Chapter 26 NOVA: a list-oriented computer 319
These buffers should be equivalent in length to the number of words on a track of the rotating memory.
The loading and unloading of the buffers to and from the rotating memory is dependent on the timing of the rotating memory, whereas the loading and unloading of the buffers to and from the arithmetic unit is guided solely by the rate at which the arithmetic can be performed. Here again it may also be possible to take advantage of the streaming nature of the operands by designing an "assembly-line" arithmetic unit in which more than one pair of operands could be in process at the same time. With this kind of unit it may be possible to execute additions at a rate equal to the word-transfer rate from the rotating memory; however a multiplication or division of two lists may require several revolutions of the memory. The timing diagram of Fig. 5 shows several typical instructions being carried out. A certain amount of look-ahead is required, but there is ample time for this, since instructions are prepared for execution at an average rate of less than one per revolution of the rotating memory.
While a detailed cost estimate has not been made for a simple prototype NOVA, a quick estimate would be $50,000 for a head-per-track disc and $50,000 for the arithmetic and control section, making a total of $100,000. For a buffering scheme such as the one shown in Fig. 4 the cost would be considerably higher but would be offset by increased versatility.
In the previous paragraphs we have demonstrated that NOVA is capable of handling network problems at a significantly lower cost than contemporary computers, and at a comparable speed. The availability of such a machine as NOVA would stimulate further
Fig. 5. Timing diagram of buffers, rotating memory, and arithmetic unit. Dotted line shows movement of data into a device; solid line shows movement out.
interest in the one-operation, many-operand approach to computation and no doubt would uncover many other problems to which it could be applied.
Because NOVA makes it possible to easily establish neighbor-relationships between mesh points that are further away than nearest neighbors, it may be possible to develop new differencing techniques for the solution of coupled sets of differential equations. This may increase the accuracy or shorten the time required for their solution.
The memory, arithmetic, and other units needed for NOVA are commercially available now. No new technology would be required to fabricate a prototype model. In view of the potential advantages of such a machine, it seems clear that construction of a model would justify the minimal development costs.