Turing machines with restricted memory access

Multitape Turing machines which can use their storage tapes only as counters or as pushdown stores are investigated. The memory access restrictions are produced by regarding the machines as small computers (as in the formalism of Wang) and by restricting the instruction repertoires. Relationships are given linking machines which only accept or reject inputs and machines which emit output sequences as a function of their input. It is shown that without restrictions on computing time or amount of tape used that only six distinct classes of sets of strings (languages) are produced by the above memory access restrictions