Communication Across Levels of Abstraction

ion, this results in an inherent asymmetry of upward vs. downward events. The properties of downward events described in the previous section do not hold for upward events; in fact the properties of upward events are essentially complementary to those of downward events: 1. Upward events need not be yielding. While the simple formulation of Handler in Section 2 does cause the lower level to yield on an upward event, this is not a necessary constraint. One could imagine a concurrent variant which continues evaluation at the lower level of abstraction along with the higher level. This would be useful for modeling systems consisting of multiple processes and processors, as in Figure 2. 2. Upward events are definitely resumeable. Any upward event must eventually lead to a downward event, because a term at a higher level of abstraction can only execute by making requests to lower levels of abstraction. For example, upward event (b) must eventually lead to another downward event at the CPU level, because following the execution of any (non-halt) instruction, the program must execute another instruction. 3. Upward events may not autonomously modify terms at lower levels of abstraction. The only way in which a term at a higher level of abstraction may affect lower levels of abstraction is by sending a request message which triggers a downward event. For example, the only way in which a process can terminate is to make a request which causes the operating system to terminate the process. There is no way that the process in Figure 1 could have terminated itself without making a downward request. Figure 2 also illustrates how peer-to-peer communication can be modeled with language-level virtualization. By definition, peer computations should have a symmetric relationship, unlike the relationship explored above between different levels of abstraction. In effect, peers are all higher levels of computation built upon a common lower-level abstraction—the communication mechanism. Processes running on a kernel, processors in a computer, and autonomous robots in a common environment are all examples of this structure. This is, of course, why upward events should not necessarily be yielding: multiple 4 constrained