Improved methods and operating systems for use with a multi-mode microprocessor enable efficient operation in a multi-mode environment. Preferred embodiments for use with microprocessors which were not designed to switch from each mode to another mode enable multi-tasking of a mixture of programs written for different modes using the mode switching methods of the present invention. Frequently used portions of the operating system are stored in memory at locations which can be commonly addressed in all modes. Means for handling device drivers and interrupts in all modes are also provided. Preferred embodiments for use with computer systems using microprocessors such as the Intel 80286 include means for storing the operating system routines to maximize performance of the system in real mode. Auxiliary protection hardware and I/O masking hardware are also provided in alternate preferred embodiments to enhance protection during real mode operation of such systems. Means for handling interrupts in a mode switching environment and alternate embodiments to eliminate problems caused by hooking programs in a multi-tasking environment are also provided.