A systematic exploration of the relationships between different models of computation was also undertaken during this period. This included variants of the traditional Turing machine model with additional heads, tapes, and auxiliary storage devices such as stacks. Another important model introduced at about this time was the random access (or RAM) machine \(\mathfrak{A}\) (see, e.g, Cook and Reckhow 1973). This model provides a simplified representation of the so-called von Neumann architecture on which contemporary digital computers are based. In particular, a RAM machine \(A\) consists of a finite sequence of instructions (or program) \(\langle \pi_1,\ldots,\pi_n \rangle\) expressing how numerical operations (typically addition and subtraction) are to be applied to a sequence of registers \(r_1,r_2, \dots\) in which values may be stored and retrieved directly by their index.