Patent Publication Number: US-7716264-B2

Title: Method and apparatus for performing alignment shifting in a floating-point unit

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to floating-point units (FPUs) in general, and, in particular, to a method and apparatus for performing alignment shifting in a floating-point unit. 
   2. Description of Related Art 
   Floating-point numbers can be either single precision or double precision as defined by the Institute for Electrical and Electronic Engineers (IEEE) standard. Single precision floating-point numbers have one sign bit, eight exponent bits, and twenty-three mantissa bits with a one implicit bit. Double precision floating-point numbers have one sign bit, eleven exponent bits, and fifty-two mantissa bits with one implicit bit. 
   The computation logic for floating-point numbers can typically be divided into two types: multiply-add/subtract and distinct multiply and add/subtract. One of the more common methods associated with multiply-add/subtract computation logic is based on three operands A, B and C to provide the operation A*B+C. In order for a floating-point addition/subtraction to take place, the mantissas of two floating-point numbers must be aligned, which is commonly performed by an alignment shifter. 
   Referring now to the drawings, and specifically to  FIG. 1 , there is depicted a conventional alignment shifter. As shown, an alignment shifter  100  includes a shift amount calculator  111 , a shifter  112 , a limiter  113 , and a multiplexor  114 . Exponent EA of operand A, exponent EB of operand B and exponent EC of operand C enter shift amount calculator  112  and limiter  113  through a line  115 , a line  116 , and a line  117 , respectively. 
   Shift amount calculations are then performed after shift amount calculator  111  has received exponents EA, EB and EC. The right-shift amount is subsequently communicated to shifter  112  via a line  118 . The mantissa MC of operand C from a line  119  is right-shifted by shifter  112  accordingly. After all the shifting have been completed, the data are sent to multiplexor  114  via a line  121 . Limiter  13  provides control signals to multiplexor  114  via a line  122  to allow for multiplexor  114  to provide the necessary correction for overflow or underflow. 
   One problem associated with conventional alignment shifters, such as alignment shifter  100 , is under-utilization. Typically, a double-precision alignment shifter can handle either one single precision floating-point number or one double precision floating-point number. In a vectored floating-point implementation, such as single instruction multiple data (SIMD), it would be more efficient for the same double-precision alignment shifter to simultaneous align two single precision floating-point numbers without adding any delay to the critical path. 
   Consequently, it would be desirable to provide an improved method for performing alignment shifting such that all the resources of a double-precision alignment shifter can be fully utilized. 
   SUMMARY OF THE INVENTION 
   In accordance with a preferred embodiment of the present invention, an alignment shifter includes a shift amount calculator, a set of first level shifters and a set of second level shifter. The shift amount calculator generates one shift amount under a double-precision mode and two shift amounts under a single-precision mode. The first level shifters can concurrently receive two double-precision mantissas under the double-precision mode or two single-precision mantissas under the single-precision mode. The first level of shifts performs small shifts concurrently on the two double-precision mantissas according to the single shift amount, or on the two single-precision mantissas according to the two shift amounts. The second level shifters performs large shifts on outputs from the first level shifters to generate one double-precision floating-point result or two single-precision floating-point results. 
