1. In a data processing system including means for storing programs, CPU means responsive to the programs for processing the data, input/output devices, an input/output structure responsive to the programs for communicating information between the system and the external devices and an interrupt means responsive to operation of the system for interrupting the execution of a current operation in response to an interrupt request and executing an interrupt operation to service the interrupt request, wherein the input/output structure of the system has ports to the input/output devices which occupy a first range of the system address space and the programs include requests for input/output operations directed to the ports in the first range of the system address space, emulation means for allowing the execution of programs written for another system by emulating the input/output structure of the other system wherein the input/output devices of the other system have ports located within a second range of address space and the programs written for the other system include requests for input/output operations directed to ports in in the second range of the address space, the emulation means comprising: input/output request detection means responsive to the input/ouput requests, means for storing routines for directing operation of the system input/output structure in a manner as to emulate operation of the input/output structure of the other system, emulation interrupt means for detecting foreign input/output requests that includes means responsive to the input/output request detection means for detecting requests not normally recognized by the system input/output structure and providing corresponding non-maskable interrupts, means responsive to the non-maskable interrupts and to the corresponding foreign input/output requests for selecting and initiating corresponding input/output emulation routines, means responsive to the non-maskable interrupts for storing information pertaining to the foreign input/output requests, the means for selecting and initiating emulation routines being further responsive to the stored input/output request information in selecting the corresponding input/output emulation routines.