Patent GB-808440-A (GB)
Title: Improvements in or relating to electronic computing machines
Inventor: YANDELL RONALD PERCY BAWDEN
IPC: G06F7/491
Filed: 1956-05-30
Abstract: 808,440. Digital electric calculating-apparatus. POWERS-SAMAS ACCOUNTING MACHINES Ltd. April 26, 1957 [May 30, 1956], No. 16794/56. Class 106 (1). Electronic computing apparatus for effecting division comprises means to effect division by two of binary-coded numbers read from dividend and divisor stores, add/subtract means connected to the divide-by-two means and operable to form partial dividends and partial divisors each having a predetermined number of fractional decimal places, a trial number store having an initial state of unity of predetermined sign, the trial number being passed to the add/ subtract means where a partial divisor is subtracted or added according to whether the trial number sign is positive or negative, a quotient store into which partial dividends are added under control of trial-number-sign test means, and programming means for controlling a repetitive sequence of operations, the operations being terminated when the partial dividend or the partial divisor is reduced to zero. General. The computer described is generally similar to that described in Specification 799,550 and comprises a magnetic drum MD, Fig. 6, in which digits are recorded in parallel in the 1, 2, 4, 8 code. Sterling or decimal amounts read from the drum may be conveyed via highways H1, H2, to adding unit AU whose output is returned to the drum via scale-of-notation-correction circuit CR and highway H3. The amount on H1 may be passed to shift circuits SC including a gating matrix responsive to control potentials on lines 35, 36, 37 to perform the operations ¸ 2, X10 and ¸ 10 respectively. The amount on H2 may be routed through complementer CMP, e.g. for effecting subtraction. In the division operation particularly described, the dividend and divisor are initially registered in immediate access recirculating loop stores 0/2, 0/3, similar to those described in Specification 799,550, and comprising portions of the drum (not shown separately in Fig. 6), further immediate access stores 0/4, 0/5 being employed for storing the quotient and a trial number respectively (see Fig. 1). The computer includes programming apparatus (Fig. 7, not shown) similar to that of Specification 803,734 and comprising main and subroutine programme units each storing 120 instructions selected by corresponding counters which are normally stepped sequentially by end-of-arithmetic signals. Each instruction comprises an order or function, and two addresses (SA and SB). For a transfer instruction, address position SB contains a number (TN) which is added in the appropriate counter. In the main programme unit, the instructions are stored on interchangeable cards or printed circuits, but in the sub-routine unit, some of the instructions, including those for the division operation, are permanently connected in the computer. Division. The complete permanent division sub-routine comprises 30 instructions (described in detail in the Specification). Before the subroutine is entered, the dividend (D), which may be a sterling or decimal amount, and divisor (R) are placed in stores 0/2, 0/3 and, if necessary, R is tested to ensure that it is not zero. At the commencement of the sub-routine, store O/4 is cleared, and - 1 is entered into 0/5. R is then tested for sign; if it is negative, both D and R are complemented so that the sign of the quotient (Q) is the sign of D. If D is then negative, it is complemented and a sign flip-flop (Fig. 18, not shown) is set. If R is less than 1 both R and D are repeatedly multiplied by 10 (by circuits SC) until R lies between 1 and 10. The new divisor is then multiplied repeatedly by 10 until a significant digit appears in the highest denomination of 0/3, and trial number "- 1 " in 0/5 being similarly multiplied. In the example illustrated in Fig. 1, R = 2.84 which is multiplied by 10<SP>9</SP>, so that - 10<SP>9</SP> is the initial value in 0/5. During each step of the division calculation, Fig. 1, the dividend and divisor values in 0/2, 0/3 are divided by 2 (by circuits Sc) and the divisor value is added into 0/5. If the number in 0/5 is then positive the last-added value is subtracted, but if the number in 0/5 is still negative the corresponding dividend value is added into 0/4. This corresponds to multiplying D by a binary fraction representing the reciprocal of R, viz. where coefficients K 1 , K 2 , &c., each have value " 0 " or " 1 " as indicated. The calculation steps end when the dividend value has become smaller than the lowest denomination (10<SP>-6</SP>) within the capacity of the machine (see column 1) when the quotient value in 0/4 (column 4), which will be correct to 4 decimal places, after being complemented if the sign flip-flop has been set, is transferred to 0/2. In an alternative method, the initial trial number in 0/5 is positive, and the divisor values are subtracted therefrom.