# Patent

## Title
Method and apparatus for decimal number addition using hardware for binary number operations

## Abstract

According to embodiments of the subject matter disclosed in this application, decimal floating-point additions and/or decimal fixed-point additions may be implemented using existing hardware for binary number operations. The implementation can be carried out in software, in hardware, or in a combination of software and hardware. Pre-calculated constants that are approximations to negative powers of 10 and stored in binary format may be used for rounding addition results to a designated precision by multiplying the results with a pre-calculated constant. Additionally, several parts of a decimal multiplication may be carried out in parallel. Furthermore, a simple comparison with a constant instead of an expensive remainder calculation may be used for midpoint detection and exactness determination.

## Background
BACKGROUND
1. Field
This disclosure relates generally to processor systems, and more specifically but not exclusively, to methods and apparatus for arithmetic operations performed by processors.
2. Description
Decimal floating-point additions are typically implemented in software because there is no specialized hardware for decimal floating-point additions in most modern processors. A floating-point number (D) is normally represented by a coefficient (C) and an exponent (E), i.e., D=C·10^E, where both C and E are decimal integers. The precision of such a representation may be determined by the number of decimal digits in C, with more decimal digits in C resulting in more precise representation of D. Assume that coefficients and exponents of decimal floating-point numbers are stored in a binary format in a computer. A typical process for calculating an addition between two decimal floating-point numbers (e.g., D1=C1·10^E1, and D2=C2·10^E2) is as follows: convert coefficients (C1 and C2) and exponents (E1 and E2) from binary to decimal; perform a decimal integer addition between C1·10^(E1−E2) (assume that E1>=E2) and C2; round exact decimal addition result correctly to the destination precision; and then convert the coefficient and exponent of the final results from decimal back to binary.
Although it would be possible to store coefficients and exponents of decimal floating-point numbers in a decimal format (e.g., Binary-Coded Decimal (BCD) format) all the time, then the advantage offered by existing instructions or circuitry that operate on binary numbers could not be taken of directly. In the above process for decimal floating-point addition, conversion back and forth between decimal and binary formats and decimal integer addition consume a number of clock cycles. More importantly, the rounding operation for the addition result, which typically uses remainder-based calculation, is very expensive in terms of clock cycles. Thus, it is desirable to have faster approaches for decimal floating-point additions so that the overall performance of a computing system can be improved.



BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the disclosed subject matter will become apparent from the following detailed description of the subject matter in which:
FIG. 1 shows one example computing system where decimal number additions may be performed using hardware for binary number operations, according to the subject matter disclosed in the present application;
FIG. 2 is a block diagram of an example decimal addition module that performs decimal number additions with rounding to the nearest number within the pre-determined precision using hardware for binary number operations, according to the subject matter disclosed in the present application;
FIG. 3 is a flowchart illustrating an example process for performing decimal number additions with rounding to the nearest number within the pre-determined precision using hardware for binary number operations, according to the subject matter disclosed in the present application;
FIG. 4 is a block diagram of an example decimal addition module that performs decimal number additions with rounding down using hardware for binary number operations, according to the subject matter disclosed in the present application;
FIG. 5 is a flowchart illustrating an example process for performing decimal number additions with rounding down using hardware for binary number operations, according to the subject matter disclosed in the present application; and
FIG. 6 is a flowchart illustrating an example process for performing decimal number additions with rounding up using hardware for binary number operations, according to the subject matter disclosed in the present application.



