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Chapter 10

One-Level Storage System1
T. Kilburn / D. B. G. Edwards / M. J. Lanigan / F. H. Sumner

Summary After a brief survey of the basic Atlas machine, the paper describes an automatic system which in principle can be applied to any combination of two storage systems so that the combination can be regarded by the machine user as a single level. The actual system described relates to a fast core store-drum combination. The effect of the system on instruction times is illustrated, and the tape transfer system is also introduced since it fits basically in through the same hardware. The scheme incorporates a "learning" program, a technique which can be of greater importance in future computers.

1. Introduction

In a universal high-speed digital computer it is necessary to have a large-capacity fast-access main store. While more efficient operation of the computer can be achieved by making this store all of one type, this step is scarcely practical for the storage capacities now being considered. For example, on Atlas it is possible to address 106 words in the main store. In practice on the first installation at Manchester University a total of 105 words are provided, but though it is just technically feasible to make this in one level it is much more economical to provide a core store (16,000 words) and drum (96,000 words) combination.

Atlas is a machine which operates its peripheral equipment on a time division basis, the equipment "interrupting" the normal main program when it requires attention. Organization of the peripheral equipment is also done by program so that many programs can be contained in the store of the machine at the same time. This technique can also be extended to include several main programs as well as the smaller subroutines used for controlling peripherals. For these reasons as well as the fact that some orders take a variable time depending on the exact numbers involved, it is not really feasible to "optimum" program transfers of information between the two levels of store, i.e., core store and drum, in order to eliminate the long drum access time of 6 msec. Hence a system has been devised to make the core drum store combination appear to the programmer as a single level of storage, the requisite transfers of information taking place automatically. There are a number of additional benefits derived from the scheme adopted, which include relative addressing so that routines can operate anywhere in the store, and a "lock out" facility to prevent interference between different programs simultaneously held in the store.

2. The Basic Machine

The arrangement of the basic machine is shown in Fig. 1. The available storage space is split into three sections; the private store which is used solely for internal machine organization, the central store which includes both core and drum store, in which all words are addressed and is the store available to the normal user, and finally the tape store, which is the conventional backing-up large capacity store of the machine. Both the private store and the main core store are linked with the main accumulator, the B-store, and the B-arithmetic unit. However the drum and tape stores only have access to these latter sections of the machine via the main core store.

The machine order code is of the single address type, and a comprehensive range of basic functions are provided by normal engineering methods. Also available to the programmer are a number of extra functions termed "extracodes" which give automatic access to and subsequent return from a large number of built-in subroutines. These routines provide

1 A number of orders which would be expensive to provide in the machine both in terms of equipment and also time because of the extra loading on certain circuits. An example of this is the order:

Shift accumulator contents +n places where a is an integer.

1IRE Trans., EC-11, vol. 2, April 1962, pp. 223-235

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