340 Part 4 The instruction-set processor level: special-function processors
Section 3 Processors defined by a microprogram
For example, four registers are not sufficient to enable a succession of products to be accumulated without the transfer of intermediate results to the store, since the accumulator must be clear at the beginning of a multiplication. The addition of one register enables the accumulation of products to be provided for in the micro-programme. If this register is associated with the outlet from the store, it also enables some of the waiting time for storage access to be eliminated. To do this the micro-programme is arranged to call for a number from the store as soon as it is known that the number will be required and to continue with other necessary micro-operations before finally proceeding to use the number. The 'dynamic stop' would occur just before the number is required for use. Another way of saving time is to arrange, in the case of those orders which permit it, for the next order to be extracted from the store before the operation currently being performed has been completed.
The minimum number of registers required in the control register unit of the machine for the simplest mode of operation is three. If extra registers are provided facilities similar to those provided by the B-lines in the machine at Manchester University could be included in the micro-programme.
6. Microprogramming applied to serial machines
All the discussion so far has been with reference to parallel machines because the technique described in this paper is most adapted to that type of machine. It is, however, possible to design a serial machine along the same lines. In a parallel computer with an asynchronous arithmetical unit every gate requires only one kind of wave-form to operate it and the timing of that wave-form is not critical. In a serial machine, on the other hand, different gates require different wave-forms and the same gate may require different wave-forms at different times; further, all these wave-forms must be critically timed. These complications may be handled by including in the micro-control unit a third matrix, C, for selecting the appropriate wave-form for each micro-order. The main wave-form, routed by the decoding tree and matrix A, opens a gate which is fed by a wave-form selected by matrix C. This enables a wave-form of correct duration to be applied to any selected gate in the arithmetical or control sections of the machine.
WilkM51a; BoutE63; FlynM67; GreeJ64, 66; MercR57; Patz67; RosiR69; TuckS67; WilkM58b, 69; WebeH67