DETAILED DESCRIPTION
According to embodiments of the subject matter disclosed in this application, decimal floating-point additions and/or decimal fixed-point additions may be implemented using existing hardware for binary number operations. The implementation can be carried out in software (e.g., using machine instructions for the existing underlying hardware for binary operations), in hardware (e.g., re-using existing circuitry for operations on binary numbers), or in a combination of software and hardware. Particularly, pre-calculated constants that are approximations to negative powers of 10 and stored in binary format may be used for rounding addition results to a designated precision. As a result, a rounding operation may be implemented as a multiplication between an addition result and a pre-calculated constant. Additionally, several parts of a decimal addition may be carried out in parallel. Furthermore, a simple comparison with a constant instead of an expensive remainder calculation may be used for determining whether an addition result is a midpoint between two consecutive decimal floating-point numbers (for additions with rounding to the nearest number within the pre-determined precision) and determining whether the rounded result is an exact representation of the real addition result.
Reference in the specification to “one embodiment” or “an embodiment” of the disclosed subject matter means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosed subject matter. Thus, the appearances of the phrase “in one embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment.
FIG. 1 shows one example computing system 100 where decimal number additions may be performed using hardware for binary number operations. Computing system 100 may comprise one or more processors 110 coupled to a system interconnect 115. Processor 110 may one or more processing cores. At least one processing core may have a decimal addition module 120 that uses existing hardware for binary number operations. The decimal addition module may be implemented in hardware (e.g., as a part of the arithmetic and logic unit (ALU)) by re-using existing circuitry for binary number operations, in software by using instructions for the existing underlying binary hardware, or in a combination of hardware and software.
The decimal addition module may include pre-calculated binary approximations to certain negative powers of 10 and store these pre-calculated binary numbers as constants in a table. When rounding an addition result to a pre-determined precision, a pre-calculated constant corresponding to the pre-determined precision may be looked up in a table and used to multiply with the addition result. Using these pre-calculated constants, the conventional expensive division operations are replaced with less expensive binary multiplication operations. Additionally, several parts of a decimal addition may be carried out in parallel. Moreover, midpoint detection (i.e., determining whether an addition result is a midpoint between two consecutive decimal floating-point numbers for additions with rounding to the nearest number within the pre-determined precision) and exactness detection of the final addition results may be performed by using a simple comparison with a constant instead of more expensive remainder calculations.
The computing system 100 may also include a chipset 130 coupled to the system interconnect 115. Chipset 130 may include one or more integrated circuit packages or chips. Chipset 130 may comprise one or more device interfaces 135 to support data transfers to and/or from other components 160 of the computing system 100 such as, for example, BIOS firmware, keyboards, mice, storage devices, network interfaces, etc. Chipset 130 may be coupled to a Peripheral Component Interconnect (PCI) bus 170. Chipset 130 may include a PCI bridge 145 that provides an interface to the PCI bus 170. The PCI Bridge 145 may provide a data path between the processor 110 as well as other components 160, and peripheral devices such as, for example, an audio device 180 and a disk drive 190. Although not shown, other devices may also be coupled to the PCI bus 170.
Additionally, chipset 130 may comprise a memory controller 125 that is coupled to a main memory 150. The main memory 150 may store data and sequences of instructions that are executed by the processor 110 or any other device included in the system. The memory controller 125 may access the main memory 150 in response to memory transactions associated with the processor 110, and other devices in the computing system 100. In one embodiment, memory controller 150 may be located in processor 110 or some other circuitries. The main memory 150 may comprise various memory devices that provide addressable storage locations which the memory controller 125 may read data from and/or write data to. The main memory 150 may comprise one or more different types of memory devices such as Dynamic Random Access Memory (DRAM) devices, Synchronous DRAM (SDRAM) devices, Double Data Rate (DDR) SDRAM devices, or other memory devices.
Although FIG. 1 shows one example computing system, the decimal addition module may be used in any processing unit of any system. For example, the decimal addition module may be used in a symmetric multiprocessor system (SMP), a chip multiprocessor system (CMP), or any other multiprocessor systems.
