Abstract:
A programmable logic device is programmed to perform arithmetic operations in an internal format that, unlike known standard formats that store numbers in normalized form and require normalization after each computational step, stores numbers in unnormalized form and does not require normalization after each step. Numbers are converted into unnormalized form at the beginning of an operation and converted back to normalized form at the end of the operation. If necessary to avoid data loss, a number may be normalized after an intermediate step.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to performing floating point arithmetic operations in programmable logic devices (PLDs), including the use of specialized processing blocks, which may be included in such devices, to perform floating point operations. 
     As applications for which PLDs are used increase in complexity, it has become more common to design PLDs to include specialized processing blocks in addition to blocks of generic programmable logic resources. Such specialized processing blocks may include a concentration of circuitry on a PLD that has been partly or fully hardwired to perform one or more specific tasks, such as a logical or a mathematical operation. A specialized processing block may also contain one or more specialized structures, such as an array of configurable memory elements. Examples of structures that are commonly implemented in such specialized processing blocks include: multipliers, arithmetic logic units (ALUs), barrel-shifters, various memory elements (such as FIFO/LIFO/SIPO/RAM/ROM/CAM blocks and register files), AND/NAND/OR/NOR arrays, etc., or combinations thereof. 
     One particularly useful type of specialized processing block that has been provided on PLDs is a digital signal processing (DSP) block, which may be used to process, e.g., audio signals. Such blocks are frequently also referred to as multiply-accumulate (“MAC”) blocks, because they include structures to perform multiplication operations, and sums and/or accumulations of multiplication operations. 
     For example, a PLD sold by Altera Corporation, of San Jose, Calif., under the name STRATIX® II includes DSP blocks, each of which includes four 18-by-18 multipliers. Each of those DSP blocks also includes adders and registers, as well as programmable connectors (e.g., multiplexers) that allow the various components to be configured in different ways. In each such block, the multipliers can be configured not only as four individual 18-by-18 multipliers, but also as four smaller multipliers, or as one larger (36-by-36) multiplier. In addition, one 18-by-18 complex multiplication (which decomposes into two 18-by-18 multiplication operations for each of the real and imaginary parts) can be performed. In order to support four 18-by-18 multiplication operations, the block has 4×(18+18)=144 inputs. Similarly, the output of an 18-by-18 multiplication is 36 bits wide, so to support the output of four such multiplication operations, the block also has 36×4=144 outputs. 
     The arithmetic operations to be performed by a PLD frequently are floating point operations. However, to the extent that known PLDs, with or without DSP blocks or other specialized blocks or structures, including the aforementioned STRATIX® II PLD, can perform floating point operations at all, they operate in accordance with the IEEE754-1985 standard, which requires that values be normalized at all times because it implies a leading “1”. However, that leads to certain inefficiencies as described below. 
     SUMMARY OF THE INVENTION 
     The present invention relates to PLDs having improved floating point operation capabilities. In particular, the present invention carries out floating point operations without normalization, although the results may be normalized if IEEE754-1985 compliance is required. In addition, normalization may be performed in intermediate steps if loss of data might otherwise result. 
     Therefore, in accordance with the present invention, there is provided a method of configuring a programmable logic device to perform floating point operations on values formatted in accordance with a standard requiring a first mantissa size and a first exponent size. The method includes configuring logic of the programmable logic device to reformat those values to have a second mantissa size larger than that first mantissa size, configuring logic of the programmable logic device to perform those operations on the reformatted values to compute a result, and configuring logic of said programmable logic device to reformat the result in accordance with the standard to the first mantissa size. 
     A programmable logic device so configured, and a machine-readable data storage medium encoded with software for performing the method, are also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  is a schematic representation of conversion of numbers according to the IEEE754-1985 standard format to a preferred embodiment of a format according to the present invention; 
         FIG. 2  is a schematic representation of conversion of numbers according to a preferred embodiment of a format according to the present invention to the IEEE754-1985 standard format; 
         FIG. 3  is a schematic representation of the configuration of a simple addition in accordance with the invention; 
         FIG. 4  is a schematic representation of the configuration of a somewhat more complicated addition in accordance with the invention; 
         FIG. 5  is a schematic representation of the configuration of a preferred embodiment of an arithmetic logic unit in accordance with the invention; 
         FIG. 6  is a schematic representation of the configuration of a preferred embodiment of a multiplier in accordance with the invention; 
         FIG. 7  is a cross-sectional view of a magnetic data storage medium encoded with a set of machine-executable instructions for performing the method according to the present invention; 
         FIG. 8  is a cross-sectional view of an optically readable data storage medium encoded with a set of machine executable instructions for performing the method according to the present invention; and 
         FIG. 9  is a simplified block diagram of an illustrative system employing a programmable logic device incorporating the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Floating point numbers are commonplace for representing real numbers in scientific notation in computing systems. Examples of real numbers in scientific notation are:
 
