Abstract:
In a floating point arithmetic execution unit, an additional adder unit and a selection network are added to the apparatus typically performing the arithmetic floating point function. The additional apparatus permits certain processes forming part of arithmetic operations to be executed in parallel. For selected arithmetic operations, the final result can be one of two values typically related by an intermediate shifting operation. By performing the processes in parallel and selecting the appropriate result, the execution time can be reduced when compared to the execution of the process in a serial implementation. The fundamental arithmetic operations of addition, subtraction, multiplication and division can each have the execution time decreased using the disclosed additional apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to data processing systems and, more particularly, to apparatus in the central processing unit that performs the manipulation of the data signal groups determined by the instruction sequence generally referred to as floating point operations. 
     2. Description of the Related Art 
     In the modern data processing system, groups of data signals can be chosen to represent the sign (+), the fraction or numeric value (A,B), and the position of the binary point of a numeric value (E A , E B ). This representation is referred to as a floating point representation of a numeric quantity. (According to convention, the floating point fraction field is normalized to provide a non-zero quantity in the most significant bit position of the fraction field.) The floating point representation of a numeric quantity has the advantage that a wide range of values can be accommodated with the same degree of accuracy. However, the manipulation of the floating point quantities, because of the structure of the representation of a number, is slower than manipulation of the typical representation of numeric quantities in which the binary point has a predefined location. 
     Referring to FIG. 1, the selected components of a typical data processing system are shown. The data processing system generally includes a central processing unit 10 wherein the manipulation of data signal groups according to instruction sequences or programs are performed, a main memory unit 12 wherein the software programs (and associated tables) utilized by the central processing unit is stored, and at least one interface unit 11 that provides secondary storage for data and program signal groups and permits communication by the central processing unit with external devices. The central processing unit 10 includes an instruction unit 13 that responds to instruction signal groups by enabling predetermined apparatus in the proper sequence for performing the activity defined by the instruction. In particular, the instruction unit 13 controls the execution unit 14 which performs the operations, on data signal groups applied thereto, defined by the instruction signal groups. In the execution unit 14, an arithmetic unit 15 typically performs the arithmetic operations defined by the instruction signal group. Where feasible, a separate floating point arithmetic unit 16 is included. A separate floating point arithmetic unit is utilized because the floating point arithmetic operation execution time is relatively lengthy and requires special circuit configurations. The present invention relates to the floating point arithmetic unit. 
     Addition and Subtraction 
     Considering first the addition and subtraction operations, a flow chart for these operations according to the related art is shown in FIG. 2. In step 200, the determination is made as to whether the required floating point operation is an effective add or an effective subtract operation. When the macro operation is an addition operation and the signs of the two quantities to be added are the same or when the macro operation is a subtraction operation and the signs of the two quantities to be subtracted are different, the operation is considered an effective add operation and the sequence beginning with step 210 is executed. When the requested macro operation is addition operation and the signs of the two quantities to be added are different or when the macro operation is subtraction operation and the signs of the two quantities to be subtracted are the same, the operation is an effective subtract operation and the sequence beginning with step 220 is executed. In the sequence beginning with step 210, the exponents of the two quantities are compared and the difference between the exponents, DELTA E, is determined in step 211. In step 212, the fraction having the smaller exponent is moved to the right by the number of positions DELTA E determined in the previous step, the procedure aligning the fraction of the two quantities A and B. (In the remainder of this discussion, the quantities A and B will be used to denote aligned fractions.) In step 213, the fractions A and B are added and, in step 214, a shift to the right of the resulting fraction is performed when an overflow is detected as a result of the process. In step 215, a rounding operation is performed when necessary. It will be clear to those skilled in the art that step 211, the determination of the exponential DELTA E, step 212, the alignment of the fractions, step 213, the addition of the fractions, and step 215, the rounding step, involve potentially time consuming operations by the floating point arithmetic unit. An effective add operation is illustrated in Table 1A. 
     
