报告摘要：The signal calculus for event-based synchronous language is used for specification and programming of embedded systems, which adopts broadcasting communication and follows the so-called synchronous hypothesis. Our intention is to develop an algebra for linking semantic theories ofreactions. In this paper, we mainly investigate instantaneous signal calculus (I-calculus) which contains all conceptually instantaneous reactions, i.e., zero-time reactions. The delay-time reactions will be researched in the follow-up work. To explore the semantic definition of instantaneous signal calculus, a set of algebraic laws is provided to reduce all instantaneous reactions to a normal form algebraically, which exposes the internal implicit dependence explicitly. Consequently, that two differently written reactions happen to mean the same thing can be proved from the equations of an algebraic presentation. Based on the algebra, we give several important concepts and properties concerning the distinct features of instantaneous reactions and derive an observation-oriented denotation semantics with respect to the algebraic semantics. Thus the equality of two differently written reaction is algebraically provable if and only if the two reactions are equivalent with respect to the denotational semantics.