3.14159265 10 ×100 (π)
 
2.718281828 10 ×10 0  (e)
 
0.000000001 10  or 1.0 10 ×10 −9  (seconds in a nanosecond)
 
3155760000 10  or 3.15576 10 ×10 9  (seconds in a century)
 
     The first two examples are real numbers in the range of the lower integers, the third example represents a very small fraction, and the fourth example represents a very large integer. Floating point numbers in computing systems are designed to cover the large numeric range and diverse precision requirements shown in these examples. Fixed point number systems have a very limited window of representation which prevents them from representing very large or very small numbers simultaneously. The position of the notional binary-point in fixed point numbers addresses this numeric range problem to a certain extent but does so at the expense of precision. With a floating point number the window of representation can move, which allows the appropriate amount of precision for the scale of the number. 
     Floating point representation is generally preferred over fixed point representation in computing systems because it permits an ideal balance of numeric range and precision. However, floating point representation requires more complex implementation compared to fixed point representation. 
     The IEEE754-1985 standard is commonly used for floating point numbers. A floating point number includes three different parts: the sign of the number, its mantissa and its exponent. Each of these parts may be represented by a binary number and, in the IEEE754-1985 format, have the following bit sizes: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Sign 
                 Exponent 
                 Bias 
                 Mantissa 
               
               
                   
               
             
             
               
                 Single 
                 1 bit 
                  8 bits 
                  −127 
                 23 bits 
               
               
                 Precision 
                 [31] 
                 [30 . . . 23] 
                   
                 [22 . . . 00] 
               
               
                 32-Bit 
                   
                   
                   
                   
               
               
                 Double 
                 1 bit 
                 11 bits 
                 −1023 
                 52 bits 
               
               
                 Precision 
                 [63] 
                 [62 . . . 52] 
                   
                 [51 . . . 0] 
               
               
                 64-Bit 
               
               
                   
               
             
          
         
       
     
