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136 Part 1½ Fundamentals Section 3½ Computers of Historical Significance

2 The more complex mathematical operations, e.g., sin x, log x, etc.

3 Control orders for peripheral equipments, card readers, parallel printers, etc.

4 Input-output conversion routines.

5 Special programs concerned with storage allocation to different programs being run simultaneously, monitoring routines for fault finding and costing purposes, and the detailed organization of drum and tape transfers.

All this information is permanently required and hence is kept in part of the private store termed the "fixed store" [Kilburn and Grimsdale, 1960] which operates on a read only" basis. This store consists of a woven wire mesh into which a pattern of small "linear" ferrite slugs are inserted to represent digital information. The information content can only be changed manually and will tend to differ only in detail between the different versions of the Atlas computer. In Muse this store is arranged in two units each of 4096 words, a unit consisting of 16 columns of 256 words, each word being 50 bits. The access time to a word in any one column is about 0.4 m sec. If a change of column address is required, this figure increases by about 1 m sec due to switching transients in the read amplifiers. Subsequent accesses in the new column revert to 0.9 m sec. The store operates in conjunction with a subsidiary core store of 1024 words which provides working space for the fixed store programs, and has a cycle time of about 1.8 m sec. There are certain safeguards against a normal machine user gaining access to addresses in either part of the private store, though in effect he makes use of this store through the extracode facility.

The central store of the machine consists of a drum and core store combination, which has a maximum addressable capacity of about 106 words. In Muse the central store capacity is about 96,000 words contained on 4 drums. Any part of this store can be transferred in blocks of 512 words to/from the main core store, Which consists of four separate stacks, each stack having a capacity of 4096 words.

The tape system provides a very large capacity backing store for the machine. The user can effect transfers of variable amounts of information between this store and the central store. In actual fact such transfers are organized by a fixed store program which initiates automatic transfers of blocks of 512 words between the tape store and the main core store. The system can handle eight tape decks running simultaneously, each producing or demanding a word on average every 88 m sec.

The main core store address can thus be provided from either the central machine, the drum, or the tape system. Since there is no synchronization between these addresses, there has to be a priority system to allocate addresses to the core store. The drum has top priority since it delivers a word every 4 m sec, the tape next priority since words can arise every 11 m sec from 8 decks and the machine uses the core store for the rest of the available time. A priority system necessarily takes time to establish its priority, and so it has been arranged that it comes into effect only at each drum or tape request. Thus the machine is not slowed down in any way when no drum or tape transfers take place. The effect of drum and tape transfers on machine speed is given in Appendix 1.

To simplify the control commands given to the drum, tape, and peripheral equipment in the machine, the orders all take the form b ® S or s ® B and the identification of the required command register is provided by the address S. This type of storage is clearly widely scattered in the machine but is termed collectively the V-store.

In the central machine the main accumulator contains a fast adder [Kilburn, et al., 1960b] and has built-in multiplication and division facilities. It can deal with fixed or floating point numbers and its operation is completely independent of the B-store and B-arithmetic unit. The B-store is a fast core store (cycle time 0.7 m sec) of 120 twenty-four bit words operating in a word selected partial flux switching mode [Edwards et al., 1960]. Eight "fast" B lines are also provided in the form of flip-flop registers. Of these, three are used as control lines, termed main, extracode, and interrupt controls respectively. The arrangement has the advantage that the control numbers can be manipulated by the normal B-type orders, and the existence of three controls permits the machine to switch rapidly from one to another without having to transfer control numbers to the core store. Main control is used when the central machine is obeying the current program, while the extracode control is concerned with the fixed store subroutines. The interrupt control provides the means for handling numerous peripheral equipments which "interrupt" the machine when they either require or are providing information. The remaining "fast" B lines are mainly used for organizational procedures, though B124 is the floating point accumulator exponent.

The operating speed of the machine is of the order of 0.5 ´ 106 instructions per second. This is achieved by the use of fast transistor logic circuitry, rapid access to storage locations, and an extensive overlapping technique. The latter procedure is made possible by the provision of a number of intermediate buffer storage registers, separate access mechanisms to the individual units of core store and parallel operation of the main accumulator and B-arithmetic units. The word length throughout the machine is 48 bits which may be considered as two half-words of 24 bits each. All store transfers between the central machine, the drum and tape stores are parity checked, there being a parity digit associated with each half-word. In the case of transfers within the central store (i. e., between main core store and drum) the parity digits associated with a given word are retained throughout the system. Tape transfers are parity checked when information is

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