Patent GB-975191-A (GB)
Title: Electronic data processing apparatus
IPC: G06F7/50
Filed: 1961-06-30
Abstract: 975,191. Floating point arithmetic. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 30, 1961 [July 14, 1960], No. 23694/61. Heading G4A. Control circuits are added to the computor described in Specification 968, 546 to enable a floating point add/subtract operation to be performed on decimal numbers normalized to consist of a two-digit exponent and an eight-digit signed mantissa. The computor consists of a magnetic core memory 101 addressed by an instruction address register 105 and positions 6 to 9 of program register (PR) 103 into which the one-address instructions are read from memory. An operand is read from the memory address specified by the program register into arithmetic register (AR) 108. For a floating point operation the other operand has been previously entered in Accumulator Register 1 (A1) 109. An arithmetic bus (AB) 114 connects various one word registers each constructed of magnetic cores. Transfer between registers is normally parallel by word and consists in reading out a first register to capacitors comprising the AB and reading from the capacitors into the second register. Word buffer register (WBR) 112 and A1 can be connected to form a single shift register as can A1 and accumulator register 2 (A2) 110. An adder 115 operates serially by digit on the binary-coded decimal digits input on channels 1 and 2. Digits sent to the adder via channel 2 can be complemented in T/C 116 if subtraction is wanted. The position of the highest significant digit of work in the AB can be determined by the scanner described in Specification 890, 953, and this position is noted by setting a magnetic core in a ten core single core (per stage) shift register (SCSR) which is not shown or described in detail, although it is described in Specification 968, 546, mentioned above. Floating point add/subtract. The operand in A1 is subtracted from that in AR to determine the exponent different and the larger of the two operands. If necessary the two are interchanged so that the larger is in Al. The operation required is then performed, taking account of the signs of the mantissas, and the result placed in A1 and A2 in normalized form, A1 containing the eight most significant digits and the exponent, and A2 the eight least significant and the exponent of A1 less eight. The exponent is a number ranging from 00 to 99 and is equal to the power of the mantissa plus 50. An exponent 50 thus represents that the decimal point is located immediately to the left of the mantissa. The operation is performed in at most seven steps controlled by a floating point ring (not shown). If the exponents of the operands are equal the eight digit mantissa in AR is added (or subtracted) to the eight digit mantissa in A1. If the exponents differ by 1 to 7 a number of low order digits equal to the exponent difference are added (subtracted) to zero into A2, the remainder into A1. If the exponents differ by 8 to 15 some or all of the digits in AR are added (subtracted) into A2. If the exponents differ by more than 15 the answer is the larger of the operands and is set in Al, A2 being set to + 0. Positions 4 and 5 of the program register 103 are a field length register which determines what field of the AR is to be operated on at any time. The field length, as will be explained below, is determined by the exponent difference. If the sum of the exponents is greater than 99 overflow is indicated and the operation stops, the next step being determined by a handswitch on the console. If the difference of the exponents is less than 00, A1 and A2 are both set to + 0 and the next instruction taken. The seven possible steps in a floating point add/subtract operation are as follows (it is assumed that addition has been ordered): (FP1) The operand specified by the instruction is set in AR. The contents of A1 are transferred to A3 (accumulator register 111) and to the Auxiliary register (AUR) 113. The signs of the operands are set in latches (Fig. 32) to determine whether they are like or unlike and if it will be necessary to perform the operation required by the instruction or its inverse to get the correct algebraic result. AUR is subtracted from AR, only the exponent difference being stored (in PR positions 6 to 9). It is determined if the A1 mantissa is less than AR mantissa (carry from highest order) or if the two mantissas are equal (operation gives all zero result), and additionally if the A1 exponent is lower than or equal to the AR exponent. If A1 is smaller than AR interchange takes place, AR being transferred to A1 and A3 to AR. The range of the exponent difference is also determined and expressed as EDO (zero difference), EDLOW (difference 1 to 7), EDHIGH (difference 8 to 15) and EDOOR (out of range-greater than 15). From the value of the units digit of the exponent difference which is set in SCSR the field in AR to be first operated on is set in the field register, positions 4 and 5 of PR. If EDO the field is 29, all the mantissa of the AR is to be added to Al. If EDLOW the field is x9 where x is the 10's complement of the number held in SCSR, and the operation will first take place on A2, at present holding zero. At a later stage (step FP4) the remainder of the field is added into A1. If EDHIGH none of AR will be added into A1 but some or all of AR will be added into A2. The field is then 2x where x is the 7's complement of the number in SCSR. If EDOOR the operand in AR is neglected and a jump is made to FP2. All this action may be nullified by the exponent in PR being the complement of its true value. This will be so if there is no carry set in the adder as the final digit is entered in PR, and a RECOMP latch will be set. If this latch is not set and EDO is the exponent condition a jump is made to FP4; if EDLOW or HIGH is the condition a jump is made to FP3. (FP2) The exponent difference in PR is complemented if the RECOMP latch is set, and the value examined as in FP1 to set an appropriate field in PRfield register. If EDOOR has been found in FP1 or is found now the number in A1 is the required result and the operation stops. (FP3) A1 is sent to A3 to preserve the exponent. The field of AR now set in the field register is added to zero and is shifted into A2 from position 0. The field register is now set to specify the rest of the AR which unless the ED is HIGH is to be added to A1. If ED is HIGH FP4 is taken only if the instruction specifies subtraction and a borrow has to be made from the lowest order of A1. (FP4) The required field of AR is added into A1, including the exponent digit positions 0 and 1 which appear as zeros to the adder. The result is shifted into A1 from position 0. H is noted if the sum extends into position 1 of A1. (FP5) Normalization. A1 is read out to AB where its highest significant digit is determined and a core of SCSR representing a number one less than the order of this digit is set. A1 is transferred to WBR and to AR, A2 to Al. The value in SCSR is subtracted from 8 and is sent to section 2 of AUR. A1 is linked to WBR to form a shift register which is shifted left at the same time as SCSR. When SCSR shifts from its last position shifting is stopped. There are some modifications of this procedure to allow for overflow of the mantissa in A1 into position 1, or for A1 being all-zero. A1 is transferred to A2 and WBR to A1. The field register is set to 01. (FP6) Corrected exponent to A1 The number in section 2 of AUR, which represents the number of shifts made less 2, is subtracted from the exponent of the higher of the two operands, which was placed in AR at the beginning of FP5. The answer is entered in A1 at position 0 and shifted right, the lowest digits of A1 being shifted into A2. If mantissa overflow had occurred 01 is added to the exponent at this stage instead of the subtraction taking place, since clearly normalization would not have been necessary. If the new exponent is greater than 99 a light goes on on the console, if less than 00 a latch is set. (FP7) Eight is subtracted from the exponent computed during FP6 which had also been entered in PR from where it is now taken. The answer is shifted into positions 0, 1 of A2. If the exponent is less than 00, A2 is set to zero, and if the A1 exponent had also been found to be less than 00, A1 is set to zero.