The ATLAS Computing System is the latest product of the collaboration that has existed for over ten years between the staffs of the Computing Laboratory at Manchester University and the Computer Department of Ferranti Ltd. This collaboration has been responsible for the earlier Ferranti Mark I, Mark I* and Mercury computers, all of which have proved successful in a large range of installations.
The ATLAS computer is as fast and as versatile a computer as it is possible to build with currently available solid-state components. Professor T. Kilburn and his staff at Manchester University, have been engaged on research into very high speed transistor circuits for computers for many years, and engineers from Ferranti Ltd, have been assisting in this work. In addition, the experience gained in programming and systems design of previous computers has been used to organise these elements into the ATLAS system. The result is a computer having many features and facilities that make it a considerable advance in computer design.
The first ATLAS system is being manufactured by Ferranti Ltd. for Manchester University, and will be known at the University as MUSE. Subsequent ATLAS machines may differ from MUSE only in the size of store and number of peripheral equipments attached to the central computer. MUSE itself will have a wide variety of peripheral attachments, including magnetic tape mechanisms, punched paper tape equipment, punched card equipment, printers and graphical displays. The initial planning and consideration given to attaching peripheral equipments ensures that the ATLAS system will be a general-purpose computer in the true sense, equally capable of mathematical and scientific calculations and commercial data-processing work.
The remainder cf this brief description is concerned with the principal features of the ATLAS system that make it such a considerable improvement over previously produced machines.
ATLAS is a parallel computer operating at very high speeds. The ability to overlap the obeying of instructions, and simultaneous reference to more than one storage register makes it impossible to give exact times for obeying individual instructions. However, average times for addition and multiplication (of either two 48-bit floating point,or two 40-bit fixed point numbers) are 1.1 and 3.5 microseconds, respectively. Thus, in typical programmes nearly one million instructions can be obeyed in a second.
These high speeds are achieved by the combination of fast transistor circuits, a magnetic core store with a cycle time of 2 microseconds (with facilities for simultaneous access to several registers), an original and ingenious method of fast carry propagation in the adders, and a generally well-conceived organisation of the central computer.
An extensive list of over 500 instructions is available to the programmer. About one third of these instructions, those covering the more common fixed and floating point arithmetic operations and red tape operations, are basic instructions, which are obeyed directly, while the remainder are concerned with a system known as Extraoode. Extracode instructions cover a wide range of more complicated operations, provide for the evaluation of simple functions (such as sines, cosines, and logarithms) and organise routines for handling data. To the user of the machine the two types of instruction, basic and extracode, are indistinguishable, but in the computer an extracode instruction causes an appropriate sequence of basic instructions to be obeyed, entry to and exit from the sequence (or subroutine) being carried out automatically by the central control of the computer. The extracode method makes available to the programmer a far wider range of instructions than would otherwise be possible without unduly complicating (and hence slowing down) the arithmetic unit.
The high speed of ATLAS makes it a suitable computer for problems that also demand a large internal store for their efficient solution. The designers of ATLAS have made allowance in the address part of instructions for a store of up to some two million words to be directly referred to. The word length of the machine is 48 binary digits. Certain instructions refer to the full length 48-bit word, others to 24-bit half-words, and others can refer directly to individual six-bit characters (8 per word).
The range of possible store addresses is divided into several independent parts. For general use by the programmer there is a main store comprising magnetic core registers and magnetic-drums. Up to one million words can be addressed in the main store. However, a typical ATLAS system might have 16,000 words in core registers and some 100,000 words on magnetic drums. The core store has a cycle time of two microseconds and to give an effective cycle time of less than this, several independent access systems are provided for 4,000 word sections of this store.
The organisation of the main store is such that it enables the programmer to benefit from what appears to him to be a very large store, supplied at a fraction of the cost that would result if all the storage were in magnetic cores. The main store is regarded as made up of blocks, each of 512 registers. A programmer may refer to an individual word by a unique block number and a position within the block. Associated with each block (or page) in the core store there is a page-address register which holds the block number allocated to information currently held in that page. Similarly there is a list of block numbers associated with blocks (or sectors) held on the magnetic drums. An individual word can be extracted from the main store (to go the accumulator, for example) only when the block containing that word is currently held in the core store. The decoding process is to compare the block part of the address with each of the block numbers held in the page-address registers. If agreement is found, the appropriate word is extracted so that the instruction may proceed. If no agreement is found, then a drum transfer routine is automatically entered so that the appropriate block may be transferred to the core store, at the same time transferring from the core store to the drum an unwanted block. When this exchange has been effected the appropriate word is extracted from the core store so that the instruction may proceed.
