1. A receiver for digital communications comprising: a digital signal processor; an error correction coprocessor operatively coupled to the digital signal processor, said coprocessor being controlled by said digital signal processor, said coprocessor comprising an extremum cost index register; means for calculating costs for received encoded information associated with paths through a trellis representing possible states of said encoded information sequence, said means producing an extremum cost; means operatively coupled to the error correction coprocessor for writing said extremum cost of a state into said extremum cost index register; and means for comparing an index of said extremum cost state with indices of permitted states known to be transmitted, and when said index is not among the indices of permitted states known to be transmitted, writing the index of one of said extremum cost permitted state into said extremum cost index register.