# Patent

## Title
Arithmetic circuit, arithmetic processing apparatus and method of controlling arithmetic circuit

## Abstract

An arithmetic circuit calculates a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-085882 filed on Apr. 8, 2011, with the Japanese Patent Office, the entire contents of which are incorporated herein by reference.
FIELD
The disclosures herein relate to an arithmetic circuit, an arithmetic processing apparatus, and a method of controlling an arithmetic circuit.
BACKGROUND
An accounting system for processing accounts in banks and some type of scientific computation may require numerical error to be small. To this end, multiple-precision numerical representation or variable-precision numerical representation may be employed. In such a case, a single integer may express a sign and an exponent. Further, a digit string separate from the sign and the exponent expressed by the single integer may often be used to represent a mantissa. When such numerical representation is employed, integer calculation may often be utilized to implement arithmetic operation between numerical values.
In contrast, study has been underway on a method for implementing multiple-precision or variable-precision floating-point arithmetic by use of fixed-precision floating-point arithmetic. A hardware processing unit is often available for fixed-precision floating-point arithmetic. The use of such a hardware processing unit can improve processing speed compared to the case in which all processes are performed by software. For example, there is a library that performs multiple-precision binary floating-point arithmetic by use of double-precision floating-point arithmetic. In such a method, a single number is represented by a set of fixed-precision floating-point numbers, which may be referred to as an “unvalued sum” because the set is used as it is, without adding up the individual numbers. Arithmetic operation between different sets may be performed to implement a high-precision arithmetic operation (i.e., four arithmetic operations).
In fixed-precision floating-point arithmetic, the result of arithmetic operations may not always be accurate. For example, providing an accurate sum of two fixed-precision floating-point numbers may require that the result of arithmetic be expressed by two fixed-precision floating-point numbers. An algorithm to obtain such an accurate sum is expressed as follows.

two_sum(X,Y)

z=fl(X+Y)

w=fl(z−X)

v=fl(z−w)

z1=fl(Y−w)

z2=fl(v−X)

zz=fl(z1−z2)

return(z,zz)

Here, fl(X+Y) indicates the result obtained by mapping the true value of X+Y onto the floating-point number, i.e., the result obtained by expressing this value within the limited precision of the floating-point number. Two values z and zz obtained by the above-noted two_sum function accurately satisfies the following: z+zz=X+Y. Value z represents the most significant part of X+Y within the precision of the fixed-precision floating-point number format, and zz represents a remainder that is left unexpressed by the precision of the fixed-precision floating-point number format.

Attending to rounding that occurs at the time of mapping will be described by taking as an example the rounding to nearest with ties away from 0, which is an exemplary rounding method used in decimal numbers. For the sake of simplicity, the precision of a fixed-precision floating-point number is assumed to be two decimal digits. In this case, the sum of 20000 and −1 will be calculated by two_sum as follows.

X=20000

Y=−1

z=fl(X+Y)=20000

w=fl(z−X)=0

v=fl(z−w)=20000

z1=fl(Y−w)=−1

z2=fl(v−X)=0

zz=fl(z1−z2)=−1

Through the above-noted calculations, values z and zz are obtained such as to accurately satisfy the following: z+zz=X+Y.

In the above-noted algorithm, floating-point arithmetic operations are performed as many as six times in order to obtain a single sum “z+zz”. This algorithm for obtaining an accurate sum is frequently used in order to implement multiple-precision or variable-precision floating-point arithmetic by use of fixed-precision floating-point arithmetic. Accordingly, it may be desired to speed up this accurate-sum algorithm.
RELATED-ART DOCUMENTS
Non-Patent Document


    [Non-Patent Document 1] T. Dekker, A Floating-Point Technique for Extending the Available Precision, Numer. Math. vol. 18, pp. 224-242, 1971.
    [Non-Patent Document 2] D. Priest, Appendix A: Algorithms for Arbitrary Precision Floating Point Arithmetic, pp. 111-124, On Property of Floating Point Arithmetics: Numerical Stability and the Cost of Accurate Computations, PhD thesis, University of California, Berkeley, November 1992.
    [Non-Patent Document 3] Yozo Hida, Xiaoye S. Li, David H. Bailey, Library for Double-Double and Quad-Double Arithmetic, 29 Dec. 2007.


SUMMARY
According to an embodiment, an arithmetic circuit for calculating a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision includes: a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa; a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa; a first shift-amount generating unit to generate as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; a second shift-amount generating unit to generate as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; a first shift unit to calculate a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount; a second shift unit to calculate a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount; an add-subtract unit to produce a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and to produce a carry obtained by the add-subtract operation performed with respect to the first post-shift mantissa and the second post-shift mantissa; a post-shift-exponent generating unit to calculate a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa; a corrected-exponent generating unit to generate a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry; a first normalization unit to calculate a normalized exponent that is obtained by normalizing the corrected exponent, and to calculate a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and a second normalization unit to calculate a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, equal in length to the number of digits of the predetermined fixed precision, of the normalized double-precision mantissa, and to calculate a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.
According to an embodiment, an arithmetic processing apparatus includes an arithmetic circuit to calculate a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision, and an instruction control unit to decode a correction value calculating instruction for calculating the correction value for the value that is obtained by an add-subtract operation of two values and that is expressed in the predetermined fixed precision. The arithmetic circuit includes: a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa; a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa; a first shift-amount generating unit to generate as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; a second shift-amount generating unit to generate as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; a first shift unit to calculate a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount; a second shift unit to calculate a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount; an add-subtract unit to produce a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and to produce a carry obtained by the add-subtract operation; a post-shift-exponent generating unit to calculate a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa; a corrected-exponent generating unit to generate a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry; a first normalization unit to calculate a normalized exponent that is obtained by normalizing the corrected exponent, and to calculate a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and a second normalization unit to calculate a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, expressed in the predetermined fixed precision, of the normalized double-precision mantissa, and to calculate a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.
According to an embodiment, a method of controlling an arithmetic circuit for calculating a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision is provided wherein the arithmetic circuit includes: a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa; and a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa. The method includes: generating, by use of a first shift-amount generating unit of the arithmetic circuit, as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and generating as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; generating, by use of a second shift-amount generating unit of the arithmetic circuit, as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and generating as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count; calculating, by use of a first shift unit of the arithmetic circuit, a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount; calculating, by use of a second shift unit of the arithmetic circuit, a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount; producing, by use of an add-subtract unit of the arithmetic circuit, a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and producing a carry obtained by the add-subtract operation; calculating, by use of a post-shift-exponent generating unit of the arithmetic circuit, a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa; generating, by use of a corrected-exponent generating unit of the arithmetic circuit, a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry; calculating, by use of a first normalization unit of the arithmetic circuit, a normalized exponent that is obtained by normalizing the corrected exponent, and calculating a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and calculating, by use of a second normalization unit of the arithmetic circuit, a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, expressed in the predetermined fixed precision, of the normalized double-precision mantissa, and calculating a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.
According to at least one embodiment of the present disclosures, an arithmetic method is provided to obtain at high speed an accurate sum of fixed-precision floating-point numbers.
The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a drawing illustrating a table of specific examples of Oracle-numbers;
FIG. 2 is a drawing illustrating an example of the configuration of a computer system;
FIG. 3 is a drawing illustrating the configuration of the oraclenum64 format;
FIG. 4 is a drawing illustrating the configuration of an oraclenum64 number having a length of 9 bytes;
FIG. 5 is a drawing illustrating an example of the configuration of an arithmetic device that can perform arithmetic directly on oraclenum64 numbers;
FIG. 6 is a drawing illustrating an example of the configuration of an exponent-&-mantissa arithmetic circuit;
FIG. 7 is a drawing illustrating an example of the configuration of a normalization circuit;
FIGS. 8A and 8B are drawings illustrating an example of the configuration of a leading zero counting circuit;
FIG. 9 is a drawing illustrating an example of the configuration of an internal-form-conversion circuit;
FIG. 10 is a drawing illustrating an example of the configuration of an external-form-conversion circuit;
FIG. 11 is a drawing illustrating a specific example of the addition of fixed-precision floating-point numbers;
FIG. 12 is a drawing illustrating another specific example of the addition of fixed-precision floating-point numbers;
FIG. 13 is a drawing illustrating a method of dividing an Oracle-number having a length of 21 bytes into three parts;
FIG. 14 is a drawing illustrating a method of generating oraclenum64 numbers corresponding to the three divided mantissa parts, respectively;
FIG. 15 is a drawing illustrating an example of the configuration of a circuit that performs a get_z arithmetic operation;
FIG. 16 is a drawing illustrating a specific example of the get_z arithmetic operation;
FIG. 17 is a drawing illustrating another specific example of the get_z arithmetic operation;
FIG. 18 is a drawing illustrating yet another specific example of the get_z arithmetic operation;
FIGS. 19A and 19B are drawings illustrating examples of an addition operation in a case where the absolute values of two inputs are separated by more than the number of digits of the format being used;
FIG. 20 is a drawing illustrating an example of the configuration of a circuit that performs a get_zz arithmetic operation;
FIG. 21 is a drawing illustrating a specific example of the get_zz arithmetic operation;
FIG. 22 is a drawing illustrating another specific example of the get_zz arithmetic operation;
FIG. 23 is a drawing illustrating yet another specific example of the get_zz arithmetic operation;
FIG. 24 is a drawing illustrating a circuit-diagram symbol for two_sum;
FIG. 25 is a drawing illustrating an example of the circuit that obtains the sum of an oraclenum64 number and triple-oraclenum64 numbers;
FIG. 26 is a drawing illustrating an algorithm for obtaining the sum of two sets of triple-oraclenum64 numbers;
FIG. 27 is a drawing illustrating connections between three two_sum operators for removing overlaps;
FIG. 28 is a drawing illustrating an example of sign-matched Priest's renormalization;
FIG. 29 is a drawing illustrating an example of calculation that produces a set of strongly-normalized numbers from the number set normalized by sign-matched Priest's renormalization;
FIG. 30 is a drawing illustrating an example of the configuration of circuit that performs a scale_next(X, Y) operation for performing quantization;
FIG. 31 is a drawing illustrating an example of the configuration of a circuit that performs a scale_next operation without converting a mantissa into an internal format;
FIG. 32 is a drawing illustrating a table demonstrating three types of methods for specifying NUMBER-type precision in the Oracle-Database;
FIGS. 33A through 33C are drawings illustrating examples of algorithms that generate a number having “5” only at the digit position at which rounding occurs;
FIG. 34 is a drawing illustrating an example of the configuration of a circuit that performs a get_comma5 arithmetic operation;
FIG. 35 is a drawing illustrating an example of the configuration of a circuit that performs a truncate operation;
FIG. 36 is a drawing illustrating an example of the configuration of a mask-value generating circuit;
FIG. 37 is a drawing illustrating an example of the configuration of a magnitude-comparison error check circuit for p_d and p_s;
FIGS. 38A and 38B are drawings illustrating an example of the circuit configuration of a sign arithmetic unit;
FIG. 39 is a drawing illustrating an example of the arithmetic operation for obtaining the length of an Oracle-number;
FIG. 40 is a drawing illustrating an example of the configuration of a circuit that performs an expand operation;
FIGS. 41A and 41B are drawings illustrating an example of the configuration of a trailing zero counting circuit;
FIG. 42 is a drawing illustrating an example of the configuration of a digit-select calculating circuit;
FIG. 43 is a drawing illustrating an example of the configuration of a fixed-precision floating-point number adding and subtracting unit; and
FIG. 44 is a drawing illustrating an example of the configuration of an exponent-&-mantissa mask calculating circuit.