                       TABLE IA______________________________________(Example of an Effective Add Operation)______________________________________A = 0.10001     E.sub.A = N  Aligned operandB = 0.10000     E.sub.B = N  Aligned operandR = 1.00001     E.sub.R = N  Sum before normalizationR = 0.100001     E.sub.R = N + 1                  Sum after normalization0.000001           Rounding constantR = 0.10001     E.sub.R = N + 1                  Final result after rounding______________________________________ 
    
     Referring once again to FIG. 2, the effective subtract operation, indicated by step 220 begins with step 221, determination of DELTA E, the difference between the exponents of the two quantities. In step 222, the fractions are aligned by shifting the fraction with the smaller exponent to the right by DELTA E positions, DELTA E having been determined in the previous step. The aligned fractions are then subtracted in step 223. In step 224, when the result of the subtraction is a negative quantity, the 2&#39;s complement of the value determined in step 223 is computed. In step 225, the position of the leading 1 is detected and, in step 226, the fraction is normalized and the exponent value corrected for the normalization. In step 227, the fraction is rounded off, if necessary. It will be clear to those familiar with floating point operations that all the steps, 221 through 227, can require a relatively lengthy period for execution. 
     Multiplication 
     The floating point multiplication operation can be understood by the processes implemented for the fields of the quantities to be multiplied. 
     
         S.sub.R =S.sub.1 XOR S.sub.2 
    
     
         E.sub.R =E.sub.1 +E.sub.2 
    
     
         F.sub.R =F.sub.1 *F.sub.2 
    
     where S is the sign field, E is the field of the argument of the exponent and F is the fraction field. The multiplication operation has a relatively simple process flow. The major time consuming step portion of the operation is the multiplication of the two fractions. In most applications, the multiplication of the two fractions is expedited by using a large array of adder circuits. One of the most frequently used techniques to perform the fraction multiplication is to couple the adder circuits in a carry-save configuration and eliminate the carry propagation times. One consequence of using a carry-save configuration is that the result consists of a redundant form and is comprised of two vectors. These vectors are called the final Sum and Carry vectors, and a carry propagation addition is required at the end of the array in order to change the result to a standard format. This standard format result may not be normalized, in which case a shifting process must be performed in which the fraction quantity is shifted one position prior to a final rounding step. The final rounding step is implemented by adding a one to the standard format result and may require a carry propagation addition. The flow of the fraction processing is indicated in FIG. 3. In step 301, the core operation of multiplying the fractions using the carry-save adder configuration is implemented. In step 302, the final Sum and Carry vectors determined by the carry-save adder configuration are added. The resulting sum is normalized, when necessary, in step 303. And finally, in step 304, a rounding process is performed. This rounding process is implemented by adding a carry in bit to the previous result at the round position, i.e. one bit position lower than the least significant bit position. The operation has two potentially time consuming operations in addition to the core multiplication step 301: the adding step 302 and the rounding step 304. These steps may each require a full carry propagation addition and are performed in series, thus adding two lengthy operations to the overall execution time. 
     Division 
     The floating point division operation can be understood by the following processes on the fields of the quantities to be divided. 
     
