We present a unified framework for the specification and analysis of mode-change protocols used in multi-mode realtime systems. We propose a highly expressive formalism, called MCP, to model the system behavior during mode transitions, and show how various existing mode change protocols can be described as MCPs. The explicit representation of the MCP model provides a means to analyze the system state during a mode transition as well as during an intra-mode execution. We introduce the concept of feasibility with respect to the MCP model, and give a decidable method for checking the feasibility of a MCP for a given multi-mode system. The formalization of mode change behaviors using the MCP model allows a range of mode change protocols to be modeled, evaluated, and optimized to the specific operations and performance requirements of the system. Besides feasibility analysis, it is also possible to analyze other system behaviors (e.g., delay between modes, buffer backlog) using automata verification techniques. Our framework can also be used to describe mode change semantics of multi-mode systems whose modes/transitions have different criticality levels, or of systems composed of multiple multi-mode components that require different mode change protocols.