   All features and advantages of the present invention will become apparent in the following detailed written description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of an alignment shifter, according to the prior art; and 
       FIG. 2  is a block diagram of an alignment shifter, in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   In accordance with a preferred embodiment of the present invention, a multi-mode alignment shifter is utilized to support both single-precision and double-precision operations. The multi-mode alignment shifter includes two modes, namely, a single-precision mode and a double-precision mode. The operations of the multi-mode alignment shifter can be divided into three main steps: 
   Step 1: Bit Arrangement and Exponent Difference 
   Since exponents and mantissa are in different fields, they need to be arranged (or multiplexed) differently for single precision and double precision floating-point numbers. In double-precision mode, two copies of a 53-bit mantissa are identical. In single-precision mode, the first of two 24-bit single-precision mantissas is loaded into the left 24 bits of the first 53-bit copy of the mantissa with the remaining 29 bits set to zero, and the second of two 24-bit single-precision mantissas is loaded into the second copy of the 53-bit mantissa shifted right by N positions with the other bits set to zero. The variable N is selected so that the multiplier decimal point of the double-precision result is at the same place as the multiplier decimal point of the single-precision result. For the present embodiment, N is set to 29, as follows: 
   A double-precision floating-point number having mantissa=53 bits+2 safety bits+106 bit product: 
   0 . . . 52 53 54 44 56 . . . 57 58 59 . . . 160 
   A single-precision floating-point number having mantissa=24 bits+2 safety bits+48 bit product: 
   0 . . . 23 24 25 26 27 . . . 28 29 30 . . . 73 
   The double-precision multiplier decimal point is located after bit position 56, and the single-precision multiplier decimal point is located after bit position 27. Thus, N=56−27=29. 
   There are two parallel calculations of the exponent difference (Ex,Ey). In double-precision mode, Ex and Ey preform the same calculation in parallel. In single precision mode, Ex and Ey preform different calculations. The Ey calculation belongs to the second set of single-precision exponents. Since the mantissa is shifted N bit positions in the value fed into the shifters, no special modifications are required to the shifter controls. The Ex calculation belongs to the first set of single precision exponents. In the final result the single-precision result for the first operand will be in discontinuous fields. The first half of the field is in positions [0:23], and no modification is required for the shift controls for that field. The second half of the field is in positions [104:151]. Bit  24  moves to bit position 104, so the value of 80 needs to be added to the shift controls for multiplexor in this field in single-precision mode. Since the value 80 is evenly divisible by 16, the timing critical controls to shifters for the small shifts of 0-15 do not need to be adjusted. The adjustment only needs to take place for the less timing critical controls for the larger shifts. 
   Step 2: Small Shifts in Parallel 
   Mantissas are shifted using the calculated exponent difference. The least significant bits (LSBs) of the shift amount are available first so the LSB shifts can begin before the most significant bit (MSB) shift amounts are available. The first several shifts are performed in parallel. In the double-precision mode, the two shifts use the same data. In the single-precision mode, some zeroes are padded into the data. There are two parallel single-precision shifts for the small (LSB) shift amounts. The right shift is for distances 0-15. 
   Step 3: Large Shifts 
   In double-precision mode, the bits are arranged so that there is only one result. In single-precision mode, the bits are arranged so that there are two results. The data for the shifters that belong to the first single-precision result are fed only from copy 1 of the small shift results. The data for the shifters that belong to the second single-precision result are fed only from copy 2 of the small shift results. The result for the second single-precision operation is in contiguous fields in the middle of the double-precision result. The shift controls for this field require no modification since the adjustment of N positions was made in step 1. The multiplier decimal point for this single-precision field is the same as the multiplier decimal point for the double-precision field. The result for the first single-precision field is split into two non-adjacent fields separated by 80 positions. 
   The first part of the sp_result — 1_field starts in positions 0, and no modification is required for the controls to those shifters. The second part of the sp_result — 1_field starts 80 positions after the ending of the first half of the sp_result — 1_field, so that in single-precision mode, the value 80 must be added to the controls to those fields. Since 80 is evenly divisible by 16, and the second level multiplexors shift by multiples of 16, all of the entire adjustment only needs to be applied to the controls of the second level multiplexors for the second part of the single-precision_result — 1 field. The hardware for performing large shifts is shared, and there is no additional multiplexing required. As in the prior art, after the large shifts, the result still needs to be modified for the right shift overflow and right shift underflow cases. 