         S.sub.R =S.sub.1 XOR S.sub.2 
    
     
         E.sub.R =E.sub.1 -E.sub.2 
    
     
         F.sub.R =F.sub.1 /F.sub.2 
    
     where S is the sign field, E is the field of the argument of the exponent and F is the fraction field. The floating point operation of division has a relatively simple flow. The major time consuming process is finding the quotient of the two fractions. One frequently used algorithm in implementing the determination of the quotient is the &#34;non-restoring&#34; division. This algorithm requires performing a series of subtractions and additions depending on whether a partial remainder is positive or negative. This core operation is followed by a plurality of terminal operations. The first terminal operation is the determination of the difference between the number of ordinary subtract steps, designated as N S  and the number of negative subtract steps, designated N A . Additional process steps that are required include possible normalization of the result, rounding, and the possible rounding induced denormalization. Referring to FIG. 4, the steps in the floating point division operation are illustrated. In step 401, the core operation in the floating point division operation, i.e. the division of the fractions, is implemented while computing N S  and N A . In step 402, the difference N S  -N A , which defines the effective number of subtractions that were performed, is determined. The result is normalized, if necessary, in step 403. The maximum shift required for this normalization is one bit position. In step 404, the resulting fraction is rounded. It will be clear to those skilled in the art that, in addition to the core procedure of step 401, steps 402 and 404 may require full carry propagation addition and, by being performed in series, add two potentially lengthy procedures to the execution time. 
     FEATURES OF THE INVENTION 
     It is an object of the present invention to provide an improved data processing system. 
     It is another feature of the present invention to provide an improved central processing unit for a data processing unit. 
     It is yet another feature of the present invention to provide an improved execution unit for a data processing system. 
     It is still another feature of the present invention to provide improved apparatus and method for execution of floating point operations in a data processing system. 
     It is a more particular feature of the present invention to provide a method and apparatus for improved execution of the basic floating point arithmetic operations by parallel execution of certain procedures. 
     SUMMARY OF THE INVENTION 
     The aforementioned and other features are obtained, according to the present invention, by providing an additional adder circuit or unit to be operated in parallel with an adder circuit of the floating point arithmetic unit. In addition, a selector circuit is included to select which adder circuit has the correct computational result. In the flow charts of the typical floating point arithmetic operation, the operation sequence appears to require a previous result before certain procedures (e.g. the rounding procedure) can be implemented. However, a more careful analysis indicates that this linear flow diagram results in only two possible results. The second adder circuit, which along with the original adder circuit can combine the rounding constant to an addition or subtraction process, permits the two possible results of the operation to be determined in parallel. The selector circuit receives signals that indicate which adder has the correct resulting value, typically based on a presence of an overflow bit. Each of the arithmetic operations, the addition operation, the subtraction operation, the multiplication operation or the division operation, can have an increased execution speed as a result of the parallel processing capability. 
     These and other features of the present invention will be understood upon reading of the following description along with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of selected components of a data processing system. 
     FIG. 2 is a flow diagram for the floating point operations of addition and subtraction according to the related art. 
     FIG. 3 is a flow diagram for the floating point operation of multiplication according to the related art. 
     FIG. 4 is a flow diagram for the floating point operation of division according to the prior art. 
     FIG. 5 is a flow diagram of the execution of a floating point effective add operation according to the present invention. 
     FIG. 6 is a block diagram of the additional apparatus required to implement the floating point effective addition operation according to the present invention. 
     FIG. 7 is a flow diagram illustrating the floating point effective subtract operation according to the present invention. 
     FIG. 8 is a flow diagram illustrating the fractin floating point multiplication operation according to the present invention. 
     FIG. 9 is a flow diagram illustrating the fraction floating point division operation according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Detailed Description of the FIGS. 
     Referring now to FIG. 5, the procedure for performing an improved effective add operation according to the present invention is illustrated. As with the sequence illustrated in FIG. 2 beginning with step 210, the subsequent steps 211 and 212, the determination of DELTA E and the alignment of the fractions providing aligned fractions A and B, are the same. In the process of the present invention, the subsequent activity is the simultaneous execution of steps 513A and 513B. In step 513A, the (aligned) fractions A and B are divided by 2 and the resulting quantities, A/2 and B/2, and the rounding constant are added. Simultaneously in step 513B, the fractions A and B and the rounding constant are added together. Then in step 514, the result of the appropriate process result, i.e. the result of step 513A or 513B, is selected based on the presence or absence of an overflow bit from the process of step 513B. 