     The exponent preferably is an unsigned binary number which, for the single precision format, ranges from 0 to 255. In order to represent a very small number, it is necessary to use negative exponents. To achieve this the exponent preferably has a negative bias associated with it. For single-precision numbers, the bias preferably is −127. For example a value of 140 for the exponent actually represents (140−127)=13, and a value of 100 represents (100−127)=−27. For double precision numbers, the exponent bias preferably is −1023. 
     As discussed above, according to the standard, the mantissa is a normalized number—i.e., it has no leading zeroes and represents the precision component of a floating point number. Because the mantissa is stored in binary format, the leading bit can either be a 0 or a 1, but for a normalized number it will always be a 1. Therefore, in a system where numbers are always normalized, the leading bit need not be stored and can be implied, effectively giving the mantissa one extra bit of precision. Therefore, in single precision format, the mantissa typically includes 24 bits of precision. 
     However, the IEEE754-1985 standard requires continuous normalization—i.e., normalization after every step of a multistep computation—to maintain the leading “1” to preserve accuracy. This is expensive in terms of PLD resources, as each normalization operation requires two steps—(1) finding the position of the “1”, and (2) shifting the fractional part to get a leading “1” (which is then eliminated, because it is implied). 
     In accordance with the invention, there is no implied leading “1”, so that normalization is not required. Although this requires that one bit of precision be given up, because all bits must be kept, rather than implied, this greatly reduces the required logic, particularly shifting logic, and therefore the latency of the floating point operations. Moreover, in a PLD that already has dedicated arithmetic circuits, such as multipliers and/or adders, that are capable of handling the extra bits, there is no additional cost in terms of logic resources to handle those extra bits. 
     Preferably, the floating point representation in accordance with the invention uses a signed fractional component, with greater precision. Some operations may be configured in general-purpose logic of the programmable logic device. However, multiplication, at least, is more efficiently performed in a dedicated multiplier such as may be available in the aforementioned DSP block. The extra precision in accordance with the invention requires large multipliers, which heretofore have consumed more resources than the shifting logic required for normalization. However, in the aforementioned STRATIX® II PLDs, as well as those described in copending, commonly-assigned U.S. patent applications Ser. Nos. 11/447,329, 11/447,370, 11/447,472 and 11/447,474, all filed Jun. 5, 2006, 11/426,403, filed Jun. 26, 2006, and 11/458,361, filed Jul. 18, 2006, each of which is hereby incorporated herein in its respective entirety, large dedicated multipliers are available, and are more efficient than shifting logic. This allows the efficient use of a signed fractional component. 
     Specifically, according to a preferred method according to the invention for configuring a programmable logic device to perform floating point operations, the programmable logic device preferably is configured so that floating point values in accordance with a first format, such as the IEEE754-1985 standard format, preferably are converted to an internal format for calculation purposes, and are reconverted to the standard format upon completion of the operations. 
     Whereas the IEEE754-1985 standard format includes a 24-bit unsigned mantissa (23 bits plus the implied “1”) and an 8-bit exponent, the internal format according to the invention preferably includes a 32-bit signed mantissa and a 10-bit exponent. When converting from the standard 24-bit format to the 32-bit format of the invention, the implied leading “1” of the mantissa is made explicit and preferably is initially positioned at the 28th bit location. This leaves the four most significant bits of the 32-bit number available for overflows as operations progress. For example, 16 additions could be performed before any overflow would consume all four bits. Similarly, because the original standard representation is only 24 bits wide, the four least significant bits also are available for any underflows that may occur. 
     If this method is implemented on the aforementioned STRATIX® II PLD, or on a PLD of any of the above-incorporated patent applications, which include DSP blocks capable of 36-bit multiplications, then the multiplications of the 32-bit mantissas can be accomplished within the 36-bit dedicated multipliers, making the multipliers more efficient. Moreover, the mantissa size could be expanded to 36 bits if necessary or desired. However, the invention could be implemented even where no dedicated multipliers are available, using programmed general-purpose logic. Moreover, if dedicated multipliers are available, but are only large enough for IEEE754-1985-compliant operations, computations other than multiplications could be performed in programmed general-purpose logic, with the multiplications being performed in the dedicated multipliers. In such a case, the values would have to be renormalized before each multiplication step, but would not have to be normalized for other steps either before or after a multiplication step, except at the end of the operation. 