DESCRIPTION OF EMBODIMENTS
In the following, embodiments of the invention will be described with reference to the accompanying drawings. In each of the drawings, the same or corresponding elements are referred to by the same or corresponding symbols or numbers, and a description thereof will be omitted as appropriate.
The embodiments described below uses, as an example of numerical-number expression, Oracle-numbers (registered trademark) which are the numerical number type used in the Oracle database (registered trademark), and provide hardware for high-speed calculation of Oracle-numbers. A description will be first given of the Oracle-number representation. What is described herein can be checked by using the SQL interpreter (i.e., Structured Query Language interpreter) of the Oracle database.
An Oracle-number is represented in a variable-precision data format including up to a maximum of 21 bytes. The first byte stores a sign and an exponent, and the following bytes store a mantissa. The mantissa extends up to a maximum of 20 bytes.
The Oracle-number format is a data format for representing a floating-point decimal number. Mainly because of the efficiency of memory utilization, the mantissa has two digits of a decimal number in each byte. Matching this notation, the exponent stores an exponent number in respect of a radix of 100. A number represented by the Oracle-number format can be expressed as follows.
number=±(M00 . M01 M02 . . . )*100^(exp) Here, M00, M01, M02, and so on represent the data of respective bytes, i.e., the first byte, the second byte, the third byte, and so on, of the mantissa extending up to a maximum of 20 bytes. Since the mantissa is sectioned in units of two digits of a decimal number, the mantissa can be regarded as having 20 digits of a centesimal number. An Oracle-number is normalized without exception when viewed as a centesimal number. Under no circumstances, does the M00 part (i.e., the first byte of the mantissa) become zero.
The first byte of the Oracle-number format (i.e., the first byte of the entire number) contains a sign and an exponent, which are encoded as follows.
In the case of number>0: First Byte=exp+193
In the case of number==0: First Byte=128
Otherwise: First Byte=62−exp
The mantissa in the second byte and onwards contain M00, M01, and so on in the respective bytes thereof. In each byte, different encoding methods are used as illustrated below, depending on the sign of the expressed numerical number.
In the case of number>0: Mantissa's n-th Byte=M(n−1)+1
In the case of number==0: No Mantissa
Otherwise: Mantissa's n-th Byte=101−M(n−1)
With these encoding methods, 0×00 never appear in any byte of the mantissa since Mn ranges from 0 to 99. When the number to be expressed can be expressed by a short mantissa, this Oracle-number becomes shorter than 21 bytes. Namely, trailing zeros are not permitted in the mantissa of an Oracle-number. In the case of a negative number, a terminator of 102 (0×66) is stored in the last byte in order to indicate the tail end of the mantissa when the mantissa is shorter than 20 bytes.
The use of the encoding scheme described above in the Oracle-number format ensures that the magnitude relationship as viewed as byte strings, i.e., the magnitude relationship based on comparison utilizing the C-language standard function memcmp, be the same as the magnitude relationship between values expressed in the Oracle-number format.
FIG. 1 is a drawing illustrating a table of specific examples of Oracle-numbers. In the expression of 10E+0(=10×1000), for example, the exponent is 193 (=0+193), and the mantissa is 11 (=10+1). In the expression of 10E+1(=10×1001), for example, the exponent is 194 (=1+193), and the mantissa is 2 (=1+1). In the expression of a negative number of −10E−130(=−10×100−65), for example, the exponent is 127 (=62−(−65)), and the mantissa is 91 (=101−10). In the expression of a negative number of −10E−129(=−10×100−64), for example, the exponent is 126 (=62−(−64)), and the mantissa is 100 (=101−1). For the negative numbers, the terminator “102” is attached as the last byte. Further, the positive infinite number Inf and the negative infinite number −Inf are assigned to special byte strings as illustrated in the table.
FIG. 2 is a drawing illustrating an example of the configuration of a computer system. The computer system illustrated in FIG. 2 includes a processor 110 and a memory 111. The processor 110 serving as an arithmetic processing apparatus includes a secondary cache unit 112, a primary cache unit 113, a control unit 114, and an arithmetic unit 115. The primary cache unit 113 includes an instruction cache 113A and a data cache 113B. The arithmetic unit 115 includes a register 116, an arithmetic controlling unit 117, and an arithmetic device 118. The arithmetic device 118 includes an arithmetic circuit 119. In FIG. 2 and the subsequent drawings, boundaries between functional blocks illustrated as boxes basically indicate functional boundaries, and may not correspond to separation in terms of physical positions, separation in terms of electrical signals, separation in terms of control logic, etc. Each functional block may be a hardware module that is physically separated from other blocks to some extent, or may indicate a function in a hardware module in which this and other blocks are physically combined together. Each functional block may be a module that is logically separated from other blocks to some extent, or may indicate a function in a module in which this and other blocks are logically combined together.
The above-noted computer system is an exemplified information processing apparatus utilizing a CPU (central processing unit), and is used to implement hardware for performing arithmetic on Oracle-numbers. In so doing, it may be preferable to add a new function without making extensive modifications to the system configuration. Namely, an effort is made to achieve an implementation method that can do away with as many modifications as possible caused by addition of new functions. For example, in a currently used CPU, the arithmetic device typically has two operand inputs and one output, with each operand having a data length of 8 bytes (i.e., 64 bits). It may be preferable not to change this configuration in order to reduce the extent of hardware modifications.
In the processor 110, the cache memory system is implemented as having a multilayer structure in which the primary cache unit 113 and the secondary cache unit 112 are provided. Specifically, the secondary cache unit 112 that can be accessed faster than the main memory is situated between the primary cache unit 113 and the main memory (i.e., the memory 111). With this arrangement, the frequency of access to the main memory upon the occurrence of cache misses in the primary cache unit 113 is reduced, thereby lowering cache-miss penalty.
The control unit 114 issues an instruction fetch address and an instruction fetch request to a primary instruction cache 113A to fetch an instruction from this instruction fetch address. The control unit 114 controls the arithmetic unit 115 in accordance with the decode results of the fetched instruction to execute the fetched instruction. The arithmetic controlling unit 117 operates under the control of the control unit 114 to supply data to be processed from the register 116 to the arithmetic device 118 and to store processed data in the register 116 at a specified register location. Further, the arithmetic controlling unit 117 specifies the type of arithmetic performed by the arithmetic device 118. Moreover, the arithmetic controlling unit 117 specifies an address to be accessed to perform a load instruction or a store instruction with respect to this address in the primary cache unit 113. Data read from the specified address by the load instruction is stored in the register 116 at a specified register location. Data stored at a specified location in the register 116 is written to the specified address by the store instruction.
A description will be first given of the definition of an oraclenum64 format that is a subset of the Oracle-number format. An oraclenum64 is an Oracle-number in which the significant length of the mantissa is shorter than or equal to 7 bytes.
FIG. 3 is a drawing illustrating the configuration of the oraclenum64 format. Data 121 representing an oraclenum64 number can be accommodated in an 8-byte-length register. When an Oracle-number having a total length, including a sign and an exponent, of shorter than 8 bytes is stored in a register as an oraculenum64 number, data is packed to the left as illustrated as bytes 122 in FIG. 3. The remaining part on the left has bytes 123 each having a value of 0×00 stored as many as there are remaining bytes.
FIG. 4 is a drawing illustrating the configuration of an oraclenum64 number having a length of 9 bytes. As illustrated in FIG. 4, data 125 representing a negative Oracle-number having a length of 9 bytes has a mantissa 126 having a length of 7 bytes followed by the last byte (i.e., ninth byte) in which a terminator 127 (0×66) is stored. This Oracle-number is an oraclenum64 number since the significant length of the mantissa 126 is 7 bytes. This is required for a sign-reversed number of an oraclenum64 number to be always an oraclelum64 number.
FIG. 5 is a drawing illustrating an example of the configuration of an arithmetic device that can perform arithmetic directly on oraclenum64 numbers. The arithmetic device illustrated in FIG. 5 corresponds to part of the arithmetic circuit 119 illustrated in FIG. 2. The arithmetic device illustrated in FIG. 5 includes an input-X register 131, an input-Y register 132, internal-form-conversion circuits 133 and 134, an exponent-&-mantissa arithmetic circuit 135, selectors 136 and 137, shifters 138 and 139, an exponent adder 140, and an absolute-value adder 141. The arithmetic device further includes a normalization circuit 142, a rounding circuit 143, an external-form-conversion circuit 144, and an output-Z register 145. The internal-form-conversion circuits 133 and 134 and the external-form-conversion circuit 144 may be specially designed such that both the oraclenum64 format and the IEEE754-decimal64 format can be processed. In FIG. 5, the inputs and the output are floating-point numbers having the same format and the same precision. The input data may not be normalized data. The output data is normalized. The input data is provided in the oraclenum64 format, and, also, unnormalized data are processable. The output data is output in the oraclenum64 format without exception.
Each of the internal-form-conversion circuits 133 and 134 divides an input into a sign, an exponent, and a mantissa, thereby converting the input value representation into an internal format. The sign, exponent, and mantissa of the input X are referred to as a sign-X, an exponent-X, and a mantissa-X, respectively. The sign, exponent, and mantissa of the input Y are referred to as a sign-Y, an exponent-Y, and a mantissa-Y, respectively.
The exponent-&-mantissa arithmetic circuit 135 receives the exponent-X and the exponent-Y as well as the mantissa-X and the mantissa-Y. The exponent-&-mantissa arithmetic circuit 135 compares the exponent-X and the exponent-Y in terms of their magnitudes. Based on the result of magnitude comparison, the exponent-&-mantissa arithmetic circuit 135 generates a select signal such that the mantissa (i.e., first mantissa) associated with the larger exponent is supplied to the shifter 138 and that the mantissa (i.e., second mantissa) associated with the smaller exponent is supplied to the shifter 139. The exponent-&-mantissa arithmetic circuit 135 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 135 outputs the count indicative of the number of leading zeros in the first mantissa as the amount of shift to the left for the shifter 138 if the absolute value of a difference between the exponent-X and the exponent-Y is larger. The exponent-&-mantissa arithmetic circuit 135 outputs the absolute value of a difference between the exponent-X and the exponent-Y as the amount of shift to the left for the shifter 138 if the absolute value of a difference between the exponent-X and the exponent-Y is smaller. Here, the count indicative of the number of leading zeros is equal to the number of consecutive zeros appearing from the most significant digit in the mantissa.
The exponent-&-mantissa arithmetic circuit 135 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 225 outputs the absolute value of a difference between the exponent-X and the exponent-Y minus the count indicative of the number of leading zeros in the first mantissa as the amount of shift to the right for the shifter 139 if the absolute value of a difference between the exponent-X and the exponent-Y is larger. The exponent-&-mantissa arithmetic circuit 135 outputs zero as the amount of shift to the right for the shifter 139 if the absolute value of a difference between the exponent-X and the exponent-Y is smaller. The exponent-&-mantissa arithmetic circuit 135 further outputs as an exponent the smaller exponent plus the above-noted amount of shift to the right.
The shifter 138 shifts the supplied mantissa to the left according to the specified amount of shift. The shifter 139 shifts the supplied mantissa to the right according to the specified amount of shift. The results of shifts by these shifters are input into the absolute-value adder 141.
In the case of subtraction, one of the mantissas is inverted, and a carry is input into the absolute-value adder 141. When a digit overflow is detected as a result of addition performed by the absolute-value adder 141, a value that is shifted to the right by one digit is output. At the same time, a carry is supplied from the absolute-value adder 141 to the exponent adder 140, so that the supplied carry is added to the exponent.
When a digit underflow is detected as a result of addition performed by the absolute-value adder 141, a value that is shifted to the left by one digit is output. At the same time, a signal indicative of digit underflow is supplied from the absolute-value adder 141 to the exponent adder 140, so that subtraction is performed with respect to the exponent. In the case of multiplication and division, a loop arithmetic circuit reutilizing a result of addition may be used.
The normalization circuit 142 receives the result of addition, and outputs a normalized arithmetic result. The rounding circuit 143 rounds the normalized arithmetic result. The external-form-conversion circuit 144 converts the rounded normalized arithmetic result into an external format for provision to the output-Z register 145.
FIG. 6 is a drawing illustrating an example of the configuration of the exponent-&-mantissa arithmetic circuit. The exponent-&-mantissa arithmetic circuit 135 illustrated in FIG. 6 includes a comparison circuit 151, an absolute-value adder 152, selectors 153 and 154, an adder 155, a leading zero counting circuit 156, selectors 157 and 158, and an adder 159.
The comparison circuit 151 compares the exponent-X and the exponent-Y in terms of their magnitudes, and generates a select signal such that the mantissa (i.e., first mantissa) associated with the larger exponent is supplied to the shifter 138 and that the mantissa (i.e., second mantissa) associated with the smaller exponent is supplied to the shifter 139. The absolute-value adder 152 calculates an absolute value of a difference between the exponent-X and the exponent-Y. The leading zero counting circuit 156 counts the number of leading zeros in the selected mantissa. The adder 155 compares the absolute value of a difference between the exponent-X and the exponent-Y and the count indicative of the number of leading zeros in the selected mantissa, thereby outputting a select signal responsive to the result of the comparison.
The selector 138 outputs the count indicative of the number of leading zeros as the amount of shift to the left for the shifter 138 if the absolute value of a difference is larger. The selector 158 outputs the absolute value of a difference as the amount of shift to the left for the shifter 138 if the absolute value of a difference is smaller.
The selector 157 outputs the absolute value of a difference minus the count indicative of the number of leading zeros as the amount of shift to the right for the shifter 139 if the absolute value of a difference is larger. The selector 157 outputs zero as the amount of shift to the right for the shifter 139 if the absolute value of a difference is smaller.
The adder 159 receives the smaller exponent from the selector 153. The adder 159 outputs as an exponent a value that is obtained by adding the amount of shift to the right to the received smaller exponent.
FIG. 7 is a drawing illustrating an example of the configuration of the normalization circuit. The normalization circuit 142 illustrated in FIG. 7 includes a leading zero counting circuit 160, a shift-amount correcting circuit 161, a left shifter 162, an exponent arithmetic unit 163, bit shifters 164 and 165, and a selector 166.
The mantissa is input by use of a data width having an extra one digit attached to the most significant digit when taking into account a digit overflow. The leading zero counting circuit 160 receives a mantissa excluding the extra digit situated immediately above the most significant digit, and counts the number of leading zeros to output a count indicative of the number. The shift-amount correcting circuit 161 receives the count indicative of the number of leading zeros and the least significant bit of the exponent. When an XOR (i.e., exclusive OR) operation between the least significant bit of the exponent and the least significant bit of the count indicative of the number of leading zeros (which indicates whether the count is an odd number or an even number) produces a value of “1”, the shift-amount correcting circuit 161 outputs as the amount of shift to the left a value that is obtained by subtracting 1 from the count indicative of the number of leading zeros. When the above-noted XOR value is 0, the shift-amount correcting circuit 161 outputs the count indicative of the number of leading zeros as the amount of shift to the left.
The left shifter 162 receives the amount of shift and a mantissa excluding the extra digit situated immediately above the most significant digit, and outputs a value that is obtained by shifting the mantissa to the left by the specified amount of shift. The selector 166 selects the result of the shift to the left output from the left shifter 162 when the extra digit situated immediately above the most significant digit of the input is “0”. Further, the selector 166 selects a value obtained by shifting to the right by two digits the input mantissa inclusive of the extra digit situated immediately above the most significant digit upon concurrent occurrence of both a condition that the extra digit situated immediately above the most significant digit of the input is “1” and a condition that the least significant bit of the exponent is “0”. Moreover, the selector 166 selects a value obtained by shifting to the right by one digit the input mantissa inclusive of the extra digit situated immediately above the most significant digit upon concurrent occurrence of both a condition that the extra digit situated immediately above the most significant digit of the input is “1” and a condition that the least significant bit of the exponent is “1”. The value selected by the selector 166 is output as a mantissa.
The exponent arithmetic unit 163 receives the exponent, the amount of shift, and the most significant digit of the input mantissa. In the case of the most significant digit of the input mantissa being “0”, the exponent arithmetic unit 163 outputs as an exponent a value that is obtained by subtracting the amount of shift from the exponent. Upon concurrent occurrence of both a case of the most significant digit of the input mantissa being “1” and a case of the least significant bit of the exponent being “0”, the exponent arithmetic unit 163 outputs as an exponent a value that is obtained by adding 2 to the exponent. Upon concurrent occurrence of both a case of the most significant digit of the input mantissa being “1” and a case of the least significant bit of the exponent being “1”, the exponent arithmetic unit 163 outputs as an exponent a value that is obtained by adding 1 to the exponent.
FIGS. 8A and 8B are drawings illustrating an example of the configuration of the leading zero counting circuit. As illustrated in FIG. 8A, the leading zero counting circuit includes a conversion circuit 160. The conversion circuit 160 receives the mantissa as input data, and generates output data from the input data in accordance with the table illustrated in FIG. 8B. This output data is the count indicative of the number of leading zeros, and represents the count by a binary number. In the table, the symbol “X” at the leftmost position indicates a non-zero value, and other Xs indicate a “don't care” value. 0s are the zeros that are subjected to counting.
FIG. 9 is a drawing illustrating an example of the configuration of the internal-form-conversion circuit. The internal-form-conversion circuit 133 or 134 illustrated in FIG. 9 includes selectors 170 through 174, adders 175 and 176, and binary-to-decimal conversion circuits 177 and 178.
The input data is divided into a sign S, an exponent EXP, and a mantissa M01, M02, . . . . The sign S is comprised of one bit, which is output as sign data as it is. The sign indicates a positive number when it is 1.
The exponent EXP is 7 bits long. The selector 170 outputs as exponent data the exponent EXP as it is when the sign is “1”. The selector 170 outputs as exponent data an inverse of the exponent EXP when the sign is “0”.
Each of the mantissa parts M01, M02, and so on is 8 bits long. The selector 171 selects the mantissa when the sign is “1”, and selects an inverse of the mantissa when the sign is “0”. This selected value is applied to one of the inputs of the adder 175. The selector 172 selects “−1” when the sign is “1”, and selects “+101” when the sign is “0”. This selected value is applied to the other input of the adder 175. When the sign is “0”, further, a carry is input into the adder 175. The same also applies in the case of the selectors 173 and 174 and the adder 176.
The outputs of the adders 175 and 176 are converted from the binary format into the BCD format by the binary-to-decimal conversion circuits 177, respectively. The values after the BCD conversion are output as exponent data. It may be noted that values after the BCD conversion are forcibly set to zeros if the CO from the adder is “0”.
FIG. 10 is a drawing illustrating an example of the configuration of the external-form-conversion circuit. The external-form-conversion circuit 144 illustrated in FIG. 10 includes a decoder 180, a decimal-to-binary conversion circuits 181 and 182, selectors 183 through 187, and adders 188 and 189. FIG. 10 expressly illustrates each circuit portion corresponding to a respective one of the two mantissa parts BCD01 and BCD02. When the number of mantissa parts is three or more, the same circuit portion is provided for each respective one of these mantissa parts.
A 1-bit sign data input is output as it is, as sign data. The sign indicates a positive number when it is 1. An input and output exponent is 7 bits long. The selector 183 provides the input exponent as an output exponent when the sign is “1”, and provides an inverse of the input exponent as an output exponent when the sign is “0”. Each of the mantissa parts BCD01, BCD02, and so on is 8 bits long. The decimal-to-binary conversion circuits 181, 182, and so on convert BCD values into binary values.
The decoder 180 receiving a terminator selecting signal generates a terminator digit selecting signal that selects a terminator digit indicative of the digit at the tail end. The terminator digit selecting signal is distributed to the selectors 184, 185, and so on in the subsequent stage.
In respect of each of the adders 188, 189, and so on, one of the inputs thereof receives a mantissa when the sign is “1”, and receives an inverse of the mantissa when the sign is “0”. When the terminator digit selecting signal is “1”, however, zero is selected as an input. The other input receives “+1” when the sign is “1”, and receives “+101” when the sign is “0”. When the terminator digit selecting signal is “1”, however, “+101” is selected as an input. The value that is supposed to be added in this case is “+102”. The addition of the carry-in to “+101” achieves an operation equivalent to the addition of “+102” When the sign is “0”, further, a carry-in is input into the adder. The outputs of the adders 188, 189, and so on are output as respective parts of the mantissa.
FIG. 11 is a drawing illustrating a specific example of the addition of fixed-precision floating-point numbers. In FIG. 11, the exponent of the input X is denoted as Ex, the exponent of the input Y denoted as Ey, the count indicative of the number of leading zeros in the input X denoted as Lx, and the count indicative of the number of leading zeros in the input Y denoted as Ly. Further, the exponent of the output Z is denoted as Ez. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. To this end, Ex−Ey is calculated, the result of which is compared with Lx. In the example illustrated in FIG. 11, Lx is larger. Namely, the amount of shift Ex−Ey to left is within the number of leading zeros, so that only a mantissa 191 of the input X is shifted to the left. A mantissa 192 of the input Y is not shifted to the right. The amount of shift to the left is Ex−Ey, and the amount of shift to the right is 0.
The numbers whose digits are aligned as described above, i.e., the mantissa 192 and a mantissa 193 obtained by shifting the mantissa 191 to the left, are added together. Further, an addition result 194 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the left by one digit may be necessary. With such a shift, however, the exponent changes from “0” to “1”, which is an odd number. The amount of shift to the left is thus decreased by “1” to become “0”, so that the exponent remains to be “0”. Consequently, the addition result does not go through any change by normalization. The mantissa 194 and the exponent Ez obtained as the addition result are output as they are as the result of arithmetic.
FIG. 12 is a drawing illustrating another specific example of the addition of fixed-precision floating-point numbers. In this specific example, Lx is smaller than the contemplated amount of shift Ex-Ey to the left That is, the amount of shift Ex−Ey to the left is not within the number of leading zeros. Accordingly, an exponent 201 of the input X is shifted to the left by Lx, and an exponent 202 of the input Y is shifted to the right by the number of digits equal to a difference between the actual left shift amount and the contemplated left shift amount. In this case, the amount of shift to the left is Lx, and the amount of shift to the right is (Ex−Ey)−Lx.
The numbers whose digits are aligned as described above, i.e., a mantissa 201 obtained by shifting the mantissa 203 to the left and a mantissa 202 obtained by shifting the mantissa 204 to the right, are added together. In so doing, the digits that are overflowed by the shift to the right are kept. Further, an addition result 205 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the right by one digit may be necessary. With such a shift, however, the exponent Ez changes from “2” to “3”, which is an odd number. The amount of shift to the right is thus increased by “1” to become “2”, so that the exponent Ez changes from “2” to “4”.
Subsequently, rounding is performed on a normalized result 206. In this example, rounding to nearest with ties away from 0 is performed with respect to the digits that are overflowed to the right. As a result of such a rounding operation, a rounded mantissa 207 together with the corresponding exponent Ez are output as the result of an arithmetic operation.
In the following, a description will be given of a method of expressing a single Oracle-number by use of a set of plural oraclenum64 numbers.
In order to perform the calculation of Oracle-numbers each having a length of up to 21 bytes by use of arithmetic hardware for oraclenum64, an arrangement is made to express an Oracle-number by use of a set of plural oraclenum64 numbers.
FIG. 13 is a drawing illustrating a method of dividing an Oracle-number having a length of 21 bytes into three parts. As illustrated in FIG. 22, a mantissa 210 having a length of up to 20 bytes is divided into a mantissa 211 being 7 bytes long, a mantissa 212 being 7 bytes long, and a mantissa 213 being 6 bytes long.
FIG. 14 is a drawing illustrating a method of generating oraclenum64 numbers corresponding to the three divided mantissa parts, respectively. As for a0, the 8 bytes (i.e., one byte of a sign and an exponent 214 plus the 7-byte mantissa 211) at the head of the original Oracle-number is extracted to generate a number in the oraclenum64 format. In respect of a1 and a2, an arrangement is made to modify the first byte (i.e., the sign and the exponent 214) of the original Oracle-number. Specifically, the exponent E1 of a1 is set equal to E−14 where E is the exponent of the original Oracle-number for a radix of 10, thereby generating for a1 a sign and exponent part 215 being 1 byte long. The 7-byte mantissa 212 is attached to the sign and exponent part 215 to produce an oraclenum64 number corresponding to a1. The exponent E2 of a2 is set equal to E−28, thereby generating for a2 a sign and exponent part 216 being 1 byte long. The 6-byte mantissa 213 and one byte of “0” are attached to the sign and exponent part 216 to produce an oraclenum64 number corresponding to a2. The set of three oraclenum64 numbers generated in this manner will hereinafter be referred to as a triple-oraclenum64.
In the following, a description will be given of the configuration that performs the four arithmetic operations with respect to the triple-oraclenum64 format. A description will be first given of an arithmetic operation for obtaining an accurate sum of oraclenum64 numbers. The two-sum function described below is the same as formula (4.16) shown in Non-Patent Document 1. Similar methods are also disclosed on page 18 of Non-Patent Document 2 and disclosed as algorithm4 in Non-Patent Document 3.