     Referring now to FIG. 6, the additional apparatus 60 required to implement the effective add operation of the present invention is shown. The results of the alignment operation, i.e. the fractions A and B, are applied to adder circuit 61 and, the same fractions, shifted by one position, i.e. the fractions A/2 and B/2, are applied to adder circuit 62. The addition operation is performed in each adder circuit simultaneously and, during this process, the rounding constant is applied to each adder circuit. The resulting quantities determined by each of the two adder circuits are applied to the selector circuit 63. Selector circuit 63 also receives an overflow signal from adder circuit 61. The presence of the overflow bit causes the result from adder circuit 62 to be utilized, while the absence of the overflow bit causes the result from adder circuit 61 to be selected. 
     Referring next to FIG. 7, the procedure according to the present invention for performing the floating point effective subtract operation is illustrated following the determination of the effective subtract operation in step 220. As with the procedure of the related art, the difference between the exponential values, DELTA E, is determined in step 221. The DELTA E value determines the remainder of the procedure. When DELTA E=O in step 722, no aligning step is necessary. However, the resulting difference can be negative, there is never a need to round the resulting quantity and a large normalization may be necessary. Because the resulting quantity can be negative, in order to expedite the effective subtract operation, two adders are used to compute simultaneously (A-B) in step 723A and (B-A) in step 723B. A selection circuit determines the correct quantity in step 724 by selecting the positive result using the overflow bit from step 723A. In the presence of the overflow bit, the selector circuit selects the result determined by step 723B. In the absence of the overflow bit, the selecting step 724 selects the result determined by step 723A. In step 725, the leading one in the resulting fraction selected in step 724 is identified and the quantity is normalized in step 726 based on the position of the leading one. In the second situation illustrated in FIG. 7 in which DELTA E=1, the alignment procedure requires a shift of one position for the fraction with the smaller exponent, a relatively rapid process. When the aligned fraction is subtracted from the normalized fraction, the resulting quantity is always positive. However, the resulting quantity must be sometimes rounded and a large normalization can be required. Because the aligning procedure requires only a single position shift, the use of a large shifter is not necessary. Rounding is required only in the case where no normalization is required. Therefore, referring to FIG. 7, the determination that DELTA E =1 in step 732 is followed by an (rapid) alignment step 733, and a subtraction step 734. Following the subtraction step 734, either the normalization step 735A or the rounding step 735B is implemented. Thus, the steps potentially requiring a lengthy execution interval are the determination of DELTA E in step 221, the subtraction in step 734 and either the normalization step 735A or the fraction rounding in step 735B. In the third situation illustrated in FIG. 7, DELTA E&gt;1 is determined in step 721. The resulting quantity from this effective subtract situation will always be positive when the aligned fraction is subtracted from the normalized fraction. The resulting quantity may sometimes be rounded and the maximum normalization requires a shift of 1 position. Referring to FIG. 7, after DELTA E is identified to be greater than 1, in step 743, the quantities must be aligned. In step 744A, the quantity 2A-2B is determined in a first adder circuit and the rounding constant is combined during the process. Simultaneously, in step 744B, the quantity A-B is determined in a second adder circuit and the rounding constant combined therewith. In step 745, the overflow bit from the first adder circuit is used to select the correct result from the two adder circuits. If the overflow bit of step 744A is present, then the result determined by step 744B is selected. If the overflow bit is absent in step 744A, then the result determined by step 744A is selected. 
     Referring next to FIG. 8, the flow diagram for the improved floating point multiplication operation is illustrated. In step 801, the fractions are multiplied in the carry sum adder array to obtain a Sum vector and a Carry vector. In step 802A, the Carry vector and the Sum vector are added together and the rounding constant is combined therewith in a first adder circuit. Simultaneously, in step 802A, the quantity 2*(Carry vector) and 2*(Sum vector) are added together and the rounding constant is combined therewith in a second adder circuit. In step 803, the result from one of the adder circuits is selected based on the overflow bit from the second adder circuit of step 802B. If the overflow bit is present, then the sum determined by step 802A is used and is normalized. If the overflow bit is not present, then the sum determined by step 802B is selected as the final result. 
     Referring next to FIG. 9, the procedure for performing the floating point division operation according to the present invention is illustrated. In step 901, the fractions are divided and the value for the number of subtractions, N S , and the value for the number of negative subtractions, N A , are determined. In step 902A, the value N S  -N A  is determined and the rounding bit is combined during the operation. Simultaneously, in step 902B, the value of N S  /2-N A  /2 is determined and the rounding constant combined therewith. In step 903, the result from either step 902A or step 902B is selected depending on the overflow bit of the operation performed in step 902A. If the overflow bit in step 902A is present, then the result determined by step 902B is selected. If the overflow bit is absent in step 902A, then the result determined by step 902A is selected. 
     OPERATION OF THE PREFERRED EMBODIMENT 
     Addition and Subtraction 
     Referring to FIG. 5, the procedure for performing the effective add operation according to the present invention is illustrated. 
     