In the following description, a decimal floating-point addition is used for illustration purposes, but the subject matter disclosed in this application is not limited to decimal floating-point additions. A person of ordinary skill in the art can easily understand that the disclosed subject matter also applies to decimal fixed-point additions. The Institute of Electrical and Electronics Engineers (IEEE) has a standard 754-1985 for binary floating-point arithmetic that is being revised. A new revision to IEEE standard 754 is proposed to add definitions of decimal floating-point arithmetic. The proposal (IEEE 754R) defines three decimal floating-point formats with sizes of 32, 64, and 128 bits and respective precisions of p=7, p=16, and p=34 decimal digits. Additionally, it is assumed that the decimal numbers have their coefficients and exponents stored in a binary format (even if coefficients and exponents are stored in a decimal format, they can be efficiently converted to binary by taking advantage of available instruction-level parallelism). The subject matter disclosed in this application can be used by IEEE 754R, but it is not limited to or by IEEE 754R.
A decimal floating-point number D is typically represented as D=±C·10^E, where C is a positive integer having at most p decimal digits and E is an integer exponent. It is further assumed that two decimal floating-point operands D1=C1·10^E1 and D2=C2·10^E2 are different from zero with coefficients having at most p decimal digits which are stored as binary integers. For convenience of description, it is assumed that D1>=0 and E1>=E2. The addition of D1 and D2 can then be performed as, D=(D1+D2)md,p=(C1·10^(E1−E2)+C2)md,p·10^(E2)=(C′)md,p·10^(E2)=C·10^E, where (Y)md,p stands for rounding Y to p decimal digits. Rules for other cases of sign or exponent ordering combinations of D1 and D2 can be derived directly from the following description.
FIG. 2 is a block diagram of an example decimal addition module 200 that performs decimal number additions with rounding to the nearest number within the pre-determined precision using hardware for binary number operations, according to the subject matter disclosed in the present application. The decimal addition module may comprise at least one binary integer adder 210, a decimal digit number identifier 220, at least one binary integer multiplier 230, a rounding module 240, a binary shifter 250, a midpoint detector 260, and an exactness detector 270. Although not shown in this figure, the decimal addition module may also comprise a decimal-to-binary converter to convert a number from a decimal format to a binary format and/or a binary-to-decimal converter to convert a number from a binary format to a decimal format.
Additionally, the decimal addition module 200 may comprise a pre-processor (not shown in the figure) to pre-process received two operands D1 and D2. For example, the pre-processor may compare E1 and E2 (here it is assumed that E1>=E2). The comparison result is used to determine which coefficient needs to be multiplied by a non-negative power of 10 before being added to the other coefficient, for example, C1 needs to be multiplied by 10^(E1−E2) before adding with C2, according to the assumption here. The pre-processor may also compare (q1+E1) with (q2+E2), where q1 and q2 are numbers of decimal digits in C1 and C2, respectively, and can be obtained by table lookup and if necessary also simple comparisons, as described below for decimal digit number identifier 220. Assuming (q1+E1)>=(q2+E2), then the pre-processor may compute the difference, (q1+E1−q2−E2), and determine if (q1+E1−q2−E2)>=p. If it is, D2 may only cause rounding error to the sum of D1 and D2. Thus, the sum of D1 and D2 may be obtained directly from D1 rounded with value of D2. The resulting sum is inexact and may equal to D1 or (C1±10^(q1−p)·10^E1. Cases where (q1+E1−q2−E2)<=(p−1) may then be dealt with in more details according to what is described below.
Binary integer adder 210 may first perform a subtraction (e.g., E1−E2 for E1>=E2) and then pass the result to binary integer multiplier 230 that performs a multiplication (e.g., C1·10^(E1−E2)). Second, the binary integer adder may perform an addition between the result from the binary integer multiplier and one coefficient (e.g., C′=C1·10^(E1−E2)+C2). The binary integer adder may pass the result of the addition, C′, to decimal digit number identifier 220 which determines the number of decimal digits in C′. Since the number of decimal digits in a number is closely correlated with the number of valid binary digits in the number, decimal digit number determination may be achieved via a lookup table and simple comparisons if necessary. The lookup table may include corresponding relationships between the number of valid binary digits and the number of decimal digits in a number. For example, a number with 6 binary valid digits (i.e., from 100000 to 111111) has 2 decimal digits. If C′ has 6 binary digits, a simple table lookup will tell the number of decimal digits that C′ may have. However, a number with 4 binary valid digits (i.e., from 1000 to 1111) may have 1 or 2 decimal digits. Thus, if C′ has 4 valid binary digits, a simple table lookup is not sufficient to determine the number of decimal digits in C′. In this case, a simple comparison in addition to table lookup may be needed. Table lookup will tell that C′ has either 1 or 2 decimal digits; then the result is further compared with binary number “1001.” If C′ is smaller than or equals to “1001,” C′ has 1 decimal digit; otherwise, C′ has 2 decimal digits.
After the number of decimal digits in C′, q, is determined by decimal digit number identifier 220, C′ may be rounded to a pre-determined precision p by rounding module 240. First, q is compared with p. If q is not larger than p, no rounding is needed and the sum of D1 and D2 will be D=C′·10^E, with E=E2. Since no rounding is performed, D is the exact representation of the sum of D1 and D2. If q is larger than p, the rounding module computes the difference (x) between q and p, i.e., x=q−p, (this may be performed by the binary integer adder). The rounding module may obtain binary representations of 10^(−x) and (1⁄2·10^x) by looking up the table. Rounding of C′ may be done using Property 1 from U.S. patent application Ser. No. 11/133,811, which is incorporated by reference herein in its entirety. For convenience of description, this property is summarized below.










