1. A computer system for running plural operating systems under control of a control program, said computer system comprising: (a) a plurality of I/O devices; (b) a main storage partitioned to said operating systems; (c) an I/O processor, responsive to an I/O operation request by one of said operating systems which designates a subchannel number assigned to one of the I/O devices and a control signal required for execution of an I/O operation, for executing the I/O operation between said one I/O device and said main storage assigned to said one operating system; (d) an instruction processor, responsive to an I/O instruction of a first kind issued from a first one of the operating systems which designates a subchannel number and an operand of a first kind which designates an I/O operation to be executed, for providing, without interrupting the control program, said I/O processor with the subchannel number and a control signal designated by the operand of the first kind, said instruction processor comprising: (e) judging means, responsive to an I/O instruction of a second kind which includes a device address assigned to a second one of the I/O devices and an operand of a second kind which designates an I/O operation to be executed, for judging whether said I/O instruction is issued by a second one of the operating systems, and (f) I/O instruction execution means, responsive to an affirmative judging by said judging means, for providing said I/O processor with (i) a subchannel number assigned to said second one of the I/O devices and (ii) a control signal corresponding to the operand of the second kind; interruption subclass indicating means for indicating one or more interruption subclasses for direct input/output execution at the time when an operating system starts to run on an instruction processor; and TPI (test pending interruption) instruction executing means, wherein: when the operating system running on said instruction processor issues a TPI (test pending interruption) instruction for releasing an interrupt assigned with a highest priority from interrupts for said one of the input/output devices, when allowability of the interrupt for said input/output device is indicated by an interruption subclass mask, and storing identification information of the input/output device having the interrupt released from suspension in a main storage at a region designated by the TPI instruction, said TPI instruction execution means responds to the issuance of said TPI instruction to thereby release a pending interrupt having the highest priority assigned thereto from the interrupts for the input/output device of said input/output devices, the interrupt allowability of said input/output device being indicated by said operating system, and store the identification information of the input/output device having the interrupt released in the main storage of said operating system at a region designated by said TPI instruction.