A driver is the part of an I/O system used for processing of an I/O request for a specific channel. The interaction of the CPU with a channel is described through the monitor concept of Hoare and Brinch Hansen. The implementation of monitors using hardware interrupt facilities is described. The resulting device monitor is compared pared with the device processes of Wirth's Modula. The concept is illustrated through an extension to Concurrent Pascal with examples drawn from the PDP11 system. Problems of missing interrupts and power failure are also discussed.