Patent GB-1412541-A (GB)
Title: Arithmetic and logic unit
IPC: G21C17/07
Filed: 1973-07-17
Abstract: 1412541 Arithmetic and logic unit INTERNATIONAL BUSINESS MACHINES CORP 17 July 1973 [31 July 1973] 33947/73 Heading G4A An arithmetic and logic unit (Fig. 1, not shown) for operating on a pair of operands comprises circuitry (40) for controlling (1) a complementer (60) for transferring one of the operands in true form, nine's complement form or one's complement form to a first subtractor for subtraction from the second operand, (2) a borrow look ahead circuit (170) receiving the output of the complementer, the output of the first subtractor and the second operand and deriving borrow signals and (3) a second subtractor receiving the borrow signals and the output of the first subtractor. The circuit is shown in detail in Figs. 3A-3E. Complementer.-The complementer 60 transmits the input data unchanged via OR gates 64, 69, 73, 76, 84, 89, 93, 96 when AND gates, 62, 68, 72, 74, 81, 88, 92, 94 are enabled by a signal from AND gate 77. The ones complement of the data is fed to the OR gates via AND gates 61, 66, 70, 75, 81, 86, 90, 95 when AND gate 79 and inverter 98 are enabled. The nines complement is fed via AND gates 63, 65, 67, 71, 83, 85, 87, 91, 95 when both AND gate 79 and OR gate 97 are enabled. The OR gates are connected to subtractor 100, function control circuit 130 and borrow look ahead circuit 170. Subtracter.-The subtractor 100 comprises two sections for performing first the exclusive OR function A#B in exclusive OR gates 101- 108, and then feeding the result of this operation together with borrow look ahead signals from circuit 170 and a borrow in signal from the control circuitry to further exclusive OR gates 109-116. Function control circuit.-The exclusive OR gates 109-116 are connected to AND gates 131-133, 135-137, 139-141, 143-145, 147- 149, 151-153, 155-157, 159-161 in function control circuit 130. The first gate in each set 131, 135 ... is enabled by a signal on lead 41 to perform the operation A#B. The third gate in each set 133, 137 ... is enabled by a signal on lead 42 to pass the result of the exclusive OR operation A#B. The OR function is effected by enabling both the first and third sets of gates by signals on both leads 41, 42. Absence of a signal on both these leads results in the second gate in each set being enabled to perform an arithmetic operation. Borrow look ahead circuit.-The borrow look ahead circuit 170 comprises logic circuitry for deriving borrow signals for data coded in binary, binary coded decimal and zoned decimal. The borrow for example for bit position 6 is derived from OR gate 225 receiving at its input (1) the output of AND gate 174 representing A6B6, (2) the output of AND gate 224 receiving at its input A7B7 and the inverted output of exclusive OR gate 107 and (3) the output of AND gate 223 receiving at its input the inverted output of exclusive OR gate 108, the borrow-in signal from the control circuitry and the inverted output of exclusive OR gate 107. The borrow for bit 4 must be propagated through the sign and zone positions when the apparatus is operating in zoned decimal. If a sign signal exists on line 49 the signal from OR gate 218 for bit 4 is only passed via AND gate 185 and OR Gate 186 as a borrow 4 signal if the borrow-in signal is simultaneously present. A borrow-out signal is derived from the signal from OR gate 218 if a zone DEC signal exists on line 45. When the system is not operating in zoned decimal code the borrow-out signal is derived from OR gate 204 operating on the data bits A0, B0. Six correct circuit.-When the operands are in decimal or zoned decimal code a six correct circuit 250 is operative on the outputs R0-R7 of the function control circuit 130 when a borrow-out signal is present to prime gates 259, 261 in logic circuitry associated with the data 1 and data 2 output bits.