     As stated above, preferably, and ordinarily, during floating point operations in accordance with the invention, the operands remain in the format according to the invention, and are converted back to their original format only upon completion of operations. Because of the initial presence of the leading and trailing bits, as well as the larger exponent size, during operations it is possible to continue beyond conditions that might have led to overflows or underflows in the original format, because of the possibility that the accumulation of further results may reverse the overflow or underflow condition. 
     However, if during operation the accumulation of underflows or overflows reaches the point that information may be lost—e.g., there would be an overflow if the data were converted back to the standard format, or an underflow would be approached such that fewer than three significant bits beyond the required mantissa precision (i.e., in this example, fewer than 1+23+3=27 bits) would remain—it may be desirable in accordance with the invention to normalize the data at an intermediate step to prevent lost of precision. In such a case, subsequent operations preferably would not include further normalization until the final result is achieved (unless a condition again arises in which data may be lost). 
     Alternatively, if overflow or underflow is likely (e.g., there will be many operations in a calculation), then the start position of the mantissa can be changed from the 28th bit position to another position (to the right to prevent overflows; to the left to prevent underflows). The correct result can be maintained by adjusting the exponents accordingly. The larger exponent size (10 bits instead of 8 bits) allows room for the necessary exponent adjustments. 
     The examples that follow illustrate configurations, in accordance with the invention, of a programmable logic device to perform a number of different arithmetic operations. For simplicity, these examples do not show pipelining between stages, nor do they show circuitry for handling special cases, such as zero, infinity or not-a-number (NAN) situations. 
     The examples include conversions in both directions between the format of the IEEE754-1985 standard and the internal format according to a preferred embodiment of this invention. Preferred embodiments of those conversions are illustrated in  FIGS. 1 and 2 . 
       FIG. 1  shows a preferred embodiment of the conversion  10  from a value represented in the IEEE754-1985 standard format  11  to the same value represented, in the embodiment described above, in the format  12  according to the present invention. As seen, in format  11 , the value is indicated by three numbers representing the sign  110 , the mantissa  111  and the exponent  112 . As indicated above, sign  110  is one bit wide, mantissa  111  is 23 bits wide but represents 24 bits of precision because it has an implied leading “1”, and exponent  112  is eight bits wide. In format  12 , there is no separate number representing the sign, while signed mantissa  120  is 32 bits wide and exponent  121  is ten bits wide. 
     As shown in  FIG. 1 , exponent  112  converts directly to exponent  121  by the addition of two leading zeroes, as clearly the value of the exponent cannot change. The availability of two extra bits, however, provides for greater ranging of the value during internal computations, prior to conversion back to the IEEE754-1985 standard. This helps reduce the occurrence of overflows and underflows. 
     Sign bit  110  and 23-bit-wide mantissa  111  (carrying 24 bits of precision) convert to four sign bits  122  and 24-bit mantissa portion  123  with the aid of exclusive-OR (XOR)  124 . Four trailing bits  125  (because again the value cannot change in the conversion) are added to provide 32-bit mantissa  120 . The trailing bits are zeroes for positive numbers, and ones for negative numbers (which are inverted). 
     The operation of XOR  124  preferably is as follows: 
     If sign bit  110  is a “0”, then XOR  124  has no effect. If sign bit  110  is a “1” (signifying a negative number), then XOR  124  inverts the mantissa—i.e., it converts the mantissa to a one&#39;s-complement number. The actual computation requires a two&#39;s-complement number. The one&#39;s-complement number can be converted to a two&#39;s-complement number by adding a “1” to the least significant bit of the one&#39;s-complement number. An adder can be provided as part of each conversion  10 . However, such adders are very large, and because the precision of the mantissa in the format according to the present invention is greater than that of the IEEE754-1985 mantissa, it is also possible to omit this addition completely without significantly affecting the result. 
     As a third alternative, a single adder can be provided after a group of conversions  10 , which adds to the result a number equal to the total number of negative numbers within that group of conversions  10 . For example, if in an addition of eight numbers (meaning there are eight conversions), five of those numbers are negative, the value 5 10  (101 2 ) can be added to the one&#39;s-complement result to give the two&#39;s-complement result. This becomes more complicated in the case of multiplications, but can still be used where there is a local cluster of operations. 