Property 1















Let C ∈ N be a number in base b = 2, and d0·10(q−1) + d1·10(q−2) +


d2·10(q−3) + ... + d(q−2) ·101 + d(q−1) be its representation in base


B=10, where d0, d1, ... d(q−1) ∈ {0, 1, ... , 9}, d0 ≠ 0.


Let x ∈ {1, 2, 3, ..., (q−1) } and ρ = ln10/ln2 = log210.


If y ∈ N, y ≧ ceil(frac(ρ·x) + ρ·q) and Kx is the value of 10−x rounded


up (toward positive infinity) to y bits:









Kx = (10−x)RP,y = 10−x · (1 + ε), with 0 < ε < 2−(y+1)







then









floor(C·Kx) = d0·10(q−x−1) + d1·10(q−x−2) + d2·10(q−x−3) +...









+ d(q−x−2)·101 + d(q−x−1)







Note: In practice we may take y = 1+ceil(ρ·q), which makes


0<ε<2−ceil([]·q).









This property gives a precise way to “cut off” x decimal digits from the lower part of an integer C when its binary representation is available, thus avoiding the need to convert C to decimal, remove the lower x decimal digits, and then convert the result back to binary. For example, if the decimal number C=123456789 is available in binary and its six most significant decimal digits are required, Property 1 specifies precisely the constant K3≈10−3 so that floor(C·K3)=123456 with certainty. The values of Kx can be pre-calculated. Thus, only a binary integer multiplication and a logical shift right are needed for this computation.
Assume further that the full integer product C′=C1·C2 requires q decimal digits to represent. Actual rounding to p decimal digits will be necessary only if qε[p+1, 2·p]. In all rounding modes, we will use for this purpose the constant Kx≈10−x, which is pre-calculated to y bits as specified in Property 1, where x=q−p. Since qε[p+1, 2·p] for situations where rounding is necessary, all cases are covered correctly if we choose y=1+ceil(2·ρ·p). The pre-calculated values Kx may be stored as pairs (kx, ex) for all xε{1, 2, . . . , p}, where kx and ex are positive integers with Kx=kx·2^(−ex).
To round C′ to the nearest integer with p decimal digits, the following operation is performed to C′: (C′+1⁄2·10^x)·10^(−x). Binary approximation for 10^(−x) may be pre-calculated according to Property 1 and stored in a table. Similarly, binary representation of (1⁄2·10^x) may also be pre-calculated and stored in a table. Rounding module 240 may look up in tables for binary representations for 10^(−x) and (1⁄2·10^x). The rounding module may pass C′ and the binary representation of (1⁄2·10^x) to binary integer adder 210 which adds them together to produce a sum C′′=C′+(1⁄2·10^x). Binary integer multiplier 230 may then multiply C′′ and Kx to produce a product C*(=C′′·Kx) for binary shifter 250. The binary shifter shifts C* to right by ex bits to produce a floor number C=floor(C*) and a factional number f*=frac(C*)=C*−floor(C*). For example, floor(15.2)=15, frac(15.2)=0.2; floor(−15.2)=−16, frac(−15.2)=−15.2−floor(15.2)=0.8.
Midpoint detector 260 may detect whether C′ is a midpoint between two consecutive decimal floating-point numbers. This can be done by calculating a remainder R=C′−C·10^x and by comparing R with 1⁄2·10^x. If R=1⁄2·10^x, then C′ is a midpoint and should be rounded to the nearest even decimal floating-point number according to some standards such as the IEEE 754R specification (e.g., (15.5)md,2=16, (14.5)md,2=14), where (Y)md,2 represent Y is rounded to 2 decimal digits). The calculation of a remainder, however, is a fairly costly operation. According to an embodiment of the subject matter disclosed in this application, remainder calculation may be avoided and instead a single comparison to a pre-calculated constant, 10^(−p), may be used. Specifically, the midpoint detector may compare f* with 10^(−p). If f* is larger than 0 but smaller than 10^(−p) and if C is odd, then the midpoint detector decrements C by 1 so that C′ is rounded to the nearest even number; otherwise, it is rounded correctly and no adjustment of C by the midpoint detector is necessary. After midpoint detection and if needed, adjustment of C, binary integer adder 210 may add E2 and x together to produce a corresponding exponent, E, for coefficient C, i.e., E=E2+x. At this point, a rounded sum of D1 and D2, D, may be obtained with coefficient C and exponent E, i.e., D=C·10^E.
In certain situations, it may be necessary to inform a user whether E is an exact representation of the real sum of D1 and D2. If q≧p+1, E is inexact unless the x decimal digits removed from the lower part of C* are all zeros. One approach to determine whether E is exact is to compare remainder R above with 0. If R=0, E is exact; otherwise, it is not. Again the remainder calculation is fairly costly. According to an embodiment of the subject matter disclosed in this application, exactness detector 270 may use comparison operations to determine the exactness of D.
Since the value of C′ is rounded to the nearest decimal number, it is possible that C, which results from C′, may have (p+1) decimal digits rather than the pre-determined p digits. Thus, the exactness detector first determines whether C=10^p, i.e., whether there is rounding overflow. If C=10^p, C has (p+1) decimal digits, i.e., there is rounding overflow. Then the exactness detector adjusts C to 10^(p−1) and accordingly increases E by 1. Second, the exactness detector determines whether (f*−1⁄2) is larger than 0 but smaller than the pre-calculated constant, 10^(−p), or whether f* is larger than 1⁄2 but smaller than (10^(−p)+1⁄2). If it is, D is an exact representation of the real product of D1 and D2; otherwise, D is an inexact result.
FIG. 3 is a flowchart illustrating an example process 300 for performing decimal number additions with rounding to the nearest number within the pre-determined precision using hardware for binary number operations, according to the subject matter disclosed in the present application. At block 302 two exponents, E1 and E2, may be received; and at block 304 two coefficients, C1 and C2, corresponding to E1 and E2, respectively, may be received. At block 305, received exponents and coefficients of D1 and D2 may be preprocessed. The numbers of decimal digits in C1 and C2, q1 and q2, respectively, may be identified. A comparison between (q1+E1) and (q2+E2) and a comparison between E1 and E2 may be made. Assuming that comparison results show that (q1+E1)>=(q2+E2) and E1>=E2, (q1+E1−q2−E2) may be computed and compared with p. If (q1+E1−q2−E2)>=p, the sum of D1 and D2 may be obtained by rounding D1 with the value of D2, i.e., D=(D1+D2)md,p=D1 or ((C1±10^(q1−p)·10^E1); and process 300 may end at block 307. If (q1+E1−q2−E2)<=(p−1), process 300 may continue.
At block 306, a binary integer multiplication, C1·10^(E1−E2), may be performed and C′ may be calculated via a binary integer addition, i.e., C′=C1·10^(E1−E2)+C2. At block 308, the number of decimal digits in C′, q, may be determined based on the number of binary digits in C′ and if necessary, also by a comparison operation, as disclosed above along with FIG. 2. The corresponding relationship between the number of decimal digits and the number of binary digits in C′ may be stored in a table.
At block 310, a comparison between q and the pre-determined precision p is made. If q<=p, no rounding is needed and a final addition result D (=D1+D2) may be obtained at block 314 with coefficient being C′ and exponent being E2. This result D is an exact representation of the real sum of D1 and D2. Process 300 may end at block 316. If q>p, x=q−p is calculated at block 312. At block 318, binary representation for (1⁄2·10^x) may be obtained by looking up a table that includes pre-calculated constants. At block 320, an addition, i.e., C′′=C′+(1⁄2·10^x), may be performed. While operations involved in blocks 318 and 320 are performed, binary representation for Kx=10^(−x) may be obtained at block 322 by looking up a table that stores pre-calculated constants. At block 324, a binary integer multiplication, C*=C′′·Kx may be performed. At block 326, C* may be shifted to right by ex bits to obtain a floor number C=floor(C*) and a fractional number f*=frac(C*). Operations in blocks 312 and 318-326 together perform rounding operations for C′ according to Property 1 as disclosed above.
A comparison may be made between 0, f*, and 10^(−p) at block 328 and a check whether C is even may be made at block 330, both for midpoint detection. If 0<f*<10^(−p) and if C is odd, then C may be decreased by 1 at block 332 so that C′ is rounded to the nearest even number; otherwise, C′ is rounded correctly and no adjustment of C is necessary. At block 334, an addition result of (D1+D2), after rounding and midpoint detection, may be obtained, i.e., D=C·10^(E2+x). At block 336, whether C has more than p decimal digits after rounding (i.e., whether there is rounding overflow) may be detected by comparing C with 10^p. If C=10^p, C overflows after rounding and adjustments for C and E may be made at block 338, i.e., changing C from 10^p to 10^(p−1) and changing E from (E2+x) to (E2+x+1). At block 340, comparisons are made to determine whether D from block 336 or from block 338 is an exact representation of the real sum D1+D2, i.e., whether x digits removed from the lower part of C* during rounding are all zeros. If (f*−1⁄2) is larger than 0 but smaller than 10^(−p) (or if f* is larger than 1⁄2 but smaller than (10^(−p)+1⁄2)), that result is exact may be outputted at block 344; otherwise, that result is inexact and may be outputted at block 342. Process 300 may end at block 346.
Process 300 as shown in FIG. 3 for decimal floating-point addition in the round-to-nearest mode using hardware for binary number operations may be summarized as Property 2 below.