two_sum(X,Y)

z=fl(X+Y)

w=fl(z−X)

v=fl(z−w)

z1=fl(Y−w)

z2=fl(v−X)

zz=fl(z1−z2)

return(z,zz)

Here, fl(X+Y) indicates the result obtained by mapping the true value of X+Y onto the floating-point number, i.e., the result obtained by expressing this value within the limited precision of the floating-point number. Two values z and zz obtained by the above-noted two_sum function accurately satisfies the following: z+zz=X+Y. Value z represents the most significant part of X+Y within the precision of the fixed-precision floating-point number format, and zz represents a remainder that is left unexpressed by the precision of the fixed-precision floating-point number format.

Attending to rounding that occurs at the time of mapping will be described by taking as an example the rounding to nearest with ties away from 0, which is an exemplary rounding method used in decimal numbers. For the sake of simplicity, the precision of a fixed-precision floating-point number is assumed to be two decimal digits. In this case, the sum of 20000 and −1 will be calculated by two_sum as follows.

X=20000

Y=−1

z=fl(X+Y)=20000

w=fl(z−X)=0

v=fl(z−w)=20000

z1=fl(Y−w)=−1

z2=fl(v−X)=0

zz=fl(z1−z2)=−1

As in this example, z and zz that are obtained as results of two_sum may have different signs. Rounding down may be used as a rounding operation. In such a case, z+zz may differ from X+Y as in the following example when significant digits do not overlap between X and Y.

X=20000

Y=−1

z=fl(X+Y)=19000

w=fl(z−X)=−1000

v=fl(z−w)=20000

z1=fl(Y−w)=990

z2=fl(v−X)=0

zz=fl(z1−z2)=990

In the following, it will be shown that the calculation of zz becomes easier by using a new rounding operation as described below, with an assumption that the absolute value of X is greater than or equal to the absolute value of Y:
(1) rounding to nearest with ties away from 0 is performed when there is no significant digit overlap between X an Y; and
(2) rounding down is performed when there is a significant digit overlap between X an Y.
When significant digits are consecutively arranged between X and Y, such a case is included in (2) even though no significant digit overlap is in existence between X and Y.
An advantage of using the above-noted rounding operation will be described by use of a specific example. As in the above-noted example, the precision of a fixed-precision floating-point number is assumed to be two decimal digits. The example to be described is directed to a case in which significant digits are consecutively arranged between X an Y. If fl performs rounding to nearest with ties away from 0 rather than the special rounding operation described above, calculation will become as follows.

