Systems and techniques for transferring the flow of control between agents. In one general aspect, a computer program product includes instructions operable to cause data processing apparatus to: store data on a first call stack, the first call stack corresponding to and being used by a first agent; suspend the first agent's use of the first call stack; enable a second agent to access the data by using the first call stack; and resume the first agent's use of the first call stack after the second agent is no longer using the first call stack. According to another aspect, a computer program product includes a first set of instructions that is operable to store and access data on a shared call stack; and a second set of instructions that is operable to alternate execution of the first set of instructions between a first agent and a second, distinct agent.