Atlas 1 is a large computing system which can include a variety of peripheral equipments, and an extensive store. All the activities of the system are controlled by a program called the supervisor. Several types of store are used, and the addressing system enables a virtually unlimited amount of each to be included. The primary store consists of magnetic cores with a cycle time of two microseconds, which is effectively reduced by multiple selection mechanisms. The core store is divided into 512 word pages; this is also the size of the fixed blocks on drums and one-inch magnetic tapes. The core store and drum store are addressed identically, and drum transfers are performed automatically as described in Section 3. There is a fixed store which consists of a wire mesh into which ferrite slugs are inserted; it has a fast read-out time, and is used to hold common routines, including routines of the supervisor. A subsidiary core store is used as working space for the supervisor. The V-store is a collective name given to various flip-flops throughout the computer, which can be read, set, and re-set by reading from or writing to particular store addresses.
The accumulator performs floating point arithmetic on 48-bit numbers, of which 8 bits are the exponent. There are 128 index registers, or B-lines, each 24 bits long; in the instruction code, which is of the one address type, each instruction refers to two B-lines which may modify the address and the average instruction time is around three microseconds. There are three control registers, referred to as main control, extracode control, and interrupt control, which are also B-lines 127, 126 and 125. Main control is used by object programs. When main control is active, access to the subsidiary store , V-store, or to other programs sharing the machine, is prevented by hardware, and this makes it possible to ensure that object programs cannot interfere with the supervisor or with each other. The fixed store contains about 250 subroutines which can be called in from an object program by single instructions called extracodes. When these routines are being obeyed, extracode control is used; extracode control is also used by the supervisor, which requires access to the private stores. Interrupt control is used in short routines within the supervisor which deal with peripheral equipment. These routines are entered at times dictated by the peripheral equipments; the program using main or extracode control is interrupted, and continues when the peripheral equipment routine is completed.
Details of the various Atlas 1 installations are given in the appendix to this document; the supervisor program described below is of sufficient generality to handle any configuration through the minor adjustment of parameters.