X=2000

Y=52

z=2100

w=100

v=2000

z1=−48

z2=0

zz=−48

If the above-noted new rounding operation is used, calculation will become as follows.

X=2000

Y=52

z=2000

w=0

v=2000

z1=52

z2=0

zz=52

The use of the new rounding operation described above provides the following advantages. In the case of (1), the absence of overlap eliminates an arithmetic operation for rounding to nearest with ties away from 0 after the calculation of X+Y, and eliminates an arithmetic operation for obtaining z1 that would otherwise be necessary to correct w generated by rounding to nearest with ties away from 0. In the case of (2), the precision of X+Y is guaranteed to be no more than twice the number of significant digits, and only a rounding-down operation is used as rounding, which makes it easier to provide hardware for performing the arithmetic operation.
As described above, the computation of zz becomes easier by comparing the absolute values of two inputs, by classifying the case at hand to either (1) or (2) described above, and by performing the new rounding operation. Further, the provision of hardware circuits for performing get_z(x, y) and get_zz(x, y) to obtain z and zz, respectively, makes it possible to perform two_sum at high speed as described below.

two_sum_fast(x,y)

z=get_z(x,y)

zz=get_zz(x,y)

return(z,zz)

FIG. 15 is a drawing illustrating an example of the configuration of a circuit that performs the get_z arithmetic operation. The arithmetic device illustrated in FIG. 15 corresponds to part of the arithmetic circuit 119 illustrated in FIG. 2. The arithmetic device illustrated in FIG. 15 includes an input-X register 221, an input-Y register 222, internal-form-conversion circuits 223 and 224, an exponent-&-mantissa arithmetic circuit 225, selectors 226 and 227, shifters 228 and 229, an exponent adder 230, and an absolute-value adder 231. The arithmetic device further includes selectors 232 and 233, a normalization circuit 234, an external-form-conversion circuit 235, and an output-Z register 236. In FIG. 15, the same or corresponding elements as those in the circuit of FIG. 5 are referred to by the same or corresponding numerals. In FIG. 15, the inputs and the output are floating-point numbers having the same format and the same precision. The input data may not be normalized data. The output data is normalized. The input data is provided in the oraclenum64 format, and, also, unnormalized data are processable. The output data is output in the oraclenum64 format without exception.
Each of the internal-form-conversion circuits 223 and 224 divides an input into a sign, an exponent, and a mantissa, thereby converting the input value representation into an internal format. The sign, exponent, and mantissa of the input X are referred to as a sign-X, an exponent-X, and a mantissa-X, respectively. The sign, exponent, and mantissa of the input Y are referred to as a sign-Y, an exponent-Y, and a mantissa-Y, respectively.
The exponent-&-mantissa arithmetic circuit 225 receives the exponent-X and the exponent-Y as well as the mantissa-X and the mantissa-Y. The exponent-&-mantissa arithmetic circuit 225 compares the exponent-X and the exponent-Y in terms of their magnitudes. Based on the result of magnitude comparison, the exponent-&-mantissa arithmetic circuit 225 generates a select signal such that the mantissa (i.e., first mantissa) associated with the larger exponent is supplied to the shifter 228 and that the mantissa (i.e., second mantissa) associated with the smaller exponent is supplied to the shifter 229. The exponent-&-mantissa arithmetic circuit 225 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 225 outputs the latter as the amount of shift to the left for the shifter 228 if the former is larger. The exponent-&-mantissa arithmetic circuit 225 outputs the former as the amount of shift to the left for the shifter 228 if the former is smaller.
The exponent-&-mantissa arithmetic circuit 225 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 225 outputs the absolute value of a difference between the exponent-X and the exponent-Y minus the count indicative of the number of leading zeros in the first mantissa as the amount of shift to the right for the shifter 229 if the absolute value of a difference between the exponent-X and the exponent-Y is larger. The exponent-&-mantissa arithmetic circuit 225 outputs zero as the amount of shift to the right for the shifter 229 if the absolute value of a difference between the exponent-X and the exponent-Y is smaller. The exponent-&-mantissa arithmetic circuit 225 further outputs as an exponent the smaller exponent plus the above-noted amount of shift to the right.
The exponent-&-mantissa arithmetic circuit 225 further checks whether the absolute value of {(exponent-X−the count indicative of the number of leading zeros in mantissa-X)−(exponent-Y−the count indicative of the number of leading zeros in mantissa-Y)} is larger than or equal to 14. If this absolute value is larger than or equal to 14, the exponent-&-mantissa arithmetic circuit 225 generates a select signal such that the first mantissa and the exponent corresponding to the first mantissa are selected by the selectors 232 and 233, respectively. As a result, in the case of the absolute value being larger than or equal to 14, the first mantissa is supplied to the normalization circuit 234 by bypassing the shifter 228 and the absolute-value adder 231.
The shifter 228 shifts the supplied mantissa to the left according to the specified amount of shift. The shifter 229 shifts the supplied mantissa to the right according to the specified amount of shift. The results of shifts by these shifters are input into the absolute-value adder 231.
In the case of subtraction, one of the mantissas is inverted, and a carry is input into the absolute-value adder 231. When a digit overflow is detected as a result of addition performed by the absolute-value adder 231, a value that is shifted to the right by one digit is output. At the same time, a carry-out is supplied from the absolute-value adder 231 to the exponent adder 230, so that the supplied carry is added to the exponent.
When a digit underflow is detected as a result of addition performed by the absolute-value adder 231, a value that is shifted to the left by one digit is output. At the same time, a signal indicative of digit underflow is supplied from the absolute-value adder 231 to the exponent adder 230, so that subtraction is performed with respect to the exponent.
The selectors 232 and 233 select the exponent and mantissa of the addition result, respectively, or select the exponent corresponding to the first mantissa and the first mantissa, respectively, depending on the select signal generated by the exponent-&-mantissa arithmetic circuit 225. The normalization circuit 234 receives the exponent and the mantissa selected by the selectors 232 and 233, respectively, and outputs a normalized arithmetic result. The external-form-conversion circuit 235 converts the normalized arithmetic result into an external format for provision to the output-Z register 236.
FIG. 16 is a drawing illustrating a specific example of the get_z arithmetic operation. In FIG. 16, the exponent of the input X is denoted as Ex, the exponent of the input Y denoted as Ey, the count indicative of the number of leading zeros in the input X denoted as Lx, and the exponent of the output Z denoted as Ez. The arithmetic operation performed in this example is a get_z arithmetic operation 240 illustrated in FIG. 16. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost, which is impossible. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. To this end, Ex−Ey is calculated, the result of which is compared with Lx. In the example illustrated in FIG. 16, Lx is larger. Namely, the amount of shift Ex−Ey to left is within the number of leading zeros, so that only a mantissa 241 of the input X is shifted to the left. A mantissa 242 of the input Y is not shifted to the right. The amount of shift to the left is Ex−Ey, and the amount of shift to the right is 0.
The numbers whose digits are aligned as described above, i.e., a mantissa 243 obtained by shifting the mantissa 241 to the left and a mantissa 244 unchanged from the mantissa 242, are added together. Further, an addition result 245 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count indicative of the number of leading zeros is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the left by one digit may be necessary. With such a shift, however, the exponent changes from “0” to “1”, which is an odd number. The amount of shift to the left is thus decreased by “1” to become “0”, so that the exponent remains to be “0”. In this example, there is no change brought about by normalization. Further, upper-order digits of the normalized mantissa 246 are selected. When the number of digits in the mantissa of the input format is 14, 14 digits from the most significant digit are treated as the upper-order digits, with the remaining digits from the 15-th digit position and onwards being treated as lower-order digits,
As a result of the above-noted arithmetic operation, a mantissa 247 together with the corresponding exponent Ez are output as the result of an arithmetic operation.
FIG. 17 is a drawing illustrating another specific example of the get_z arithmetic operation. The arithmetic operation performed in this example is a get_z arithmetic operation 250 illustrated in FIG. 17. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost, which is impossible. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. To this end, Ex−Ey is calculated, the result of which is compared with Lx. In the example illustrated in FIG. 17, Lx is smaller. That is, the amount of shift Ex−Ey to the left is not within the number of leading zeros. Accordingly, an exponent 251 of the input X is shifted to the left by Lx, and an exponent 252 of the input Y is shifted to the right by the number of digits equal to a difference between the actual left shift amount and the contemplated left shift amount. In this case, the amount of shift to the left is Lx, and the amount of shift to the right is (Ex−Ey)−Lx.
The numbers whose digits are aligned as described above, i.e., a mantissa 253 obtained by shifting the mantissa 251 to the left and a mantissa 254 obtained by shifting the mantissa 252 to the right, are added together. In so doing, the digits that are overflowed by the shift to the right are kept. Further, an addition result 255 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the right by one digit may be necessary. With such a shift, however, the exponent Ez changes from “2” to “3”, which is an odd number. The amount of shift to the right is thus increased by “1” to make the exponent equal to “2”, so that the exponent changes from “2” to “4”. Further, upper-order digits of the normalized mantissa 256 obtained in this manner are selected. When the number of digits in the mantissa of the input format is 14, 14 digits from the most significant digit are treated as the upper-order digits, with the remaining digits from the 15-th digit position and onwards being treated as lower-order digits,
As a result of the above-noted arithmetic operation, a mantissa 257 is output as the result of the arithmetic operation, and the corresponding exponent Ez is output as the exponent of the result of the arithmetic operation.
FIG. 18 is a drawing illustrating yet another specific example of the get_z arithmetic operation. The arithmetic operation performed in this example is a get_z arithmetic operation 260 illustrated in FIG. 18. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. However, when there is a difference of 14 digits or more between the two inputs obtained by aligning digits, i.e., when there is no overlap at all, the result of an arithmetic operation can be obtained without actually performing the arithmetic operation. This is because the get_z arithmetic operation serves to obtain the upper-order digits of the result of an arithmetic operation. Namely, the one of the two inputs that has the greater value becomes the upper-order digit number when there is no overlap between the two inputs.
In order to check whether this condition is satisfied, (Ex−Lx)−(Ey−Ly) is calculated. Finding that the result of the calculation is 14 or larger warrants a determination that the above-noted condition is satisfied. A value of 14 is the number of digits of a mantissa in the format being used. In this example, the above-noted condition is satisfied, i.e., there is no overlap between a mantissa 261 and a mantissa 262. In this case, the mantissa 261 is passed to the next stage as the result of addition.
Further, a passed result 263 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count indicative of the number of leading zeros is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the left by three digits may be necessary. With such a shift, however, the exponent changes from “16” to “13”, which is an odd number. The amount of shift to the left is thus decreased by “1” to become “2”, so that the exponent changes from “16” to “14”. The result of normalization obtained in this manner is output as the result of an arithmetic operation inclusive of the mantissa 264 and the corresponding exponent Ez.
It may be noted that zz represents a remainder that is left unexpressed by the fixed precision during the addition operation. In the case of a multiplication operation, an instruction to output zz is generally used. In the case of multiplication, z and zz are values whose digits are consecutive to each other. In the case of addition, however, z and zz are not the values whose digits are consecutive to each other when the absolute values of two inputs are separated by more than the number of digits of the format being used. FIG. 19A illustrates an example of an addition operation in a case where the absolute values of two inputs are separated by more than the number of digits of the format being used. In this first case, z and zz may be output by performing an arithmetic operation according to the normal operation procedure. However, the absence of an overlap between these values may be utilized so that the one having the smaller absolute value can be used as zz without performing any arithmetic operation. This makes it possible to easily produce zz.
In the case of multiplication, further, the signs of the inputs do not affect the value of the mantissa in the result of an arithmetic operation. There is thus no need to consider the signs when obtaining zz. In the case of addition, however, a special operation may need to be performed in the addition of two numbers having different signs (i.e., the subtraction of two numbers having the same sign) when the absolute values of two inputs are separated as described above. FIG. 19B illustrates an example of addition of two values having different signs in a case where the absolute values of two inputs are separated from each other. In this second case, the use of the normal operation procedure results in a non-zero value being generated due to digit borrow as illustrated as “A” in FIG. 19B at the digits that are originally filled with zeros between the two inputs. As a result, the precision required to represent zz increases by a number equal to the number of digits between the two inputs. In this second case, therefore, an arrangement is made to output the one having the smaller absolute value as zz in the same manner as in the case 1 described above, without performing the arithmetic operation according to the normal operation procedure.
In this manner, the outputting of zz in the case of addition involves considering cases that are not considered in the case of multiplication. The circuit configuration in the case of addition is thus different from the zz output circuit used in the case of multiplication in that some modifications are made such as adding a bypass circuit.
