COMPUTER SYSTEM

PURPOSE:To execute I/O instructions on the basis of plural virtual device addresses, by providing a means, which selects dynamically a prefix area in accordance with the flag of an opeating virtual computer and an actual device address, and a means which converts dynamically a virtual device address to an actual device address. CONSTITUTION:A CPU1 is provided with prefix registers 5,000 of virtual computers (VM) and a register 4,000 where the number of an operating VM is stored. A register 7,000 is provided where the start address of the conversion table for the conversion of virtual device addresses to actual device addresses is stored. This register is set by a control program. Areas where prefix values are stored exist in registers 44 corresponding to devices in a channel 100, and an actual memory is accessed while performing always the prefix conversion with these prefix values.