       FIG. 2  shows the conversion  20  from a value represented in the format  12  according to the present invention to the same value represented in the IEEE754-1985 standard format  11 . As seen, in block  21 , the absolute value of mantissa  120  is taken, because the mantissa is signed in the format according to the invention, but unsigned in the IEEE784-1985 format. After that, the conversion operates similarly to the conversion under the IEEE754-1985 standard. Thus, in block  22  the number of leading zeroes is counted to find the implied leading “1”. The mantissa is then left-shifted in block  23  by the number found in block  22 . Any necessary rounding is performed in block  24 . To convert exponent  121 , an offset adjustment is subtracted from the exponent by subtractor  25  to account for the position of the implied leading “1” during the original conversion  10  to the format according to the invention. The offset adjustment typically is 4 10  (100 2 ) a shown, but if the mantissa size or position of the leading “1” on conversion changes, the offset adjustment to the exponent would change as well to compensate. 
     In  FIGS. 3-6 , “IF” (internal format) refers to a preferred embodiment (as described above) of the format according to the present invention, while “IEEE754” refers to the format according to the IEEE754-1985 standard. 
       FIG. 3  shows a simple case  30  of configuring logic to add two floating point numbers a and b to obtain their floating point sum c. In this case  30 , a first block  31  of logic preferably is configured from programmable logic to convert a from the IEEE754-1985 standard format to the internal format, and a second block  32  of logic preferably is configured from programmable logic to convert b from the IEEE754-1985 standard format to the internal format. Blocks  31  and  32  preferably are substantially identical. An arithmetic logic unit (ALU)  33  preferably is configured from programmable logic to add the values of a and b in the internal format. A block  34  preferably is configured to convert the result c back to IEEE754-1985 standard format from the internal format. 
     ALU  33 , operating in the internal format, is simpler than a corresponding ALU operating in the IEEE754-1985 standard format because it does not have include the shifting logic needed to deal with the implied leading “1” and therefore is about half the size of a standard-format ALU. However, conversion  34  adds about the same amount of logic as ALU  33 , while conversions  31 ,  32  add a negligible amount of logic. Therefore, in this simple example of adding two numbers, the net result is approximately the same under either format in terms of circuit size and latency, because one standard-format ALU is replaced with two blocks each about half the size of the standard-format ALU. 
       FIG. 4  shows the more complex case  40  of adding together four numbers a, b, c and d. In this case  40 , blocks  41 - 44  of logic preferably are configured from programmable logic to respectively convert a, b, c and d from the IEEE754-1985 standard format to the internal format. A first arithmetic logic unit (ALU)  45  preferably is configured from programmable logic to add the values of a and b in the internal format. ALU  46  similarly may be configured from programmable logic to add c and d in the internal format. ALU  47 , also similarly configured from programmable logic may be provided to add the intermediate sums from ALU  45  and ALU  46 . Block  34  preferably is configured from programmable logic to convert the result e back to IEEE754-1985 standard format from the internal format. 
     As in case  30 , each of conversions  41 - 44  adds negligible logic, while each of ALUs  45 - 47 , as well as conversion  34 , adds half the logic of a standard-format ALU. Using the standard format, the operation of case  40  could have been performed with three standard-format ALUs, while in case  40 , it is performed with four blocks (three ALUs  45 - 47  and conversion  34 ) approximating in total the size of two standard-format ALUs. Thus, the circuit is about two-thirds the size using the format according to the invention as compared to the standard format. 
       FIG. 5  shows a preferred embodiment of the configuration of an ALU  50  for addition using the format according to the invention, which may be used as ALU  33 ,  45 ,  46  or  47 . ALU  50  is similar to the first portion of a standard-format ALU. However, there is no normalization after the addition, which is where the size savings occurs. The two exponents  51 ,  52  are compared at  53 , and the result is used to control multiplexers  54 ,  55  to right-shift the mantissa  56 ,  57  of the smaller number. Depending on the signs of the inputs and the ADDSUB control signal  58 , none, one, or both of the numbers are one&#39;s-complemented at  59 , then added at  500 . The larger of the two input exponents  51 ,  52  is the resultant exponent  501 . In a preferred embodiment, ALU  50  may be configured from programmable logic, but alternatively some of the components, such as adder  500 , may be provided as dedicated circuitry. 
       FIG. 6  shows a preferred embodiment of the configuration of a multiplier  60  for multiplication using the format according to the invention. Multiplier  60  is similar to the first portion of a standard-format multiplier. Multiplication circuitry  61  may be configured from programmable logic but preferably may be provided as dedicated circuitry as described above. As in the addition case, there is no normalization after the multiplication, resulting in size savings as compared to a standard-format multiplication. However, for multiplication, the normalization range is more limited—typically only one bit instead of 23 bits—so the savings are smaller. Small additional savings result from the elimination of the need for rounding, which is required in the standard format. 