FIG. 20 is a drawing illustrating an example of the configuration of a circuit that performs the get_zz arithmetic operation. The arithmetic device illustrated in FIG. 20 corresponds to part of the arithmetic circuit 119 illustrated in FIG. 2. In FIG. 20, the same or corresponding elements as those of FIG. 15 are referred to by the same or corresponding numerals. The arithmetic device illustrated in FIG. 20 includes an input-X register 221, an input-Y register 222, internal-form-conversion circuits 223 and 224, an exponent-&-mantissa arithmetic circuit 225A, selectors 226 and 227, shifters 228 and 229, an exponent adder 230, and an absolute-value adder 231. The arithmetic device further includes selectors 232 and 233, a normalization circuit 234, an external-form-conversion circuit 235, an output-Z register 236, and a normalization circuit 270.
Each of the internal-form-conversion circuits 223 and 224 divides an input into a sign, an exponent, and a mantissa, thereby converting the input value representation into an internal format. The sign, exponent, and mantissa of the input X are referred to as a sign-X, an exponent-X, and a mantissa-X, respectively. The sign, exponent, and mantissa of the input Y are referred to as a sign-Y, an exponent-Y, and a mantissa-Y, respectively.
The exponent-&-mantissa arithmetic circuit 225A receives the exponent-X and the exponent-Y as well as the mantissa-X and the mantissa-Y. The exponent-&-mantissa arithmetic circuit 225A compares the exponent-X and the exponent-Y in terms of their magnitudes. Based on the result of magnitude comparison, the exponent-&-mantissa arithmetic circuit 225A generates a select signal such that the mantissa (i.e., first mantissa) associated with the larger exponent is supplied to the shifter 228 and that the mantissa (i.e., second mantissa) associated with the smaller exponent is supplied to the shifter 229. The exponent-&-mantissa arithmetic circuit 225A compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 225A outputs the count indicative of the number of leading zeros in the first mantissa as the amount of shift to the left for the shifter 228 if the absolute value of a difference between the exponent-X and the exponent-Y is larger. The exponent-&-mantissa arithmetic circuit 225A outputs the absolute value of a difference between the exponent-X and the exponent-Y as the amount of shift to the left for the shifter 228 if the absolute value of a difference between the exponent-X and the exponent-Y is smaller.
The exponent-&-mantissa arithmetic circuit 225A compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 225A outputs the absolute value of a difference between the exponent-X and the exponent-Y minus the count indicative of the number of leading zeros in the first mantissa as the amount of shift to the right for the shifter 229 if the absolute value of a difference between the exponent-X and the exponent-Y is larger. The exponent-&-mantissa arithmetic circuit 225A outputs zero as the amount of shift to the right for the shifter 229 if the absolute value of a difference between the exponent-X and the exponent-Y is smaller. The exponent-&-mantissa arithmetic circuit 225A further outputs as an exponent the smaller exponent plus the above-noted amount of shift to the right.
The exponent-&-mantissa arithmetic circuit 225A further checks whether the absolute value of {(exponent-X−the count indicative of the number of leading zeros in mantissa-X)−(exponent-Y−the count indicative of the number of leading zeros in mantissa-Y)} is larger than or equal to 14. If this absolute value is larger than or equal to 14, the exponent-&-mantissa arithmetic circuit 225A generates a select signal such that the second mantissa and the exponent corresponding thereto are selected by the selectors 232 and 233, respectively. As a result, in the case of the absolute value of {(exponent-X−the count indicative of the number of leading zeros in mantissa-X)−(exponent-Y−the count indicative of the number of leading zeros in mantissa-Y)} is larger than or equal to 14 that is the number of digits of the format being used, the second mantissa is supplied to the normalization circuit 234 by bypassing the shifter 228 and the absolute-value adder 231.
The shifter 228 shifts the supplied mantissa to the left according to the specified amount of shift. The shifter 229 shifts the supplied mantissa to the right according to the specified amount of shift. The results of shifts by these shifters are input into the absolute-value adder 231.
In the case of subtraction, one of the mantissas is inverted, and a carry is input into the absolute-value adder 231. When a digit overflow is detected as a result of addition performed by the absolute-value adder 231, a value that is shifted to the right by one digit is output. At the same time, a carry is supplied from the absolute-value adder 231 to the exponent adder 230, so that the supplied carry is added to the exponent.
When a digit underflow is detected as a result of addition performed by the absolute-value adder 231, a value that is shifted to the left by one digit is output. At the same time, a signal indicative of digit underflow is supplied from the absolute-value adder 231 to the exponent adder 230, so that subtraction is performed with respect to the exponent.
The normalization circuit 270 receives the result of addition and the result of an exponent arithmetic, and outputs a normalized exponent and mantissa.
The selectors 232 and 233 select the normalized exponent and mantissa, respectively, or select the exponent corresponding to the second mantissa and the second mantissa, respectively, depending on the select signal generated by the exponent-&-mantissa arithmetic circuit 225A. The normalization circuit 234 receives the exponent and the mantissa selected by the selectors 232 and 233, respectively, and outputs a normalized arithmetic result. The external-form-conversion circuit 235 converts the normalized arithmetic result into an external format for provision to the output-Z register 236.
FIG. 21 is a drawing illustrating a specific example of the get_zz arithmetic operation. In FIG. 21, the exponent of the input X is denoted as Ex, the exponent of the input Y denoted as Ey, the count indicative of the number of leading zeros in the input X denoted as Lx, and the exponent of the output Z denoted as Ez. The arithmetic operation performed in this example is a get_zz arithmetic operation 280 illustrated in FIG. 21. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. To this end, Ex−Ey is calculated, the result of which is compared with Lx. In the example illustrated in FIG. 21, Lx is larger. Namely, the amount of shift Ex−Ey to left is within the number of leading zeros, so that only a mantissa 281 of the input X is shifted to the left. A mantissa 282 of the input Y is not shifted to the right. The amount of shift to the left is Ex−Ey, and the amount of shift to the right is 0.
The numbers whose digits are aligned as described above, i.e., a mantissa 281 obtained by shifting the mantissa 283 to the left and a mantissa 282 unchanged from the mantissa 284, are added together. Further, an addition result 285 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the left by one digit may be necessary. With such a shift, however, the exponent changes from “0” to “1”, which is an odd number. The amount of shift to the left is thus decreased by “1” to become “0”, so that the exponent remains to be “0”.
Subsequently, lower-order digits 286 of the normalized result are selected. When the number of digits in the mantissa of the input format is 14, digits from the most significant digit are treated as the upper-order digits, with the remaining digits from the 15-th digit position and onwards being treated as lower-order digits, With the selection of the lower-order digits, the exponent is decreased by 14. In this specific example, zeros are selected as the lower-order digits.
Further, selected lower-order digits 287 are normalized. Since the data of interest is zero in this example, the exponent and the mantissa existing prior to the normalization are output as they are, and serve as the result of normalization. The result of normalization is output as the result of an arithmetic operation inclusive of the mantissa 288 and the corresponding exponent Ez.
FIG. 22 is a drawing illustrating another specific example of the get_zz arithmetic operation. The arithmetic operation performed in this example is a get_zz arithmetic operation 290 illustrated in FIG. 22. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost, which is impossible. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. To this end, Ex−Ey is calculated, the result of which is compared with Lx. In the example illustrated in FIG. 22, Lx is smaller. That is, the amount of shift Ex−Ey to the left is not within the number of leading zeros. Accordingly, an exponent 291 of the input X is shifted to the left by Lx, and an exponent 292 of the input Y is shifted to the right by the number of digits equal to a difference between the actual left shift amount and the contemplated left shift amount. In this case, the amount of shift to the left is Lx, and the amount of shift to the right is (Ex−Ey)−Lx.
The numbers whose digits are aligned as described above, i.e., a mantissa 293 obtained by shifting the mantissa 291 to the left and a mantissa 294 obtained by shifting the mantissa 292 to the right, are added together. In so doing, the digits that are overflowed by the shift to the right are kept.
Further, an addition result 295 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the right by one digit may be necessary. With such a shift, however, the exponent Ez changes from “2” to “3”, which is an odd number. The amount of shift to the right is thus increased by “1” to make the exponent equal to “4”, so that the exponent changes from “2” to “4”.
Subsequently, lower-order digits 296 of the normalized result are selected. When the number of digits in the mantissa of the input format is 14, 14 digits from the most significant digit are treated as the upper-order digits, with the remaining digits from the 15-th digit position and onwards being treated as lower-order digits, With the selection of the lower-order digits, the exponent is decreased by 14.
Further, selected lower-order digits 297 are normalized. In this specific example, the mantissa is shifted to the left by two digits, and the exponent is changed from −10 to −12 through subtraction of 2. The result of normalization is output as the result of an arithmetic operation inclusive of the mantissa 298 and the corresponding exponent Ez.
FIG. 23 is a drawing illustrating yet another specific example of the get_zz arithmetic operation. The arithmetic operation performed in this example is a get_zz arithmetic operation 300 illustrated in FIG. 23. In order to maintain as high arithmetic precision as possible, a mantissa corresponding to the greater of the exponents is shifted to the left to align the digits. Shift to the left by the number of digits larger than the number of leading zeros results in the higher-order digits being lost. In consideration of this, when the amount of shift (Ex−Ey) to the left necessary to align the digits exceeds the number of leading zeros, the mantissa corresponding to the smaller of the exponents is shifted to the right to align the digits. However, when there is a difference of 14 digits or more between the two inputs obtained by aligning digits, i.e., when there is no overlap at all, the result of an arithmetic operation can be obtained without actually performing the arithmetic operation. This is because the get_zz arithmetic operation serves to obtain the lower-order digits of the result of an arithmetic operation. Namely, the one of the two inputs that has the smaller value becomes the lower-order digit number when there is no overlap between the two inputs.
In order to check whether this condition is satisfied, (Ex−Lx)−(Ey−Ly) is calculated. Finding that the result of the calculation is 14 or larger warrants a determination that the above-noted condition is satisfied. A value of 14 is the number of digits of a mantissa in the format being used. In this example, the above-noted condition is satisfied, i.e., there is no overlap between a mantissa 301 and a mantissa 302. In this case, the mantissa 302 is passed to the next stage as the result of addition.
Further, a passed result 303 is normalized. That is, the addition result is shifted to the left by the count indicative of the number of leading zeros when the count is 1 or more, and is shifted to the right when a digit overflow exists. The shift to the left may result in the exponent being an odd number. In such a case, the amount of shift to the left is decreased by one. The shift to the right may also result in the exponent being an odd number. In such a case, the amount of shift to the right is decreased by one. The exponent is adjusted in accordance with the amount of shift to the right or to the left. In the case of a shift to the left, the exponent is decreased by the amount of shift to the left. In the case of a shift to the right, the exponent is increased by the amount of shift to the right. In this specific example, a shift to the left by one digit may be necessary. With such a shift, however, the exponent changes from “0” to “−1”, which is an odd number. The amount of shift to the left is thus decreased by “1” to become “0”, so that the exponent remains to be “0”. The result of normalization obtained in this manner is output as the result of an arithmetic operation inclusive of the mantissa 304 and the corresponding exponent Ez.
FIG. 24 is a drawing illustrating a circuit-diagram symbol for two_sum. The two_sum operation described above is frequently used in the calculation of triple-oraclenum64 numbers. This two_sum operation, i.e., the operation for obtaining z and zz for representing an accurate sum of x and y, is represented by use of an operator 310 illustrated in FIG. 24.
FIG. 25 is a drawing illustrating an example of the circuit that obtains the sum of an oraclenum64 number and triple-oraclenum64 numbers. The circuit illustrated in FIG. 25 includes three two_sum operators 310 and a renormalization unit 311. An input b is a single oraclenum64 number. Inputs a0, a1, and a2 are triple-oraclenum64 numbers. Outputs s0, s1, and s2 are also triple-oraclenum64 numbers. The circuit configuration as illustrated in FIG. 25 can obtain the sum of an oraclenum64 number and triple-oraclenum64 numbers. The arithmetic operation performed by the renormalization unit 311 will be described later.
FIG. 26 is a drawing illustrating an algorithm for obtaining the sum of two sets of triple-oraclenum64 numbers. The two_sum described above may be used to obtain the sum of two sets of triple-oraclenum64 numbers. Triple_Add illustrated in FIG. 26 obtains the sum of first triple-oraclenum64 numbers a0, a1, and a2 and second triple-oraclenum64 numbers b0, b1, and b2. This arithmetic operation is the same as the one that is disclosed in section A.2 of Non-Patent Document 2. Further, this is also similar to algorithm13 and 14 disclosed in Non-Patent Document 3. The remaining ones of the four arithmetic operations can also be implemented by use of the methods disclosed in Non-Patent Documents 1 through 3.
In the following, renormalization of the result of calculation will be described. Renormalization is described on page 116 of Non-Patent Document 2, which is also referred to by Non-Patent Document 3. Here, renormalization disclosed in Non-Patent Document 2 is referred to as Priest's renormalization. The results (X0, X1, X2, X3) of an arithmetic operation prior to renormalization are generally arranged in the descending order of their absolute values. However, some digits may overlap. Further, X0 does not represent, in the fixed precision, the most significant part of X0+X1+X2+X3.
The results of renormalization performed on the arithmetic results (X0, X1, X2, X3) by use of Priest's renormalization are referred to as (a0, a1, a2) In this case, a0+a1+a2 is equal to X0+X1+X2+X3 within the range of triple, and also satisfies the following conditions.

|a0|≧|a1|≧|a2|

E(i+1)≦Ei−14

Here, Ei is the exponent of the element ai when the radix is 10. Since the precision of oraclenum64 is decimal digits, the second condition denoted above requires that the elements do not overlap.