   Referring now to the drawings and in particular to  FIG. 2 , there is depicted a block diagram of a multi-mode alignment shifter, in accordance with a preferred embodiment of the present invention. As shown, a multi-mode alignment shifter  200  includes a shift amount calculator  220 , multiplexors  202 ,  204 , shifters  210 ,  212  and shifters  270 ,  274 ,  278 . Multi-mode alignment shifter  200  differs from alignment shifter  100  (from  FIG. 1 ) in that multi-mode alignment shifter  200  can preform one double-precision floating point calculation or two single-precision floating point calculations concurrently. 
   Multi-mode alignment shifter  200  includes two modes, namely, a single-precision mode and a double-precision mode. In double-precision mode, both multiplexors  202  and  204  receive a 53-bit double-precision mantissa via a line  236 . In single-precision mode, a first 24-bit single-precision mantissa is placed inside the 53-bit wide output of multiplexor  202  via a line  232 , and a second 24-bit single-precision mantissa is placed inside the 53-bit wide output of multiplexor  204  via a line  234 , concurrently. For multiplexor  202 , the first single-precision mantissa is placed at bit positions [0:23] with [24:52] set to zero. For multiplexor  204 , the second single-precision mantissa is placed at positions [29:52] with [0:28] set to zero. This is equivalent to a right shift of N bit positions (N=29 for the present embodiment) for the second single-precision field so that if the shift amount is zero, single-precision mantissa bit  0  will be in output bit position 29, which is the 0 th  position in the output field for the second single-precision mantissa. A mode control input (not shown) is utilized to control multiplexors  202  and  204  for selecting either single-precision floating-point numbers or double-precision floating point numbers. The output of multiplexor  202  is sent to shifter  210  via a line  244 , and the output of multiplexor  204  is sent to shifter  212  via a line  246 . 
   The exponents of the product and addend of the first single-precision floating-point number (EA1, EB1, and EC1) are sent to shift amount calculator  220  via a line  238 . The exponents of the product and addend of the second single-precision floating-point number (EA2, EB2, and EC2) are sent to shift amount calculator  220  via a line  242 . The exponents of the product and addend of the double-precision floating-point number (EA, EB, and EC) are sent to shift amount calculator  220  via a line  240 . 
   After all the proper data have been received, shifting calculations can begin. Based on the mode selected, shift amount calculator  220  produces either a single shift amount under double-precision mode, or two shift amounts under single-precision mode. If double-precision mode is selected, the four LSBs of the right shift amounts (controls for shift distances 0-15) are sent to shifter  210  on a line  221 , and to shifter  212  on a line  223 . The other MSBs of the right shift amount are sent to shifter  270  on a line  225 , to shifter  274  on a line  227 , and to shifter  278  on line  229 . 
   If single-precision mode is selected, the four LSBs of the right shift amount from the second shift amount calculation is sent to shifter  212  via line  223 , and the other MSBs of the right shift amount are sent to shifter  274  via line  227 . Concurrently, the four LSBs of the right shift amount from the first shift amount calculation is sent to shifter  210  via line  221 , and the other MSBs of the right shift amount are sent to shifter  270  via line  225 . Line  229  is used to send a modified copy of the MSB shift amount to shifter  278 . The modification is to add X (80 for the present embodiment) to the shift amount, to account for the discontinuity in the first single-precision field. Since the lower four binary digits of X (80 for the present embodiment) are “0000,” no adjustment is necessary for the small shifts (0-15) preformed in the first level of multiplexors. The LSB shift amount is more timing critical than the MSB shift amount, so the overall delay is not increased by a simple manipulation of the larger shift amount bits. 
   Shifters  210  and  212  preform shifts of distances 0-15 bits; since the inputs are 53 bits wide, the right shifted output is 68 bits wide. In double-precision mode, the input data and shift mounts to shifters  210 , and  212  are identical, so the output is also identical. In single-precision mode, the input data and shift amount to shifter  210  belong to the first single-precision number, the input data and shift amount to shifter  212  belong to the second single-precision number, the input data to shifter  210  was not pre-shifted, and the input data to shifter  212  was pre-shifted right 29 positions to correlate to the position of the SP2 output field relative to the double-precision data flow. A shift amount of zero in single-precision mode will put the MSB of the addend at position 29 of the double-precision data flow because the SP2 addend was pre-shifted 29 positions by multiplexor  204 . Shifter  210  sends its 68 bit output to shifters  270  and  278  via line  211 . Shifter  212  sends its 68 bit output to shifter  274  via a line  213 . 
