1. In a virtual machine system for running a plurality of operating systems on one machine under control of a virtual machine control program (VMCP), said system including a plurality of input/output devices and a circuit for processing instructions for starting said input/output devices and for processing input/output interrupts, wherein (i) when a first instruction requiring a start of a first input/output device among said plurality of input/output devices dedicated to one running operating system and having an interruption level dedicated to said one running operating system is issued during a time when said first input/output device is in a direct input/output execution mode, said circuit starts said first input/output device to execute an input/output operation indicated by the first instruction without interrupting said VMCP, (ii) when an input/output interrupt is provided thereafter from said first input/output device, said circuit processes said input/output interrupt without interrupting said VMCP, (iii) when said first instruction is issued from said one running operation system during a time when said first input/output device is in a simulation mode or when a second instruction requiring a start of a second input/output device other than said first input/output device is issued from another running operation system, said circuit provides said VMCP with an instruction interrupt to request execution of simulation of said first or second instruction, (iv) when an input/output interrupt is issued thereafter from said first or second input/output device, said circuit is provided an input/output interrupt to said VMCP to request execution of simulation of said input/output interrupt, and (v) said VMCP executes the simulation of said first or second instruction or the simulation of an input/output interrupt in response to said instruction interrupt or said input/output interrupt provided by said circuit; a method of input/output control for changing mode of operation of said first input/output device from a simulation mode to a direct input/output execution mode, comprising the steps of: (a) detecting a status of execution of simulation be said VMCP regarding said first input/output device and a status of execution of an input/output start instruction regarding said first input/output device as issued from said VMCP; and (b) determining whether said status of execution of said simulation and said status of execution of said input/out start instruction satisfy a predetermined mode change condition so as to indicate said direct input/output execution mode to said circuit as a mode of operation of said first input/output device when a result of the determination is affirmative.