The two_sum operation previously described is used in Priest's renormalization. In order to remove overlaps, the three two_sum operators 310 connected as illustrated in FIG. 27 are used. Results (t0, t1, t2, t3) obtained in this manner do not overlap, and are arranged in the descending order of their absolute values, except for an element that is zero. These obtained results may be successively accumulated with t0 first, thereby producing (a0, a1, a2). The two_sum operation is also used in the arithmetic operation “accumulate” for performing such successive accumulation.
In general, the results (X0, X1, X2, X3) of an arithmetic operation prior to renormalization do not have the same sign, so that (a0, a1, a2) obtained by successively performing two_sum as previously described do not have the same sign. In consideration of this, when all the numbers included in a set of fixed-precision floating-point numbers obtained by Priest's renormalization have the same sign, such renormalization is referred to as sign-matched Priest's renormalization.
In the present disclosures, stronger normalization that imposes stricter conditions than Priest's renormalization imposes is used. Strongly normalized set (b0, b1, b2) satisfies the following conditions. It may be noted that strong renormalization may create a case in which b1=0 and b2≠0, so that a condition of |b0|≧|b1|≧|b2| may not be generally satisfied.

E(i+1)=Ei−14

b0, b1, b2: all have the same sign.
The use of such strong normalization ensures that a difference between the exponents of elements is equal to the precision (i.e., the number of digits) of fixed-point numbers. Because of this, it is easy to convert a strongly normalized set into the original Oracle-number expression.
In the following, a description will be first given of a process of obtaining a number set (a0, a1, a2, normalized by sign-matched Priest's renormalization from a number set (t0, t1, t2, . . . ) illustrated in FIG. 27 that do not overlap and that are arranged in the descending order of their absolute values, except for zeros. Here, fl_truncate(X+Y) indicates an arithmetic operation that truncates the sum of fixed-precision floating-point numbers X and Y to the significant digits thereof. Further, two_sum(X, Y) indicates an arithmetic operation that obtains two numbers whose significant digits do not overlap as was previously described. In the example described here, all the arithmetic operations performed in the two_sum operation are subjected to rounding to nearest with ties away from 0.

a0=fl_truncate(t0+t1)

(z0,zz0)=two_sum(t0,t1)

In the case of zz0=0, t2, t3, . . . are successively accumulated until zz0≠0 is satisfied as described below.

a0=fl_truncate(a0+t2)

(z0,zz0)=two_sum(a0,t2)

If zz0 is still equal to 0 even after accumulating all the numbers of the input number set (t0, t1, t2, . . . ), the procedure comes to an end. In the following, the procedure performed after zz0≠0 is satisfied will be described.

w0=fl_truncate(z0−a0)

Due to the characteristics of the two_sum operation, the following is satisfied.

t0+t0+ . . . +ti=z0+zz0=a0+w0+zz0

Here, t0, t1, . . . , ti are numbers accumulated until zz0≠0 is satisfied. In this case, a0 and (w0+zz0) are the same sign, and do not have any overlapping significant digit as described in the following.

(i) In the case of w0=0, zz0 is equal to the value discarded by fl_truncate(t0+t1+ . . . +ti), so that a0 and (w0+zz0) are the same sign, and do not have any overlapping significant digit.
(ii) In the case of w0≠0, w0 is a number that is the same sign as a0 and that has “1” only at the least significant one of the significant digits of a0. Further, the absolute value of zz0 is smaller than the absolute value of w0, and zz0 and w0 have different signs. Accordingly, a0 and (w0+zz0) are the same sign, and do not have any overlapping significant digit.
Subsequently, the same procedure is performed with respect to (w0, zz0, ti+1, . . . ), thereby successively obtaining a1, a2, and so on. By doing so, a1, a2, . . . will have the same sign as described in above-noted (i) and (ii).
FIG. 28 is a drawing illustrating an example of sign-matched Priest's renormalization. In this example, a fixed-precision floating-point number is comprised of 4 decimal digits for the sake of simplicity, and rounding used in the two_sum operation is rounding to nearest with ties away from 0. In FIG. 28, (t0, t1, t2, t3) do not overlap, and are arranged in the descending order of their absolute values, except for an element that is zero. These values may be successively accumulated with t0 first, thereby producing a number set (a0, a1, a2) normalized by sign-matched Priest's renormalization.
FIG. 29 is a drawing illustrating an example of calculation that produces a set of strongly-normalized numbers from the number set normalized by sign-matched Priest's renormalization. In order to achieve strong normalization, it suffices to properly quantize the second and subsequent elements in the number set normalized by sign-matched Priest's renormalization. In the example illustrated in FIG. 29, a fixed-precision floating-point number is comprised of 4 decimal digits for the sake of simplicity. As illustrated in FIG. 29, a set of strongly-normalized numbers (b0, b1, b2) is obtained from the number set (a0, a1, a2) normalized by sign-matched Priest's renormalization.
FIG. 30 is a drawing illustrating an example of the configuration of circuit that performs a scale_next(X, Y) operation for performing quantization. The circuit illustrated in FIG. 30 includes an exponent-correcting-value register 320, an input-X register 321, an input-Y register 322, internal-form-conversion circuits 323 and 324, an exponent adder 325, a shift-amount calculating circuit 326, a right shifter 327, an external-form-conversion circuit 328, and an output-Z register 329. Data input into this circuit is assumed to satisfy the following condition.

Exponent-Y≧Exponent-X+t 

Here, t is the number of digits of an oraclenum64 number. In this example, t may be 14.

Each of the internal-form-conversion circuits 323 and 324 divides an input into an exponent and a mantissa, thereby converting the input value representation into an internal format. The exponent and mantissa of the input X are referred to as an exponent-X and a mantissa-X, respectively. The exponent of the input Y is denoted as an exponent-Y. The exponent-correcting-value register 320 stores therein a predetermined fixed value t. Instead of using a register for storing this value, this fixed value may be set by use of wired logic.
The shift-amount calculating circuit 326 receives the exponent-X, the exponent-Y, and the fixed value t. The shift-amount calculating circuit 326 outputs the result of the arithmetic operation “exponent-Y−exponent-X−t” as the amount of shift. The right shifter 327 receives the amount of shift and the mantissa-X to output the value obtained by shifting the mantissa-X to the right by the amount of shift. Shifted-out digits are discarded.
The exponent adder 325 receives the exponent-Y and the fixed value t to output the result of the arithmetic operation “exponent-Y−t”. The external-form-conversion circuit 328 converts the exponent from the exponent adder 325 and the mantissa from the right shifter 327 into an external format for provision to the output-Z register 329.
FIG. 31 is a drawing illustrating an example of the configuration of a circuit that performs the scale_next operation without converting a mantissa into the internal format. In FIG. 31, the same elements as those of FIG. 30 are referred to by the same or similar numerals, and a description thereof will be omitted as appropriate. In FIG. 31, an internal-form-conversion circuit 323A is provided in place of the internal-form-conversion circuit 323, and an external-form-conversion circuit 328A is provided in place of the external-form-conversion circuit 328. In the case of the input data being provided in the oraclenum64 format, the right shifter performs an 8-bit shift for a shift amount of 2.
Each of the internal-form-conversion circuits 323A and 324 divides an input into an exponent and a mantissa, thereby converting the input value representation into an internal format. The exponent of the input X is denoted as an exponent-X. The exponent of the input Y is denoted as an exponent-Y. The exponent-correcting-value register 320 stores therein a predetermined fixed value t. Instead of using a register for storing this value, this fixed value may be set by use of wired logic.
The shift-amount calculating circuit 326 receives the exponent-X, the exponent-Y, and the fixed value t. The shift-amount calculating circuit 326 outputs the result of the arithmetic operation “exponent-Y−exponent-X−t” as the amount of shift. The right shifter 327 receives the amount of shift and the mantissa-X to output the value obtained by shifting the mantissa-X to the right by the amount of shift. Shifted-out digits are discarded.
The exponent adder 325 receives the exponent-Y and the fixed value t to output the result of the arithmetic operation “exponent-Y−t”. The external-form-conversion circuit 328A converts the exponent from the exponent adder 325 into an external format for provision to the output-Z register 329. The left-shifted mantissa output from the right shifter 327 is supplied to the output-Z register 329 as it is.
In the following, a description will be given of rounding of the triple-oraclenum64 format. A set of triple-oraclenum64 numbers (a0, a1, a2) that is subjected to rounding is assumed to be renormalized. In this assumption, renormalization may be either sign-matched Priest's renormalization or the strong renormalization.
FIG. 32 is a drawing illustrating a table demonstrating three types of methods for specifying NUMBER-type precision in the Oracle-Database (registered trademark). Here, rounding to nearest with ties away from 0 is used. In the case of the result of calculation being 1234.56, for example, the result of rounding for NUMBER(4), NUMBER(4, −2), and NUMBER(4, 1) will be as follows.

NUMBER(4)→1234

NUMBER(4,−2)→1200

NUMBER(4,1)→error

The reason why the last example produces an error is because “1234.6” that is obtained by rounding the result of calculation “1234.56” off to one decimal place is not accommodated within the limit of the 4-digit precision.

In respect of the triple-oraclenum64 format, it is preferable to achieve such a rounding operation in an efficient manner and with the provision of error detection. An arithmetic device that can be used here is the type that receives operands each having a length of 8 bytes as two inputs, and that outputs a single value having a length of 8 bytes. Such an arithmetic device is assumed to be used in the rounding operation.
An arrangement is made to implement the rounding operation for triple-oraclenum64 numbers by use of three steps as follows:
1) generating a number that has “5” only at the digit position at which rounding occurs;
2) adding the generated number to triple-oraclenum64 numbers; and
3) truncating the results at a proper digit position.
A description will be first given of an algorithm for the first step. In the following, triple-oraclenum64 numbers to be rounded are referred to as (a0, a1, a2). The value of a0 that is the most significant element of this set is expressed as follows.

a0=M*100^e 

Further, the arithmetic operation for obtaining “e” in the above equation is referred to as e(a0).

The first step will be implemented by use of different algorithms depending on the method of specifying precision. When no indication of precision is given (i.e., in the case of using NUMBER without any argument), rounding is performed such that the mantissa becomes 20 bytes or fewer.
In order to describe an algorithm in the case of precision being specified, it is convenient to express a0 by use of decimal expression. The following form is thus used.

a0=M′*10^e′

It is further assumed that the mantissa is normalized such that 10>|M′|≧0 is satisfied. Further, the arithmetic operation for obtaining e′ is referred to as e′(a0).

FIGS. 33A through 33C are drawings illustrating examples of algorithms that generate a number having “5” only at the digit position at which rounding occurs. FIGS. 33A and 33B illustrate algorithms used when precision is specified by referring to decimal digit numbers. These two algorithms can each be implemented as an arithmetic device having two inputs and one output. FIG. 33C illustrates an algorithm used when a scale (i.e., relative position from the decimal point) is specified. Since a relative position from the decimal point is specified, this calculation is independent of a0. The algorithm illustrated in FIG. 33C may be implemented by creating a program calculating the mathematical formula illustrated in FIG. 42C. Alternatively, a table in which various values of “n” and values associated to these values are stored may be provided, and a program may be created to refer to the table by use of “n” as a key.
Whether a rounded result can be accommodated within the specified precision when a scale is specified can be determined as follows. In the case of NUMBER(p, s) being specified, two values p_d and p_s are calculated by using the above-noted algorithm for specified precision and the algorithm for a specified scale.

p—d=get_comma5(a0,digits=p)

p—s=get_comma5(scale=s)

When |p_d|>|p_s| is satisfied, i.e., when the absolute value of p_d is larger than the absolute value of p_s, this satisfied condition indicates that precision is not sufficient to express the result rounded by the specified scale.

