Patent GB-815751-A (GB)
Title: Improvements in electric calculators and accumulators therefor
IPC: G06F11/10
Filed: 1955-11-18
Abstract: 815,751. Digital electric calculating-apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 18, 1955 [Nov. 23, 1954], No. 33074/55. Class 106 (1). In a device, Fig. 24, for computing the remainder obtained on dividing a given decimal number by 9, referred to in the Specification as the modulo 9 value of the number, each digit of the number is entered into an accumulator in turn together with the tens carry resulting from the entry of the immediately previous digit. For example in computing the modulo 9 value of 2897, first 7 is entered into an accumulator, then 9 is entered giving a sum digit of 6 and a carry of 1, then both 8 and 1 are entered giving a sum digit of 5 and a carry of 1, and finally both 2 and 1 are entered giving a sum of 8, which is the required modulo 9 value. The decimal digits are entered successively in binary coded form on four parallel leads, Fig. 24, into the accumulator which comprises four circulating loops-one for each bit-each including an elementary 3-input logical adder, the loop for the 1-bit, for example, being from the " sum output 121 of the 1-bit adder 108 via a delay 141 and an and gate 103 and back to one input of the adder. Gate 103 and the corresponding gates in the three other circulating paths are only open in the presence of a control signal, the absence of which zeroizes the accumulator. The accumulator also includes an arrangement of gates (middle row, Fig. 24) referred to as a rationalising network in the Specification, which whenever the four bits emerging from the adders represent 10 or more produces a decimal carry, which is entered into the 1-bit adder via a delay 127, and modifies the four binary bits so that they represent the appropriate decimal sum digit. The Specification also describes how such a device-a modulo 9 accumulator-can be incorporated in a series-mode binary-codeddecimal computer for checking each of the four rules of arithmetic. For example during addition as the two operands being summed are entered into the adder in the calculator, they are also entered into two modulo 9 accumulators 2066, 2069, Fig. 32, and as the result of the addition emerges from the adder in the calculator for transfer to a store, it also is entered into a modulo 9 accumulator 2062, Fig. 33. Thus at the end of the addition by the calculator the three modulo 9 accumulators hold respectively the modulo 9 values of the two numbers being summed and their sum as computed by the calculator. The modulo 9 values of the numbers being summed are then subtracted from the modulo 9 value of the computed sum by entering their nines complements in turn into the result modulo 9 accumulator. If the computed sum is correct this operation will always result in a 9 circulating in the result modulo 9 accumulator. Any number other than a 9 causes an error indication. In checking multiplication the modulo 9 value of the multiplicand is subtracted from the modulo 9 value of the computed product a number of times equal to the modulo 9 value of the multiplier, this being achieved either by adding the nines complement of the modulo 9 value of the multiplicand to the modulo 9 value of the computed product a number of times equal to the modulo 9 value of the multiplier if this value is less than 5, or, if the modulo 9 value of the multiplier is not less than 5 by adding the modulo 9 value of the multiplicand to the modulo 9 value of the computed result a number of times equal to the nines complement of the modulo 9 value of the multiplier. Division is checked similarly by a combination of multiplication and addition. Specifications 719,418, [Group XL (c)], 721,180, 755,441 and 807,882 are referred to.