Property 2

















q1 = number of decimal digits in C1; /* table lookup */



q2 = number of decimal digits in C2; /* table lookup */



if(q1+E1−q2−E2) >= p



{









round D1 by the value of D2; /* D2 is too small compared









with D1 */









D = C1 · 10{circumflex over ( )}E1, or D = (C1±10{circumflex over ( )}(q1−p) · 10{circumflex over ( )}E1;









}



else /* for cases where (q1+E1−q2−E2) <= (p−1) */



{









C′ = C1·10{circumflex over ( )}(E1−E2) + C2; /* binary integer multiplication



and addition; 10{circumflex over ( )}(E1−E2) may be obtained via table lookup*/



q = the number of decimal digits required to represent C′;









/* look up a table */









if q ≦ p, the result D = C′ · 10{circumflex over ( )}E2 is exact;



else /* for case where q ∈ [p+1, 2·p] */



{









x = q − p; /* the number of decimal digits to be









removed from the lower part of C′, x ∈ [1, p] */









C′′ = C′ + (1/2 · 10x); /*1/2·10x is pre-calculated */



Kx = 10−x · (1 + ε), 0 < ε < 2−ceil(2·p·p) ; /* pre-calculated









as specified in Property 1) */









C* = C′′ · Kx = C′′ · kx · 2{circumflex over ( )}(−ex); /* binary integer









multiplication with implied binary point */









C = floor(C*); /* C has p decimal digits as rounded









according to Property 1 */









f* = frac(C*); /* f* consists of lower ex bits of C* */



if 0 < f* < 10−p, /* since C* = C′′ · Kx · 2{circumflex over ( )}(−ex),



compare ex bits shifted out of C* with 10−p */



{









if C is odd, C = C − 1;









}



D = C · 10E2+x;



if C = 10p /* rounding overflow) */









D = 10p−1 · 10E2+x+1;









if 0 < (f* − 1⁄2) < 10−p, the result is exact



else the result is inexact;









}









}









Note that several parts of computations for Property 2 can be carried out in parallel, for example computation of q1+E1−q2−E2 and C′=C1·10^(E1−E2)+C2 can be carried out in parallel, and so can test for q≦p and computation of x=q−p; reading 1⁄2·10x and kx from tables; computation of C and f* and E=E2+x; and tests for 0<f*<10^(−p) and 0<f*−1⁄2<10^(−p).
FIG. 4 is a block diagram of an example decimal addition module 400 that performs decimal number additions with rounding down using hardware for binary number operations, according to the subject matter disclosed in the present application. Decimal addition module 400 may comprise at least one binary integer adder 410, a decimal digit number identifier 420, at least one binary integer multiplier 430, a rounding module 440, a binary shifter 450, and an exactness detector 460. Although not shown in this figure, the decimal addition module may also comprise a decimal-to-binary converter to convert a number from a decimal format to a binary format and a binary-to-decimal converter to convert a number from a binary format to a decimal format. Like decimal addition module 200 in FIG. 2, decimal addition module 400 may also include a pre-processor to perform similar functions as the pre-processor disclosed in the description above for decimal addition module 200.
Decimal addition module 400 works similarly to decimal addition module 200 as shown in FIG. 2, except that rounding module 440 and exactness detector 460 work differently from rounding module 240 and exactness detector 270, respectively, and there is no midpoint detector in decimal addition module 400. Since C′ is rounded down, unlike rounding module 240, rounding module 440 does not need to obtain a binary representation for (1⁄2·10^x) and to obtain C′′=C′+(1⁄2·10^x). Instead rounding module 440 directly passes Kx and C′ to binary integer multiplier 430 to obtain C*=C′·Kx. Because C′ is rounded down, there is no rounding overflow. Thus, unlike exactness detector 270, exactness detector 460 does not need to detect rounding overflow and perform adjustment to C and E accordingly when there is rounding overflow. Exactness detector 460 simply determines whether (f*−1⁄2) is larger than 0 but smaller than the pre-calculation constant, 10^(−). If it is, D from rounding module 440 is an exact representation of the real sum of D1 and D2; otherwise, D is an inexact result.
FIG. 5 is a flowchart illustrating an example process 500 for performing decimal number additions with rounding down using hardware for binary number operations, according to the subject matter disclosed in the present application. At block 505 two exponents, E1 and E2, may be received; and at block 510 two coefficients, C1 and C2, corresponding to E1 and E2, respectively, may be received. At block 512, received exponents and coefficients of D1 and D2 may be preprocessed. The numbers of decimal digits in C1 and C2, q1 and q2, respectively, may be identified. A comparison between (q1+E1) and (q2+E2) and a comparison between E1 and E2 may be made. Assuming that comparison results show that (q1+E1)>=(q2+E2) and E1>=E2, (q1+E1−q2−E2) may be computed and compared with p. If (q1+E1−q2−E2)>=p, the sum of D1 and D2 may be obtained by rounding D1 with the value of D2, i.e., D=(D1+D2)md,p=D1 or ((C1±10^(q1−p)·10^E1); and process 500 may end at block 514. If (q1+E1−q2−E2)<=(p−1), process 500 may continue.
At block 515, a binary integer multiplication, C1·10^(E1−E2), may be performed and C′ may be calculated via a binary integer addition, i.e., C′=C1·10^(E1−E2)+C2. At block 520, the number of decimal digits in C′, q, may be determined. At block 525, a comparison between q and p is made. If q<=p, no rounding is needed and a final exact addition result D (=D1+D2) may be obtained at block 530 with exponent being C′ and exponent being E2. Process 500 may end at block 535. If q>p, x=q−p is calculated at block 540. At block 545, binary representation for Kx=10^(−x) may be obtained by looking up a table that stores pre-calculated constants. At block 550, a binary integer multiplication, C*=C′·Kx may be performed. At block 555, C* may be shifted to right by ex bits to obtain a floor number C=floor(C*) and a fractional number f*=frac(C*). Operations in blocks 540-555 perform rounding operations for C′ according to Property 1 as disclosed above.
At block 560, an addition result of (D1+D2), after rounding, may be obtained, i.e., D=C·10^(E2+x). At block 565, comparisons are made to determine whether D is an exact representation of the real sum (D1+D2), i.e., whether x digits removed from the end of C* during rounding are all zeros. If f* is larger than 0 but smaller than 10^(−), that result is exact and may be outputted at block 575; otherwise, that result is inexact may be outputted at block 570. Process 500 may end at block 580.
Process 500 as shown in FIG. 5 for decimal floating-point addition in the rounding-down mode using hardware for binary number operations may be summarized as Property 3 below.










