The emergence of the Internet has sparked a tremendous explosion in the special class of systems called mission critical systems. These systems are so vital to their intended tasks that they must operate continuously. Two problems affect them: unplanned, and therefore disastrous, downtime and planned downtime for software maintenance. As the pressure to keep these systems operating continuously increases, scheduling downtime becomes complex. However, dynamically modifying the mission critical systems without disruption can reduce the need for a planned downtime.

Every executing process has an executing code tightly coupled with an associated state, which continuously changes as the code executes. A dynamic modification at this juncture involves modifying the executable code and the state present within the binary image of the associated process. An ill-timed modification can create runtime incompatibilities that are hard to rectify and eventually cause a system crash. The purpose of the research in this thesis is to examine the causes for incompatibilities and propose the design of a dynamic modification technique: Software Hot Swapping. To achieve these objectives, the researcher proposes mechanisms which these incompatibilities can prevent, examines the characteristics and the implementation issues of such mechanisms, and demonstrates dynamic modification with a simple prototype Hot Swapping program.