1. In a computer system including a given microprocessor which is specifically designed to operate in a virtual operating mode that allows a software program previously written for an earlier designed single program microprocessor to execute in a protected, paged, multi-tasking environment under a particularly designed host operating software program, said system further including means forming part of said given microprocessor and said host operating software program for executing software interrupt instructions (INTn instructions) using emulation software forming part of said host program in order to emulate the way in which said instructions would have been executed by said earlier microprocessor, the improvement comprising: (a) means for causing certain ones of the INTn instructions to be executed by means of said previously written program in cooperation with said given microprocessor and said host operating software program but not means of said emulation software.