Property 3

















q1 = number of decimal digits in C1; /* table lookup */



q2 = number of decimal digits in C2; /* table lookup */



if (q1+E1−q2−E2) >=p



{









round D1 by the value of D2; /* D2 is too small compared









with D1 */









D = C1 · 10{circumflex over ( )}E1, or D = (C1±10{circumflex over ( )}(q1−p) · 10{circumflex over ( )}E1;









}



else /* for cases where (q1+E1−q2−E2) <= (p−1) */



{









C′ = C1·10{circumflex over ( )}(E1−E2) + C2; /* binary integer multiplication



and addition; 10{circumflex over ( )}(E1−E2) may be obtained via table lookup*/



q = the number of decimal digits required to represent C′;









/* look up in a table */









if q ≦ p, the result n = C′ · 10E2 is exact;



else /* for cases where q ∈ [p+1 , 2·p] */



{









x = q − p; /* the number of decimal digits to be



removed from the lower part of C′, x ∈ [1, p] */



Kx = 10−x · (1 +  ), 0 <   < 2−ceil(2·ρ·p); /* pre-calculated









as specified in Property 1 */









C* = C′ · Kx = C′ · kx · 2{circumflex over ( )}(−ex); /* binary integer









multiplication with implied binary point */









C = floor(C*); /*logical right shift; C has p decimal









digits, calculated correctly by Property 1 */









f* = frac(C*); /* the fractional part of C*, consists of









the lower ex bits of the product C* */









D = C · 10E2+x;



if 0 < f* < 10−p, the result is exact;



else the result is inexact;









}









}









Similar to computations for Property 2, several parts of computations for Property 3 can also be carried out in parallel, for example computation of q1+E1−q2−E2 and C′=C1·10^(E1−E2)+C2 can be carried out in parallel, and so can test for q≦p and computation of x=q−p; reading 1⁄2·10x and Kx from tables; computation of C and f* and e=e2+x; and test for 0<f*<10^(−p) and encoding of the result. Also note that condition 0<f*<10^(−p) from Property 3 for exactness determination holds also if 10^(−p) is replaced by 10^(−x) or by Kx=10^(−x)·(1+ε).
FIG. 6 is a flowchart illustrating an example process 600 for performing decimal number additions with rounding up using hardware for binary number operations, according to the subject matter disclosed in the present application. A decimal addition in the rounding-up mode works very similarly to a decimal addition in the rounding-down mode, operations performed in blocks 605-675 are similar to those performed in blocks 505-575 of process 500 shown in FIG. 5. Differences between the rounding-down mode and the rounding-up are pointed out below.
Although calculation of the addition result, D, and exactness determination are identical to those carried out for the rounding-down as shown in FIGS. 4 and 5, the value D=floor(C′·Kx)·10^(E2+x)=C·10^(E2+x) is the final result only if the result calculated for the rounding-down mode is exact. If the result is inexact, then the result for the rounding-up needs to have C replaced by C=C+1 at block 680. If C=10^p (rounding overflow), then both C and E need to be adjusted at block 685, i.e., C=10^(p−1), E=E+1=E2+x+1, D=C·10^E.
In some situations, a rounding-to-zero may be mentioned/used. This mode is not treated separately here because for positive values it is identical to the rounding-down mode and for negative values it is identical to the rounding-up mode.
Although an example embodiment of the disclosed subject matter is described with reference to block and flow diagrams in FIGS. 1-6, persons of ordinary skill in the art will readily appreciate that many other methods of implementing the disclosed subject matter may alternatively be used. For example, the order of execution of the blocks in flow diagrams may be changed, and/or some of the blocks in block/flow diagrams described may be changed, eliminated, or combined.
In the preceding description, various aspects of the disclosed subject matter have been described. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the subject matter. However, it is apparent to one skilled in the art having the benefit of this disclosure that the subject matter may be practiced without the specific details. In other instances, well-known features, components, or modules were omitted, simplified, combined, or split in order not to obscure the disclosed subject matter.
Various embodiments of the disclosed subject matter may be implemented in hardware, firmware, software, or combination thereof, and may be described by reference to or in conjunction with program code, such as instructions, functions, procedures, data structures, logic, application programs, design representations or formats for simulation, emulation, and fabrication of a design, which when accessed by a machine results in the machine performing tasks, defining abstract data types or low-level hardware contexts, or producing a result.
For simulations, program code may represent hardware using a hardware description language or another functional description language which essentially provides a model of how designed hardware is expected to perform. Program code may be assembly or machine language, or data that may be compiled and/or interpreted. Furthermore, it is common in the art to speak of software, in one form or another as taking an action or causing a result. Such expressions are merely a shorthand way of stating execution of program code by a processing system which causes a processor to perform an action or produce a result.
Program code may be stored in, for example, volatile and/or non-volatile memory, such as storage devices and/or an associated machine readable or machine accessible medium including solid-state memory, hard-drives, floppy-disks, optical storage, tapes, flash memory, memory sticks, digital video disks, digital versatile discs (DVDs), etc., as well as more exotic mediums such as machine-accessible biological state preserving storage. A machine readable medium may include any mechanism for storing, transmitting, or receiving information in a form readable by a machine, and the medium may include a tangible medium through which electrical, optical, acoustical or other form of propagated signals or carrier wave encoding the program code may pass, such as antennas, optical fibers, communications interfaces, etc. Program code may be transmitted in the form of packets, serial data, parallel data, propagated signals, etc., and may be used in a compressed or encrypted format.
Program code may be implemented in programs executing on programmable machines such as mobile or stationary computers, personal digital assistants, set top boxes, cellular telephones and pagers, and other electronic devices, each including a processor, volatile and/or non-volatile memory readable by the processor, at least one input device and/or one or more output devices. Program code may be applied to the data entered using the input device to perform the described embodiments and to generate output information. The output information may be applied to one or more output devices. One of ordinary skill in the art may appreciate that embodiments of the disclosed subject matter can be practiced with various computer system configurations, including multiprocessor or multiple-core processor systems, minicomputers, mainframe computers, as well as pervasive or miniature computers or processors that may be embedded into virtually any device. Embodiments of the disclosed subject matter can also be practiced in distributed computing environments where tasks may be performed by remote processing devices that are linked through a communications network.
Although operations may be described as a sequential process, some of the operations may in fact be performed in parallel, concurrently, and/or in a distributed environment, and with program code stored locally and/or remotely for access by single or multi-processor machines. In addition, in some embodiments the order of operations may be rearranged without departing from the spirit of the disclosed subject matter. Program code may be used by or in conjunction with embedded controllers.
While the disclosed subject matter has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the subject matter, which are apparent to persons skilled in the art to which the disclosed subject matter pertains are deemed to lie within the scope of the disclosed subject matter.