In the second step, the number generated by the first step is added to the triple-oraclenum64 numbers. The algorithm illustrated FIG. 25 may be used to implement this addition.
In the third step, truncation is performed at a proper position. In this case again, the number “p” (which is either “p_d” or “p_s” corresponding to the respective precision specified cases) generated by the first step may be utilized. This number “p” is the number added for the purpose of rounding, and thus contains information indicative of the position of the digit at which rounding occurs. Accordingly, the arithmetic operation for truncating each element of the triple-oraclenum64 numbers can be defined by use of the element and “p” as two operand inputs.
With (b0, b1, b2) denoting the results of calculation in the second step, the results of rounding (c0, c1, c2) obtained in the third step are expressed as follows.

c0=truncate(b0,p)

c1=truncate(b1,p)

c2=truncate(b2,p)

FIG. 34 is a drawing showing an example of the configuration of a circuit that implements get_comma5(precision). The circuit illustrated in FIG. 34 includes a precision-p register 330, an input-X register 331, an internal-form-conversion circuit 332, a leading zero counting circuit 333, an exponent adder 334, registers 335 and 336, a selector 337, an external-form-conversion circuit 338, and an output-Z register 339.
The internal-form-conversion circuit 332 divides an input into an exponent and a mantissa, thereby converting the input value representation into an internal format. The exponent and mantissa of the input X are referred to as an exponent-X and a mantissa-X, respectively. The leading zero counting circuit 333 receives the mantissa-X, and counts the number of leading zeros in the mantissa-X. The exponent adder 334 calculates “exponent-X+1−precision-p−the count indicative of the number of leading zeros”. The selector 337 selects 5000-00 of the register 336 in the case of an output of the exponent adder 334 being an odd number, and selects 0500-00 of the register 335 in the case of an output of the exponent adder 334 being an even number.
The exponent supplied from the exponent adder 334 to the external-form-conversion circuit 338 has the most significant bit thereof being changed to zero in the case of an output of the exponent adder 334 being an odd number, and has the most significant bit thereof unchanged in the case of an output of the exponent adder 334 being an even number. The external-form-conversion circuit 338 converts the sign from the internal-form-conversion circuit 332, the exponent from the exponent adder 334, and the mantissa from the selector 337 into an external-format number for provision to the output-Z register 339.
FIG. 35 is a drawing illustrating an example of the configuration of a circuit that performs a truncate operation. The circuit illustrated in FIG. 35 includes an input-X register 340, a comma-5 register 341, internal-form-conversion circuits 342 and 343, a leading zero counting circuit 344, a mask-value generating circuit 345, a mask circuit 346, an external-form-conversion circuit 347, and an output-Z register 348.
Each of the internal-form-conversion circuits 342 and 343 divides an input into a sign, an exponent, and a mantissa, thereby converting the input value representation into an internal format. The sign, exponent, and mantissa of the input X are referred to as a sign-X, an exponent-X, and a mantissa-X, respectively. The exponent and mantissa of the input “comma5” are referred to as an exponent-c and a mantissa-c, respectively.
The leading zero counting circuit 344 receives the mantissa-c, and counts the number of leading zeros in the mantissa-c. The mask-value generating circuit 345 receives as inputs the exponent-X, the exponent-c, and the count indicative of the number of leading zeros in the mantissa-c, and generates mask data in response to these inputs. The mask circuit 346 masks the mantissa-X according to the mask data, thereby outputting a masked value as a mantissa. The external-form-conversion circuit 347 converts the sign-X and the exponent-X from the internal-form-conversion circuit 342 and the mantissa from the mask circuit 346 into an external-format number for provision to the output-Z register 348.
FIG. 36 is a drawing illustrating an example of the configuration of the mask-value generating circuit. The mask-value generating circuit 345 illustrated in FIG. 36 includes a mask-digit calculating circuit 350, a decoder 351, and selectors 352-1 through 352-14. The mask-digit calculating circuit 350 receives the exponent-X, the exponent-c, and a count Lc indicative of the number of leading zeros, and calculates a mask digit. The mask digit is obtained as 14+(exponent-c−Lc)−exponent-X when the number of digits in the mantissa of the format being used is 14. This value indicates how many digits from the least significant digit of the mask data are set to zero. Based on the result of mask-digit calculation, the decoder 351 generates digit-specific select signals The select signals generated by the decoder 351 are supplied to the selectors 352-1 through 352-14 corresponding to the respective 14 digits. In the case of the result of mask-digit calculation being “n”, the select signals are generated such that “0000” is selected for the “n” digits from the least significant digit. In the case of the result of mask-digit calculation being “0”, the select signals are generated such that “1111” is selected for all the digits. In response to the select signals, the selectors 352-1 through 352-14 select either a “1111” bit string or a “0000” bit string, and outputs the selected bit string as the mask data.
FIG. 37 is a drawing illustrating an example of the configuration of an absolute-value-comparison error check circuit for p_d and p_s. This circuit includes a p_s register 360, a p_d register 361, internal-form-conversion circuit 362 and 363, an exponent-&-mantissa arithmetic circuit 364, selectors 365 and 366, shifters 367 and 368, a sign arithmetic unit 369, an adder 370, a complementer 372, a complementer 373, and an error-flag register 371.
Each of the internal-form-conversion circuits 362 and 363 divides an input into an exponent and a mantissa, thereby converting the input value representation into an internal format. The exponent and mantissa of the input “p_s” are referred to as an exponent-X and a mantissa-X, respectively. The exponent and mantissa of the input “p_d” are referred to as an exponent-Y and a mantissa-Y, respectively.
The exponent-&-mantissa arithmetic circuit 364 receives the exponent-X and the exponent-Y as well as the mantissa-X and the mantissa-Y. The exponent-&-mantissa arithmetic circuit 364 compares the exponent-X and the exponent-Y in terms of their magnitudes. Based on the result of magnitude comparison, the exponent-&-mantissa arithmetic circuit 364 generates a select signal such that the mantissa (i.e., first mantissa) associated with the larger exponent is supplied to the shifter 367 and that the mantissa (i.e., second mantissa) associated with the smaller exponent is supplied to the shifter 368. The exponent-&-mantissa arithmetic circuit 364 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 364 outputs the latter as the amount of shift to the left for the shifter 367 if the former is larger. The exponent-&-mantissa arithmetic circuit 364 outputs the former as the amount of shift to the left for the shifter 367 if the former is smaller.
The exponent-&-mantissa arithmetic circuit 364 compares the absolute value of a difference between the exponent-X and the exponent-Y with a count indicative of the number of leading zeros in the first mantissa. The exponent-&-mantissa arithmetic circuit 364 outputs a value obtained by subtracting the latter from the former as the amount of shift to the right for the shifter 368 if the former is larger. The exponent-&-mantissa arithmetic circuit 364 outputs zero as the amount of shift to the right for the shifter 368 if the former is smaller.
The shifter 367 shifts the supplied mantissa to the left according to the specified amount of shift. The shifter 368 shifts the supplied mantissa to the right according to the specified amount of shift. The results of shifts by these shifters are input into the complementer 372 and the complementer 373.
The complementer 372 is an EOR circuit (i.e., exclusive-OR circuit) that compliments the supplied output of the left shifter by use of an inverse signal of the supplied select signal. The complementer 373 is an EOR circuit (i.e., exclusive-OR circuit) that compliments the supplied output of the right shifter by use of an inverse signal of the supplied select signal. The outputs of these complementers are input into the adder 370.
A carry is input into the adder 370, When a digit overflow is detected as a result of addition performed by the adder 370, a carry-out is supplied to the sign arithmetic unit 369.
The sign arithmetic unit 369 receives the carry-out from the adder 370. The sign arithmetic unit 369 sets an error flag equal to “1” when an inverse of the carry-out is “1”. Otherwise the error flag is zero. The error flag generated by the sign arithmetic unit 369 is stored in the error-flag register 371.
FIGS. 38A and 38B are drawings illustrating an example of the circuit configuration of the sign arithmetic unit. The sign arithmetic unit illustrated in FIG. 38A includes an inverter 380. This circuit sets the error flag to “1” when an inverse of the carry-out is “1”. The table illustrated in FIG. 38B indicates the magnitude relationships between the absolute values of “p_d” and “p_s” supplied to the absolute-value-comparison error check circuit for “p_d” and “p_s”, and indicates two output signals associated thereto. Here, |p_d| is the absolute value of “p_d”. The same applies in the case of “p_s”.
FIG. 39 is a drawing illustrating an example of the arithmetic operation for obtaining the length of an Oracle-number. The length of an Oracle-number is obtained in order to store triple-oraclenum64 numbers, obtained as the results of the arithmetic operation previously described, as a single Oracle-number in memory. The procedure illustrated in FIG. 39 is performed to obtain the length of an Oracle-number. Here, length(aX) is an arithmetic operation that obtains the number of significant digits in an oraclenum64 number. The length of the Oracle-number becomes shorter when a2 is zero, and becomes even shorter when a1 is also zero.
In the case of strongly-renormalized triple-oraclenum64 numbers, a0, a1, and a2 represents respective parts that are 7 bytes, 7 bytes, and 6 bytes, respectively, into which the mantissa of an Oracle-number is divided. Basically, the mantissas of a0, a1, and a2 may simply be catenated together to obtain an Oracle-number. It may be noted, however, that when the lengths of a0 and a1 are shorter, an arrangement is made to store these values in memory after adding trailing zeros.
FIG. 40 is a drawing illustrating an example of the configuration of a circuit that performs an expand operation. The circuit illustrated in FIG. 40 includes an input-X register 390, an internal-form-conversion circuit 391, a trailing zero counting circuit 392, a digit-select calculating circuit 393, and an output-Z register 394.
The internal-form-conversion circuit 391 receives the mantissa of the value stored in the input-X register 390, and outputs a mantissa in the internal format. The trailing zero counting circuit 392 receives the mantissa in the internal format, and obtains the number of trailing zeros from the received mantissa, followed by outputting digit select data based on the counted number of trailing zeros. The digit-select calculating circuit 393 receives the sign and mantissa of the value stored in the input-X register 390, and also receives the digit select data from the trailing zero counting circuit 392. Based on the sign and the digit select data, the digit-select calculating circuit 393 selects, on a digit-by-digit basis, either the received mantissa or zero in the oraclenum64 representation (i.e., 0×01 or 0×65) for outputting. Zero is selected and output at a digit position that is determined by the trailing zero counting circuit 392 as the position of a trailing zero. The output data is stored in the output-Z register 394.
FIGS. 41A and 41B are drawings illustrating an example of the configuration of the trailing zero counting circuit. As illustrated in FIG. 41A, the trailing zero counting circuit includes a conversion circuit 400. The conversion circuit 400 receives the mantissa as input data, and generates output data from the input data in accordance with the table illustrated in FIG. 41B. This output data is the count indicative of the number of trailing zeros, and represents the count by a binary number. In the table, the symbol “X” at the rightmost position indicates a non-zero value, and other Xs indicate a “don't care” value. 0s are the zeros that are subjected to counting.
FIG. 42 is a drawing illustrating an example of the configuration of the digit-select calculating circuit. The digit-select calculating circuit 393 illustrated in FIG. 42 includes a decoder 410 and selectors 411-1 through 411-14. The decoder 410 receives the sign and the count indicative of the number of trailing zeros, and generates a digit select signal on a digit-specific basis. The select signals generated by the decoder 410 are supplied to the selectors 411-1 through 411-14 corresponding to the respective 14 digits, for example. In the case of the count indicative of the number of trailing zeros being “n”, the select signals are generated such that “0×01” or “0×65” is selected for the “n” digits from the least significant digit. When the sign is “1” indicative of positive, “0×01” is selected. When the sign is “0” indicative of negative, “0×65” is selected. The supplied mantissa is selected as it is for the n+10-th and higher-order digits as counted from the least significant digit. The supplied mantissa, 0×01, or 0×65 selected on a digit-by-digit basis is output as mantissa data.
FIG. 43 is a drawing illustrating an example of the configuration of a fixed-precision floating-point number adding and subtracting unit. The fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 43 is obtained by adding various arithmetic functions described heretofore to the fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 5, and corresponds to part of the arithmetic circuit 119 illustrated in FIG. 2. The arithmetic operations added herein include the get_z operation illustrated in FIG. 15, the get_zz operation illustrated in FIG. 20, the scale_next operation illustrated in FIG. 30, the get_comma5 operation illustrated in FIG. 34, the truncate operation illustrated in FIG. 35, the error check operation illustrated in FIG. 37, and the expand operation illustrated in FIG. 40. The fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 43 includes a value register 450, an input-X register 451, an input-Y register 452, internal-form-conversion circuits 453 and 454, a trailing zero counting circuit 455, an exponent-&-mantissa mask calculating circuit 456, and selectors 457 and 458. The fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 43 further includes shifters 459 and 460, a digit-select calculating circuit 461, a mask circuit 462, an exponent-&-mantissa arithmetic circuit 463, an absolute-value adder 464, a normalization circuit 465, selectors 466 through 468, and a normalization circuit 469. The fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 43 further includes selectors 470 and 471, a rounding circuit 472, an external-form-conversion circuit 473, selectors 474 and 475, an error flag register 476, and an output-Z register 477.
Parts of the fixed-precision floating-point number adding and subtracting unit illustrated in FIG. 43 correspond to respective corresponding parts of the arithmetic devices described heretofore. For example, the trailing zero counting circuit 455 and the digit-select calculating circuit 461 correspond to the trailing zero counting circuit 392 and the digit-select calculating circuit 393, respectively, illustrated in FIG. 40. Also, for example, the mask circuit 462 corresponds to the mask circuit 346 of the truncate arithmetic operation illustrated in FIG. 35. Further, for example, the normalization circuit 465 corresponds to the normalization circuit 270 of the get_zz arithmetic operation illustrated in FIG. 20. For example, the normalization circuit 469 corresponds to the normalization circuit 234 of the get_z arithmetic operation illustrated in FIG. 15 and the get_zz arithmetic operation illustrated in FIG. 20. Further, for example, the selector 468 corresponds to the selector 337 of the get_comma5 arithmetic operation illustrated in FIG. 34. The exponent-&-mantissa mask calculating circuit 456 and the exponent-&-mantissa arithmetic circuit 463 correspond to a circuit unit obtained by putting together corresponding circuit units of the relevant arithmetic circuits. The operations of circuits noted above are the same as or similar to the operations of the respective corresponding circuits of the arithmetic operation circuits described heretofore. It may be noted that there is an arithmetic operation that does not need to be processed by the rounding circuit 472. In this implementation example, however, all the results of arithmetic operations are supplied to the rounding circuit 472 for the sake of reducing the number of selectors. The rounding mode may be set to zero for an arithmetic operation for which no rounding is necessary, thereby producing the same outcome as when the rounding circuit 472 is not used.
FIG. 44 is a drawing illustrating an example of the configuration of the exponent-&-mantissa mask calculating circuit. The exponent-&-mantissa mask calculating circuit 456 illustrated in FIG. 44 includes a comparison circuit 480, an absolute-value adder 481, selectors 482 through 486, leading zero counting circuit 487 and 488, adders 491 and 492, and a mask generating circuit 493. The exponent-&-mantissa mask calculating circuit 456 illustrated in FIG. 44 further includes selectors 494 through 496 and an adder 497.
The comparison circuit 480, the absolute-value adder 481, the leading zero counting circuit 487, the selectors 494 and 496, and the adder 492 correspond to the comparison circuit 151, the absolute-value adder 152, the leading zero counting circuit 156, the selectors 158 and 157, and the adder 155, respectively, illustrated in FIG. 6. It may be noted, however, that the absolute-value adder 481 receives a value responsive to an arithmetic operation of interest in addition to the exponent-X and the exponent-Y, and performs a relevant addition and subtraction operation. The absolute-value adder 481 calculates an absolute value of a difference between the exponent-X and the exponent-Y in the case of the get_z arithmetic operation or the get_zz arithmetic operation. In the case of the scale_next arithmetic operation, the absolute-value adder 481 outputs the result of the arithmetic operation “exponent-Y−exponent-X−t (i.e., number of digits of oraclenum64)” as the amount of shift. The adder 491 performs the same function as the mask-digit calculating circuit 350 of FIG. 36 performs, and receives the exponent-X, the exponent-c (i.e., the exponent of comma5), and a count Lc indicative of the number of leading zeros to calculate a mask digit. The mask generating circuit 493 corresponding to the decoder 351 and the selectors 352-1 through 352-14 illustrated in FIG. 36 generates mask data in response to the above-noted mask digit. The adder 491 further performs the same function as the exponent-&-mantissa arithmetic circuit of FIG. 15 or FIG. 20 performs, thereby checking whether the absolute value of {(exponent-X —the count indicative of the number of leading zeros in mantissa-X)−(exponent-Y—the count indicative of the number of leading zeros in mantissa-Y)} is larger than or equal to 14. When this absolute value is 14 or larger, the adder 491 generates a bypass select signal for selecting a bypass route.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

