Patent GB-987900-A (GB)
Title: Division apparatus
IPC: G06F7/52
Filed: 1962-11-14
Abstract: 987,900. Binary division apparatus. NORTH AMERICAN AVIATION Inc. Nov. 14, 1962 [Nov. 20, 1961], No. 43096/62. Heading G4A. A binary divider using the "non-restoring" method of division produces two remainder signals during each word of the division operation and includes comparing means effective during each word time (except the last) for controlling the operations during the next succeeding word time. The arrangement described provides facilities for quotient round-off and for a double length quotient if required. The apparatus described is basically similar to the binary multiplier described in Specification 987,899 and the recirculatory registers required can employ a magnetic disc memory as described in Specification 929,878. Number representation. The arrangement described operates with 7-bit fractional binary numbers, plus a sign bit which is "0" for positive, "1" for negative to the left of the binary point, negative numbers being represented by 2's complements of the corresponding positive numbers. Method employed. To check whether a division is possible, an initial quotient digit "1" is obtained if the signs of the dividend and divisor are the same, whereas if the signs are different, a "0" is obtained. The next quotient digit obtained represents the sign of the quotient. If these first two digits are alike an "overflow" is indicated and the required division cannot be performed since the divisor is smaller than the dividend. If the divisor and dividend are of like sign subtraction and shift with a quotient digit of "1" indicated take place until an over-borrow occurs, a quotient "0" is indicated and shift and addition of the divisor take place. If the divisor and dividend are of unlike signs, the additions and subtractions of the divisor likewise cause quotient "0"s and "1"s respectively but the final result has to be corrected by the addition of a "1" in the lowest order to produce the correct quotient answer. This final addition of a "1" may be combined with the round-off operation in which "1" is added if the next quotient digit is "1" and not if the next digit is "0". If a doublelength quotient is required, the quotient first obtained is not rounded, but the remainder obtained is modified by the addition of the divisor if the signs of the remainder and divisor are different. The modified remained is then employed as a dividend in a second division operation, the quotient digits obtained being appended to the first quotient. Flow diagram, Fig. 6. This shows a division operation requiring only five word times, although to obtain a 7-bit plus sign bit quotient, six word times would be required. In the first word time, a "look-ahead" circuit 75 is effective to detect the existence of an over-carry or over-borrow in the addition or subtraction of the dividend Ro and Divisor D and, in dependence also on the equality or otherwise of the signs of Ro and D, to set an indicator 80 which controls addition or subtraction in a circuit 79. During the second word time, a first addition or subtraction takes place in a circuit 76, the operation performed being controlled by an indicator 77 which indicates the first quotient digit Qo and is set in accordance with a comparison of the signs of Ro and D, to produce a first remainder R1. The first remainder R1 has the divisor D added and subtracted in the circuit 79 in dependence on the indicator 80 which indicates the second quotient digit Q1 to produce a second remainder 79 which is fed to the "look-ahead" circuit 75. Subsequent word times are similar to the second word time except that in the last word time, the output of the "look-ahead" circuit 75 in the penultimate word time is effective to control an adder 81 to add a "1" if and only if the circuit 75 indicates that the next lower quotient digit would be a "1", thereby effecting round-off. In the case of a double-length quotient being required the remainder is modified if necessary by the addition, without shift, of the divisor in a circuit 76a. Logical circuits. The embodiment described in detail in the Specification is for dividing serially a 14-bit plus sign bit dividend by a 7-bit plus sign bit divisor and includes recirculator registers including a magnetic disc memory (Fig. 1, not shown), which memory has a clock track to produce via flip-flop circuits, required timing pulses for the division. A detailed block diagram (Fig. 7, not shown) of the registers and associated logical circuits for effecting a division in the manner of Fig. 6 is described in detail in the Specification.