## Claims

1. A machine-implemented method for performing a decimal number addition between a first operand and a second operand using hardware for binary number operations, comprising:
obtaining a first coefficient and a first exponent for said first operand, and a second coefficient and a second exponent for said second operand, said first and second coefficients and said first and second exponents being integer in a binary format;
performing a binary multiplication using a binary integer multiplier and a binary addition using a binary integer adder to produce a first result based on said first and second coefficients and said first and second exponents;
rounding the number of decimal digits in said first result to a pre-determined number of digits in a designated rounding mode to obtain a second result, including determining the number of decimal digits in said first result, comparing the number of decimal digits of said first result with said pre-determined number of digits, using said first result as said second result, if the number of decimal digits in said first result is less than or equal to said pre-determined number of digits, and otherwise converting said first result to said second result, said converting including calculating a difference (x) between the number of decimal digits in said first result and said pre-determined number of digits, obtaining a binary representation of 10^(−x), if said designated rounding mode is either rounding-down or rounding-up, performing a binary integer multiplication between said first result and said binary representation of 10^(−x) to produce an intermediate result, and if said designed rounding mode is rounding-to-nearest, obtaining a binary representation of (1⁄2*10^x), performing a binary addition between said first result and said binary representation of (1⁄2*10^x), and performing a binary integer multiplication between the result of said binary addition and said binary representation of 10^(−x) to produce said intermediate result;
obtaining a final result for said decimal number addition based at least in part on said second result; and
determining whether said final result is exact.

2. The method of claim 1, wherein performing a binary multiplication and a binary addition comprises:
comparing said first exponent (E1) with said second exponent (E2);
if said first exponent is smaller than said second exponent,
performing a binary multiplication between said second coefficient and 10^(E2−E1) to produce an intermediate product, and
performing an addition between said intermediate product and said first coefficient to produce said first result; and

if said first exponent equals to or larger than said second exponent,
performing a binary multiplication between said first coefficient and 10^(E1−E2) to produce an intermediate product, and
performing an addition between said intermediate product and said second coefficient to produce said first result.

3. The method of claim 1, wherein said designated rounding mode comprises one of rounding-to-nearest, rounding-down, or rounding-up.

4. The method of claim 1, wherein obtaining a final result for said decimal number addition comprises if said second result equals to said first result,
outputting said final result with a coefficient being said first result and an exponent being the smaller of said first exponent and said second exponent; and
indicating that said final result is exact.

5. The method of claim 1, wherein converting said first result to said second result further comprises performing a binary shift operation to said intermediate result to obtain said second result and to produce a floor number and a fractional number, said binary shift operation right shifting said intermediate result by the number of bits in said binary representation of 10^(−x), said floor number being the coefficient of said second result, said fractional number including bits being shifted out from said intermediate result.

6. The method of claim 5, wherein obtaining a final result for said decimal number addition comprises:
adding x and the smaller of said first exponent and said second exponent to produce an exponent for said final result;
if said designated rounding mode is rounding-to-nearest,
performing midpoint detection based on said floor number and said fractional number,
adjusting said floor number if said first result is determined as a midpoint, and
determining whether there is rounding overflow, and if there is, adjusting said floor number and said exponent of said final result; and

producing said final result for said decimal number addition with a coefficient being said floor number.

7. The method of claim 5, wherein determining whether said final result is exact comprises:
performing comparison operations between said fractional number and a first constant and a second constant; and
if said fractional number is larger than said first constant but smaller than said second constant, indicating that said final result is exact, otherwise indicating that said final result is inexact.

8. The method of claim 7, wherein obtaining a final result for said decimal number addition further comprises if said designated rounding mode is rounding-up,
increasing the coefficient of said result by 1 if said final result is inexact;
determining whether the coefficient of said final result overflows after said increasing; and
if the coefficient of said final result overflows, adjusting the coefficient and the exponent of said final result.

9. A processor comprising:
a processor core including a decimal addition module implemented in hardware of the processor core to perform a decimal number addition between a first operand and a second operand, said decimal addition module including:
at least one binary integer adder to perform an addition to produce a first result based on a first coefficient and a first exponent of said first operand and a second coefficient and a second exponent of said second operand;
a decimal digit number identifier to determine the number of decimal digits in said first result;
a rounding module to round the number of decimal digits in said first result to a pre-determined number of digits in a designated rounding mode to obtain a second result, wherein said rounding module is adapted to determine the number of decimal digits in said first result, compare the number of decimal digits in said first result with said pre-determined number of digits, and if the number of decimal digits of said first result is more than said pre-determined number of digits, to calculate a difference (x) between the number of decimal digits in said first result and said pre-determined number of digits, and to obtain a binary representation of 10^(−x);
at least one binary integer multiplier to perform a binary multiplication between said first result and said binary representation of 10^(−x) to produce said second result, if said designated rounding mode is either rounding-down or rounding-up; and
an exactness detector to determine whether a final result is exact, said final result derived from said second result.

