Request - Based Mediated Execution

data structures are useful in any system, as a means of separating interfaces from implementations. Normally, data structures are considered as concrete within the scope of an implementation module, and abstract everywhere else that they are visible in the program. Thus an abstract data structure has global extent, although its internals are only visible within a specific scope. But when a program involves multiple levels of abstraction, it is no longer reasonable to assume high-level definitions have global extent. For example, a CPU or an OS kernel has no understanding of the classes defined in a Java program running in user mode; yet the lower levels must be able to execute on behalf of the Java program. This is because the concrete bit-level representations of the Java classes are exposed to the lower levels of the system: the JVM itself, the OS kernel, and the CPU.