This store selection process is completely automatic so that the programmer need not be aware of the physical two-level nature of the store. This one-level automatic transference from the drum will not preclude the established idea of programmed drum transfers.
A further feature of addressing the store by programmers block numbers, rather than by page and sector numbers which depend on the position of information within the store, is that several programmes can be held in the store simultaneously without any danger of one interfering with any other. This is an essential feature if timesharing of programmes is to occur.
In addition to the main store there is a fixed store. This is constructed from a woven wire mesh into, which ferrite rods are inserted, the pattern of rods corresponding to the pattern of digits to be stored. This store holds the sequences of instructions for extracode routines, organising programmes (concerned with peripheral transfers, drum transfers and time-sharing), and useful constants. The access time for the fixed store is 0.3 microseconds and the size for a typical installation is 8,000 words. A special core store (the subsidiary store) is used as working space by the fixed store. This is of similar speed to the main core store and holds 1,000 words. A further store, the V-store, comprises registers which are associated with peripheral equipments and the magnetic drums. A lock-out arrangement exists which limits the use of the subsidiary and the V-store to programmes held in the fixed store, thus safeguarding it against mis-use by incorrect programmes.
There are 128 24-bit registers which are principally for holding modifiers and performing counting and red-tape instructions. They are a natural extension of the B-registers of the earlier Manchester machines. They are made up partly of registers in a magnetic core store (with a cycle time of 0.5 microseconds) and partly of fast flip-flop registers.
The central computer is designed so that a very large number and range of peripheral equipments can be attached. Each such device fitted will have some special electronic equipment directly associated with it to control it, and to act as a small buffer store for the information transfers. The transfers of information between the main store and the small buffer stores are under the control of programmes held in the fixed store, one programme being associated with each peripheral device. These programmes are entered automatically often enough to ensure that buffer stores of a few characters or words are adequate to keep the peripheral devices operating at their maximum speeds. The amount of equipment associated with each peripheral device is thus kept to a minimum without reducing the overall speed of the system.
AMPEX FR500 magnetic tape mechanisms are used with ATLAS. The magnetic tape used is one inch wide and accommodates 16 tracks of which 12 are information tracks. The tape transport speed is 150 inches per second and the packing density for information is 300 digits per inch on each track. This gives a transfer time of 89 microseconds for each word of 48 binary digits (or, for alphabetic information, 8 characters). Eight independent tape controls are available and up to four mechanisms can be associated with each control. The speed of the computer is sufficient to enable transfers of information to take place between the main core store and up to eight mechanisms at once (one associated with each control) using only a few words of buffer store in each control. This gives an effective transfer rate of a word every 11 microseconds between magnetic tape mechanisms and the main computer.
Even though the peripheral equipments arc kept running at their maximum speed, they will sometimes cause some programmes to be held up waiting information transfers. To overcome this possible loss in efficiency the computer is designed so that several programmes can be stored in the computer at the same time. When one programme is held up, the computer can proceed automatically with other work, which may be a different part of the same programme or another programme. For example, if a major programme is held up, the machine can get on with a minor programme, such as the production of a printed record from a previously produced magnetic tape. In this.way processes which have tended to be regarded as off-line ones can be carried out while other programmes are running with little reduction in the efficiency or speed of the individual programmes.
The organisation of time-sharing of programmes (and indeed the complete organisation of programmes to be run on the machine) is dealt with automatically by a supervisory programme held in the fixed store.
All internal transfers of information will be checked by carrying a parity bit with every word. In addition, transfers of information between the central computer and peripheral equipments will be checked by means appropriate to the particular equipment (for example, double reading of punched cards, and check sums on magnetic tape).
A comprehensive automatic programming scheme is being prepared for ATLAS to cover both mathematical and data-processing applications. This scheme is much more general than the autocode system used on the Fcrranti Mercury computer. It will accept programmes written in Mercury Autocode, and also programmes written for other systems (e.g. FORTRAN, ALGOL) and can easily handle other, more complicated, codes. It will ensure that ATLAS will not only be faster than any competitive computer, but it will also be extremely simple to use.