     Instructions for carrying out the method according to this invention may be encoded on a machine-readable medium, to be executed by a suitable computer or similar device to implement the method of the invention for programming or configuring PLDs to perform arithmetic operations in accordance with the format describe above. For example, a personal computer may be equipped with an interface to which a PLD can be connected, and the personal computer can be used by a user to program the PLD using a suitable software tool, such as the QUARTUS® II software available from Altera Corporation, of San Jose, Calif. 
       FIG. 7  presents a cross section of a magnetic data storage medium  600  which can be encoded with a machine executable program that can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium  600  can be a floppy diskette or hard disk, or magnetic tape, having a suitable substrate  601 , which may be conventional, and a suitable coating  602 , which may be conventional, on one or both sides, containing magnetic domains (not visible) whose polarity or orientation can be altered magnetically. Except in the case where it is magnetic tape, medium  600  may also have an opening (not shown) for receiving the spindle of a disk drive or other data storage device. 
     The magnetic domains of coating  602  of medium  600  are polarized or oriented so as to encode, in manner which may be conventional, a machine-executable program, for execution by a programming system such as a personal computer or other computer or similar system, having a socket or peripheral attachment into which the PLD to be programmed may be inserted, to configure appropriate portions of the PLD, including its specialized processing blocks, if any, in accordance with the invention. 
       FIG. 8  shows a cross section of an optically-readable data storage medium  700  which also can be encoded with such a machine-executable program, which can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium  700  can be a conventional compact disk read only memory (CD-ROM) or digital video disk read only memory (DVD-ROM) or a rewriteable medium such as a CD-R, CD-RW, DVD-R, DVD-RW, DVD+R, DVD+RW, or DVD-RAM or a magneto-optical disk which is optically readable and magneto-optically rewriteable. Medium  700  preferably has a suitable substrate  701 , which may be conventional, and a suitable coating  702 , which may be conventional, usually on one or both sides of substrate  701 . 
     In the case of a CD-based or DVD-based medium, as is well known, coating  702  is reflective and is impressed with a plurality of pits  703 , arranged on one or more layers, to encode the machine-executable program. The arrangement of pits is read by reflecting laser light off the surface of coating  702 . A protective coating  704 , which preferably is substantially transparent, is provided on top of coating  702 . 
     In the case of magneto-optical disk, as is well known, coating  702  has no pits  703 , but has a plurality of magnetic domains whose polarity or orientation can be changed magnetically when heated above a certain temperature, as by a laser (not shown). The orientation of the domains can be read by measuring the polarization of laser light reflected from coating  702 . The arrangement of the domains encodes the program as described above. 
     Thus it is seen that a method for carrying out floating point operations, a PLD programmed to perform the method, and software for carrying out the programming, have been provided. 
     A PLD  90  programmed according to the present invention may be used in many kinds of electronic devices. One possible use is in a data processing system  900  shown in  FIG. 9 . Data processing system  900  may include one or more of the following components: a processor  901 ; memory  902 ; I/O circuitry  903 ; and peripheral devices  904 . These components are coupled together by a system bus  905  and are populated on a circuit board  906  which is contained in an end-user system  907 . 
     System  900  can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, digital signal processing, or any other application where the advantage of using programmable or reprogrammable logic is desirable. PLD  90  can be used to perform a variety of different logic functions. For example, PLD  90  can be configured as a processor or controller that works in cooperation with processor  901 . PLD  90  may also be used as an arbiter for arbitrating access to a shared resources in system  900 . In yet another example, PLD  90  can be configured as an interface between processor  901  and one of the other components in system  900 . It should be noted that system  900  is only exemplary, and that the true scope and spirit of the invention should be indicated by the following claims. 
     Various technologies can be used to implement PLDs  90  as described above and incorporating this invention. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, the various elements of this invention can be provided on a PLD in any desired number and/or arrangement. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow.