1. In a computer system having a central processing unit, a main storage and at least one I/O device, wherein a plurality of operating systems can simultaneously run under the control of a control program, a method of executing an I/O instruction using said central processing unit, comprising steps of: assigning, under control of said control program, a plurality of resident areas of said main storage which do not overlap one another to said plurality of operating systems as main memories therefore, respectively; responding to an I/O instruction issued by a running one of said plural operating systems to thereby determine an address of said main memory assigned to said running operating system which participates in an I/O operation requested by said I/O instruction without intervention of said control program; translating said address into an address of said main storage of said computer system without intervention of said control program; and executing said I/O operation by using the address resulting from said address translation.