10. The processor of claim 9, wherein said rounding module is adapted to further obtain a binary representation of (1⁄2*10(x)) and to instruct said adder to perform a binary addition between said first result and said binary representation of (1⁄2*10(x)) to produce an intermediate result, if said designated rounding mode is rounding-to-nearest.

11. The processor of claim 9, wherein said decimal addition module further including:
a pre-processor to compare said first exponent (E1) and said second exponent (E2); and
said at least one binary integer multiplier to perform a binary multiplication between said second coefficient and 10^(E2−E1) to produce an intermediate product, and to perform an addition between said intermediate product and said first coefficient to produce said first result, if said first exponent is smaller than said second exponent; and otherwise, to perform a binary multiplication between said first coefficient and 10^(E1−E2) to produce an intermediate product, and to perform an addition between said intermediate product and said second coefficient to produce said first result.

12. The processor of claim 11, wherein said at least one binary integer multiplier is further adapted to perform a binary multiplication between said first result and said binary representation of 10^(−x) to produce said second result, if said designated rounding mode is rounding-to-nearest.

13. The processor of claim 12, wherein said decimal addition module further comprising a binary shifter adapted to perform shift operations on said second result to obtain a floor number and a fractional number by right shifting said second result by the number of bits in said binary representation of 10^(−x).

14. The processor of claim 13, wherein said decimal addition module further comprising a midpoint detector, if said designated rounding mode is rounding-to-nearest, to detect whether said first result is a midpoint; and to adjust said floor number if said first result is a midpoint.

15. The processor of claim 14, wherein said adder is further adapted to add x and the smaller of said first exponent and said second exponent to produce an exponent for said final result of said decimal number addition, the coefficient of said final result being said floor number.

16. The processor of claim 15, wherein said exactness detector is adapted to adjust the coefficient and the exponent of said final result if there is rounding overflow when said designated rounding mode is rounding-to-nearest; and to determine whether said final result is exact by comparing said fractional number with at least one constant.

17. The processor of claim 16, wherein said exactness detector, if said designated rounding mode is rounding-up, is further adapted to increase the value of the coefficient of said final result by 1 if said result is inexact; to determine whether the coefficient of said final result overflows if the coefficient of said final result is increased by 1; and if the coefficient of said final result overflows, to adjust the coefficient and the exponent of said final result.

18. A computing system, comprising:
a synchronous dynamic random access memory (“SDRAM”); and
at least one processor coupled to access said SDRAM via a system interconnect, said processor having a decimal addition module, said decimal addition module including:
at least one binary integer adder to perform an addition to produce a first result based on a first coefficient and a first exponent of said first operand and a second coefficient and a second exponent of said second operand;
a decimal digit number identifier to determine the number of decimal digits in said first result;
a rounding module to round the number of decimal digits in said first result to a pre-determined number of digits in a designated rounding mode to obtain a second result, wherein said rounding module is adapted to determine the number of decimal digits in said first result, to compare the number of decimal digits in said first result with said pre-determined number of digits, and if the number of decimal digits of said first result is more than said pre-determined number of digits, to calculate a difference (x) between the number of decimal digits in said first result and said pre-determined number of digits, and to obtain a binary representation of 10^(−x);
at least one binary integer multiplier to perform a binary multiplication between said first result and said binary representation of 10^(−x) to produce said second result, if said designated rounding mode is either rounding-down or rounding-up; and
an exactness detector to determine whether a final result is exact, said final result derived from said second result.

19. The system of claim 18, wherein said rounding module is adapted to further obtain a binary representation of (1⁄2*10(x)) and to instruct said adder to perform a binary addition between said first result and said binary representation of (1⁄2*10(x)) to produce an intermediate result, if said designated rounding mode is rounding-to-nearest.

20. The system of claim 18, wherein said decimal addition module further including:
a pre-processor to compare said first exponent (E1) and said second exponent (E2); and
said at least one binary integer multiplier to perform a binary multiplication between said second coefficient and 10^(E2−E1) to produce an intermediate product, and to perform an addition between said intermediate product and said first coefficient to produce said first result, if said first exponent is smaller than said second exponent; and otherwise, to perform a binary multiplication between said first coefficient and 10^(E1−E2) to produce an intermediate product, and to perform an addition between said intermediate product and said second coefficient to produce said first result.

21. The system of claim 20, wherein said at least one binary integer multiplier is further adapted to perform a binary multiplication between said first result and said binary representation of 10^(−x) to produce said second result, if said designated rounding mode is rounding-to-nearest.

22. The system of claim 21, wherein said decimal addition module further comprising a binary shifter adapted to perform shift operations on said second result to obtain a floor number and a fractional number by right shifting said second result by the number of bits in said binary representation of 10^(−x).

23. The system of claim 22, wherein said decimal addition module further comprising a midpoint detector, if said designated rounding mode is rounding-to-nearest, to detect whether said first result is a midpoint; and to adjust said floor number if said first result is a midpoint.

24. The system of claim 23, wherein said adder is further adapted to add x and the smaller of said first exponent and said second exponent to produce an exponent for said final result of said decimal number addition, the coefficient of said final result being said floor number.

25. The system of claim 24, wherein said exactness detector is adapted to adjust the coefficient and the exponent of said final result if there is rounding overflow when said designated rounding mode is rounding-to-nearest; and to determine whether said final result is exact by comparing said fractional number with at least one constant.

26. The system of claim 25, wherein said exactness detector, if said designated rounding mode is rounding-up, is further adapted to increase the value of the coefficient of said final result by 1 if said result is inexact; to determine whether the coefficient of said final result overflows if the coefficient of said final result is increased by 1; and if the coefficient of said final result overflows, to adjust the coefficient and the exponent of said final result.