## Claims

1. An arithmetic circuit for calculating a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision, the arithmetic circuit comprising:
a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa;
a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa;
a first shift-amount generating unit to generate as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
a second shift-amount generating unit to generate as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
a first shift unit to calculate a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount;
a second shift unit to calculate a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount;
an add-subtract unit to produce a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and to produce a carry obtained by the add-subtract operation performed with respect to the first post-shift mantissa and the second post-shift mantissa;
a post-shift-exponent generating unit to calculate a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa;
a corrected-exponent generating unit to generate a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry;
a first normalization unit to calculate a normalized exponent that is obtained by normalizing the corrected exponent, and to calculate a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and
a second normalization unit to calculate a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, equal in length to the number of digits of the predetermined fixed precision, of the normalized double-precision mantissa, and to calculate a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.

2. The arithmetic circuit as claimed in claim 1, further comprising:
a first conversion unit to convert the first pre-add-subtract-operation data stored in the first input register into the first pre-add-subtract-operation data in a second format; and
a second conversion unit to convert the second pre-add-subtract-operation data stored in the second input register into the second pre-add-subtract-operation data in the second format,
wherein the first shift-amount generating unit generates as the first shift amount a first leading-zero count obtained by counting consecutive zeros from the most significant bit of the first mantissa in the second format upon detecting that an absolute value of a difference between the first exponent in the second format and the second exponent in the second format is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent in the second format and the second exponent in the second format upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is smaller than the first leading-zero count;
the second shift-amount generating unit generates as the second shift amount a value obtained by subtracting the second exponent in the second format and the first leading-zero count from the first exponent in the second format upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is smaller than the first leading-zero count; and
the post-shift-exponent generating unit calculates a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa in the second format or the second mantissa in the second format.

3. The arithmetic circuit as claimed in claim 2, wherein the second format is a binary-coded decimal format.

4. The arithmetic circuit as claimed in claim 2, further comprising a third conversion unit to convert the lower-order normalized mantissa into a lower-order normalized mantissa in the first format, and to convert the lower-order normalized exponent into a lower-order normalized exponent in the first format.

5. The arithmetic circuit as claimed in claim 1, further comprising:
a comparison circuit to make a comparison between the exponent of the first pre-add-subtract-operation data and the exponent of the second pre-add-subtract-operation data;
a first selection unit to select, as data to be shifted by the first shift unit, the mantissa of one of the first pre-add-subtract-operation data and the second pre-add-subtract-operation data, whichever has a larger exponent, based on a result of the comparison made by the comparison circuit; and
a second selection unit to select, as data to be shifted by the second shift unit, the mantissa of one of the first pre-add-subtract-operation data and the second pre-add-subtract-operation data, whichever has a smaller exponent, based on the result of the comparison made by the comparison circuit,
wherein there is no need for the first condition to be satisfied.

6. The arithmetic circuit as claimed in claim 1, wherein the post-shift-exponent generating unit further includes a bypass circuit to supply the second exponent and the second mantissa as data to be normalized by the second normalization unit, upon detecting that the value obtained by subtracting from the first post-subtraction exponent the second post-subtraction exponent is larger than or equal to the number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent the first leading-zero count, and the second post-subtraction exponent being obtained by subtracting from the second exponent the second leading-zero count,
wherein there is no need for the second condition to be satisfied.

7. An arithmetic processing apparatus, the arithmetic processing apparatus comprising:
an arithmetic circuit to calculate a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision; and
an instruction control unit to decode a correction value calculating instruction for calculating the correction value for the value that is obtained by an add-subtract operation of two values and that is expressed in the predetermined fixed precision,
wherein the arithmetic circuit includes:
a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa;
a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa;
a first shift-amount generating unit to generate as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
a second shift-amount generating unit to generate as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
a first shift unit to calculate a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount;
a second shift unit to calculate a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount;
an add-subtract unit to produce a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and to produce a carry obtained by the add-subtract operation;
a post-shift-exponent generating unit to calculate a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa;
a corrected-exponent generating unit to generate a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry;
a first normalization unit to calculate a normalized exponent that is obtained by normalizing the corrected exponent, and to calculate a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and
a second normalization unit to calculate a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, expressed in the predetermined fixed precision, of the normalized double-precision mantissa, and to calculate a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.

8. The arithmetic processing apparatus as claimed in claim 7, wherein the arithmetic circuit further includes:
a first conversion unit to convert the first pre-add-subtract-operation data stored in the first input register into the first pre-add-subtract-operation data in a second format; and
a second conversion unit to convert the second pre-add-subtract-operation data stored in the second input register into the second pre-add-subtract-operation data in the second format,
wherein the first shift-amount generating unit generates as the first shift amount a first leading-zero count obtained by counting consecutive zeros from the most significant bit of the first mantissa in the second format upon detecting that an absolute value of a difference between the first exponent in the second format and the second exponent in the second format is larger than or equal to the first leading-zero count, and to generate as the first shift amount the absolute value of a difference between the first exponent in the second format and the second exponent in the second format upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is smaller than the first leading-zero count;
the second shift-amount generating unit generates as the second shift amount a value obtained by subtracting the second exponent in the second format and the first leading-zero count from the first exponent in the second format upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is larger than or equal to the first leading-zero count, and to generate as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent in the second format and the second exponent in the second format is smaller than the first leading-zero count; and
the post-shift-exponent generating unit calculates a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa in the second format or the second mantissa in the second format.

9. The arithmetic processing apparatus as claimed in claim 8, wherein in the arithmetic circuit, the second format is a binary-coded decimal format.

10. The arithmetic processing apparatus as claimed in claim 8, wherein the arithmetic circuit further includes a third conversion unit to convert the lower-order normalized mantissa into a lower-order normalized mantissa in the first format, and to convert the lower-order normalized exponent into a lower-order normalized exponent in the first format.

11. The arithmetic processing apparatus as claimed in claim 7, wherein the arithmetic circuit further includes:
a comparison circuit to make a comparison between the exponent of the first pre-add-subtract-operation data and the exponent of the second pre-add-subtract-operation data;
a first selection unit to select, as data to be shifted by the first shift unit, the mantissa of one of the first pre-add-subtract-operation data and the second pre-add-subtract-operation data, whichever has a larger exponent, based on a result of the comparison made by the comparison circuit; and
a second selection unit to select, as data to be shifted by the second shift unit, the mantissa of one of the first pre-add-subtract-operation data and the second pre-add-subtract-operation data, whichever has a smaller exponent, based on the result of the comparison made by the comparison circuit.

12. The arithmetic processing apparatus as claimed in claim 7, wherein in the arithmetic circuit, the post-shift-exponent generating unit further includes a bypass circuit to supply the second exponent and the second mantissa as data to be normalized by the second normalization unit, upon detecting that the value obtained by subtracting from the first post-subtraction exponent the second post-subtraction exponent is larger than or equal to the number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent the first leading-zero count, and the second post-subtraction exponent being obtained by subtracting from the second exponent the second leading-zero count.

13. A method of controlling an arithmetic circuit for calculating a correction value for a value that is obtained by an add-subtract operation of two values and that is expressed in a predetermined fixed precision, the arithmetic circuit including:
a first input register to store first pre-add-subtract-operation data in a first format that includes a first mantissa of a first fixed-precision floating-point number using a base-N numbering system (N: integer larger than or equal to 2), and that includes a first exponent for the first mantissa; and
a second input register to store second pre-add-subtract-operation data in the first format that includes a second mantissa of a second fixed-precision floating-point number using the base-N numbering system, and that includes a second exponent for the second mantissa, with a first condition and a second condition being concurrently satisfied, the first condition being that a value of the second exponent is smaller than a value of the first exponent, and the second condition being that a value obtained by subtracting from a first post-subtraction exponent a second post-subtraction exponent is smaller than a number of digits of the predetermined fixed precision, the first post-subtraction exponent being obtained by subtracting from the first exponent a first leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the first mantissa, and the second post-subtraction exponent being obtained by subtracting from the second exponent a second leading-zero count that is obtained by counting consecutive zeros from a most significant bit of the second mantissa,
the method of controlling an arithmetic circuit comprising:
generating, by use of a first shift-amount generating unit of the arithmetic circuit, as a first shift amount the first leading-zero count upon detecting that an absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and generating as the first shift amount the absolute value of a difference between the first exponent and the second exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
generating, by use of a second shift-amount generating unit of the arithmetic circuit, as a second shift amount a value obtained by subtracting the second exponent and the first leading-zero count from the first exponent upon detecting that the absolute value of a difference between the first exponent and the second exponent is larger than or equal to the first leading-zero count, and generating as the second shift amount a zero upon detecting that the absolute value of a difference between the first exponent and the second exponent is smaller than the first leading-zero count;
calculating, by use of a first shift unit of the arithmetic circuit, a first post-shift mantissa that is obtained by shifting the first mantissa by the first shift amount;
calculating, by use of a second shift unit of the arithmetic circuit, a second post-shift mantissa that is obtained by shifting the second mantissa by the second shift amount;
producing, by use of an add-subtract unit of the arithmetic circuit, a double-precision mantissa that has double the precision of the predetermined fixed precision and that is obtained by performing an add-subtract operation with respect to the first post-shift mantissa and the second post-shift mantissa, and producing a carry obtained by the add-subtract operation;
calculating, by use of a post-shift-exponent generating unit of the arithmetic circuit, a post-shift exponent that is obtained by performing an add-subtract operation with respect to the second shift amount and a smaller of the first mantissa or the second mantissa;
generating, by use of a corrected-exponent generating unit of the arithmetic circuit, a corrected exponent that is obtained by performing an add-subtract operation with respect to the post-shift exponent and the carry;
calculating, by use of a first normalization unit of the arithmetic circuit, a normalized exponent that is obtained by normalizing the corrected exponent, and calculating a normalized double-precision mantissa that is obtained by normalizing the double-precision mantissa; and
calculating, by use of a second normalization unit of the arithmetic circuit, a lower-order normalized mantissa that is obtained by normalizing a lower-order portion, expressed in the predetermined fixed precision, of the normalized double-precision mantissa, and calculating a lower-order normalized exponent that is obtained by subtracting the number of digits of the predetermined fixed precision from the normalized exponent.