Virtual machines are an important component of modern portable environments such as Inferno and Java because they provide an architecture-independent representation of executable code. Their performance is critical to the success of such environments, but they are difficult to design well because they are subject to conflicting goals. On the one hand, they offer a way to hide the differences between instruction architectures; on the other, they must be implemented efficiently on a variety of underlying machines. A comparison of the engineering and evolution of the Inferno and Java virtual machines provides insight into the tradeoffs in their design and implementation. We argue that the design of virtual machines should be rooted in the nature of modern processors, not language interpreters, with an eye towards on-the-fly compilation rather than interpretation or special-purpose silicon.

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data types, each with a full range of attributes and functions to implement semantic actions, are statistics, queues and resources. (d) to provide for abstract data-typing capability using the entity class. With these the attributes can be represented for each member of a class of transaction objects flowing through a modelled system. Each entity attribute may be of any of the above-mentioned data types, except of an entity class itself. An entity class may only be declared, not also defined, in the environment; definition must take place in the simulator section. Entity attributes may be inherited. 2. For the SIMULATOR, (a) to define processes that describe an entity's activities in the modelled system, thereby forming the basis of model execution for simulation. A process contains a name, a formal parameter list (including number, type, mode of argument), an optional entity parameter, and a sequence of statements. If the optional entity parameter is not specified, the process is exogenous and can only be initiated by the start statement in the environment; otherwise, the process can be activated in the simulator, with reference to the named entity. (b) to define functions, each including a name, a formal parameter list, the function's type, and a sequence of statements. Functions are available for computational ease only. They are not illustrated in the example program of Figure 2. (c) to declare local variables and constants within processes and functions. Local constants must be of a simple type; local variables must be of a simple type or an entity type. (d) to provide processes and functions with each of the following: simple instructions (e.g. assignment, read, write); typical arithmetic, logical and relational operations; and several control constructs to aid...

Over the past decade, since Java was first introduced and integrated into the Netscape web browser, several intermediate representations have been developed that might be potentially used for mobile code applications. This paper examines the requirements for a mobile code representation, presents several examples of stack-based, tree-oriented, and proof-annotating mobile code representations, and evaluates each of these representations according to the requirements. Povzetek: Članek podaja pregled mobilnih kod. 1