Abstract: A method is disclosed for providing a single-tasking disk operating system with multitasking capability in a real-time computer system for building access control or the like. A number of reentrant, terminate-and-stay-resident application programs are loaded into a host microcomputer. As each application program loads, it passes its unique code segment and instruction pointer parameters to a look up table. For the last application program to be loaded, the program segment prefix of the application program residing in the disk operating system is copied and stored. When a timer interrupt prompts the program switching logic, the code segment and instruction pointer parameters of the then-executing program are compared with the values in the look-up table. If no match is found, program switching is postponed.