Patent GB-1385215-A (GB)
Title: Electronic digital multipliers
IPC: G06F7/42
Filed: 1972-04-04
Abstract: 1385215 Multipliers HONEYWELL INFORMATION SYSTEMS Inc 4 April 1972 [3 May 1971] 15513/72 Heading G4A A digital multiplier (Fig. 1) for multiplying a multiplicand, entered into register 56, by an M bit multiplier stored in register 14 operates by examining n + 1 bits of the multiplier and selecting a factor of the multiplicand to be fed to an adder 38 such that if the most significant bit of the examined bits is one the factor sign is negative and if the least significant bit is one the factor magnitude is increased by one, the output of the adder being fed back via an accumulator 55 to the adder as one input when the next n + 1 bits are examined. Between examination of the successive groups of bits the contents of the accumulator and register 14 are shifted by n bit positions, the operation terminating after m/n cycles. As described n = 2 and the relationship between the examined bits of the multiplier and the resultant multiplicand factor added in is in accordance with the following table:- Multiplier bits Multiplicand Factor 001 0 010 ¢ 011 ¢ 100. 1  101 -1  110 -¢ 111 -¢ 0 Initially the multiplier is multiplied by 2 (by left shift) so that its least significant bit is not "1" and consequently there is no addition to the factor magnitude. If there is a "1" bit in the most significant bit position of the first three bits it contributes a factor of - M to the input from switch 41 to the adder. When the next bits are examined, the factor of -M has been shifted 2 bits right to become -M/4 and the one bit is now in the least significant bit position resulting in +M/2 being added in to give the required resultant amount for the bit. The digits are entered in 2's complement form.  The apparatus operates under the control of signals GIN, GOS, GOM, GOF, the first of these signals controlling entry of the multiplier. During the signal GOS, register 36 is cleared and the appropriate multiplicand factor is loaded into register 40 from register 56 either unchanged or shifted one bit right, in the latter case the sign bit of the multiplicand being switched to the most significant bit position of the output bus. Control signals for this operation are derived from a logic circuit 155 receiving as inputs the output of a decoder 150 examining the three least significant bits of the multiplier register 14. For a 36 bit multiplier, seventeen identical multiplication cycles are then effected under the control of the signal GOM. During each the adder 38 generates the sum of the accumulated partial products from the register 36 and the multiplicand factor from the register 40. The sum is stored in the register 36 after right shifting by two bits, the sign being selected for the accumulated partial product in accordance with an exclusive OR operation on the sign bit output of the adder and an overflow flip-flop 44. In the 18th cycle the partial product from the output of the adder is stored in the register 40 unshifted. Under the control of the signal GOF this accumulated partial product is then transferred via the adder and register 55 to the register 56.  Instead of examining two bits of the multiplier, three bits may be examined simultaneously but a register must then be provided for storing three times the multiplicand.