection of I/O configuration on the board with jumper straps or DIP switches, but increasingly ISA bus devices were arranged for software configuration. By 1995, Microsoft Windows included a comprehensive method of enumerating hardware at boot time and allocating resources, which was called the "Plug and Play" standard.Plug and play devices can have resources allocated at boot-time only, or may be hotplug systems such as USB and IEEE 1394 (FireWire).