ABSTRACT
This chapter shows the important role played by parallel processing in a host of out-of-the-ordinary computational problems, also referred to as unconventional computations. For many computational problems, this is the largest speedup possible; that is, the speedup is at most equal to the number of processors used by the parallel computer. More generally, the computations are such that every step of the computation must obey a certain predefined mathematical constraint. An example of computations obeying a mathematical constraint is provided by a variant to the problem of sorting a sequence of numbers stored in the memory of a computer. Certain physical parameters, from the external environment surrounding the computation, become spontaneously available. Most importantly, even a computer capable of an infinite number of algorithmic steps per time unit or, more generally, a supertask machine would fail to perform the computations required by the global variable paradigm if it were restricted to execute these algorithmic steps sequentially.