                       TABLE IB______________________________________(Example of a revised effective add operation)Nonshifted     Shiftedoperands       operands(Step 513B)    (Step 513A)______________________________________A = 0.10001     E = N    A/2 = 0.010001                            E = N + 1B = 0.10000     E = N    B/2 = 0.0100001                            E = N + 1RC = 1             RC =R = 1.000011     E = N    R = 0.100010  E = N + 1(Select shifted operand by the overflow bit of step 513B. In thisexample, the result from step 513A is selected.)______________________________________ Comparing the flow diagram of FIG. 2 with the flow diagram of FIG. 5, by providing parallel processing in which the rounding process can be incorporated in the addition process, a potentially lengthy step can be eliminated and the execution of the effective add operation expedited. 
    
     In the floating point effective subtract operation of the present invention, when DELTA E =O, as will be clear to those skilled in the art, only three potentially lengthy steps are involved, the symmetric subtract operation, the leading 1 detection, and the normalization. In particular, the alignment step, the negation (2&#39;s complement) step and the rounding steps have been eliminated or are not required. Similarly, in the situation where DELTA=1, three potentially lengthy steps are required for execution of the effective subtract operation. In the situation where DELTA&gt;1, three potentially lengthy process steps can provide the requisite floating point effective subtract operation. Thus, the effective subtract operation can in general be performed in three potentially lengthy steps as opposed to seven potentially lengthy steps as the operation is performed in the related art. To achieve this economy of time, an additional adder is needed and a selector circuit. In addition, the subinstructions implementing the steps are more complex, responding to a determination as to the value of DELTA E. 
     The use of the processes of the present invention have been estimated to provide a reduction in the execution of the floating point operations of addition and subtraction operations in excess of 25%. 
     Multiplication 
     For the floating point operation, the effort expended on the core process of multiplying has resulted in the execution bottleneck being shifted to the remaining or terminal processes. In the procedure of the present invention, the two full carry propagation additions of the related art are replaced by the simultaneous execution of two processes, one of which will be the correct result of the computation. It is estimated that the implementation of the present invention will decrease the execution time for the floating point multiplication operation by an amount that can, in certain situations, be as high as 25%. 
     Division 
     A great amount of effort has been expanded in developing the techniques to provide the quotients of the fractions expeditiously. Although tremendous improvement in this process has been achieved, the quotient developement remains the principal execution bottleneck for the floating point division operation. However, the remaining portion of the operation has a significant impact on the time required for the operation. By performing the two subtractions in parallel and then selecting the appropriate result based on the overflow bit as opposed to a serial process of normalizing and rounding, the execution time for the operation has estimated to be reduced, in certain situations, by as much as 10%. 
     It will be clear that the apparatus illustrated in FIG. 6 is generally applicable to all the situations of the present invention in which the floating point arithmetic operations can be expedited by parallel processing. In the present invention, the operations are expedited in the terminal portion of the operation as opposed to the core data manipulation portion. Thus in several instances, the rounding constant can be combined prior to an actual determination of whether the calculated quantity must be normalized. Furthermore, the use of normalized quantities can place limits on the departure from normalization of the values resulting from execution of the floating point arithmetic operations. 
     The foregoing description is included to illustrate the operation of the preferred embodiment and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the foregoing description, many variations will be apparent to those skilled in the art that would yet be encompassed by the spirit and scope of the invention.