   The double-precision output positions are somewhat arbitrary in that the number of safety bits can vary. The large shift right (by multiples of 16) is preformed by shifters  270 ,  274 , and  278 . In double-precision mode, the right shift amounts to shifters  270 ,  274 , and  278  are identical and the combined effects of shifters  270 ,  274 , and  278  are to act as one large shifter. Shifter  270  correlates to double-precision output bit positions [0:28], shifter  274  correlates to double-precision output bit positions [29:102], and shifter  278  correlates to double-precision output bit positions [103:160]. 
   In single-precision mode, shifter  274  is used to create the second single-precision output result field, and its input data originates from multiplexor  204 , which comes from the second single-precision addend. The data was placed into multiplexor  204  right shifted by 29 positions so that a shift amount of 0 will place the addend MSB in position N (29 in the present embodiment) of the double-precision data flow. Position N of the double-precision data flow correlates to position 0 of the SP2 output field. In single-precision mode, shifters  270  and  278  are used to generate the SP1 result field. The field is not continuous relative to the double-precision data flow. Shifter  270  creates SP1 output bit positions [0:23] which correlate exactly to double-precision output bit positions [0:23]. Shifter  278  creates SP1 output bit positions [24:73] that corresponds to double-precision output bit positions [104:151]. Since shifter  270  correlates directly to the double-precision bit positions, the shift amount does not require any adjustment, however, since shifter  278  does not correlate to the double-precision output positions, some adjustments to the shift amount are necessary in double-precision mode. Since there is a gap of X (80 in the present embodiment) between the two parts of the output field, the shift amount needs to be adjusted by X for shifter  278 . Shift amount calculator  220  provides the shift amount to shifter  270  on line  225 , and provides the shift amount to shifter  278  on line  229 . Line  225  is the normal version of the MSBs of the SP1 shift amount calculation. Line  229  differs from  229  in that it adds X (80 in the present embodiment) to the shift amount in single-precision mode. Since X=80 is evenly divisible by 16, it does not effect the timing critical shift amount to shifter  210 . The large shift controls are not as timing critical as the small shift controls, so the manipulation of the large shift controls should not increase the delay of alignment shifter  200 . 
   Except for the width of the product, the width of the output fields is somewhat arbitrary depending on factors such as the number of safety bits used, and method for handling shift underflow and shift overflow. The targeted format is of the form [M,S,P], where M is the width of the mantissa (53 for double-precision, 24 for single-precision), S is two safety bits (arbitrary width), P is the width of the product (106 for double-precision, 48 for single-precision). The multiplier decimal point is located between the second and third product bits. The output width is (53+2+106=) 161 bits for double-precision mode, and is (24+2+48=) 74 bits for single-precision mode. It is desirable to align one of the single-precision product decimal points with the double-precision decimal point (between 56 and 57 in the data flow of the present embodiment). 
   In double-precision mode, the output bit positions are labeled [0:160]. In double-precision mode, shifter  270  produces double-precision output bit positions [0:28], shifter  274  produces double-precision output bit positions [29:102], and shifter  278  produces double-precision output bit positions [103:160]. In single-precision mode, shifter  274  produces SP2 output bit positions [0:73], shifter  270  produces SP1 output bit positions [0:23] followed by five unused bits, shifter  278  produces one unused bit followed by SP1 output bit positions [24:73] followed by nine unused bit positions. The decimal point for the double-precision product and the SP2 product are in the same position. No extra multiplexors are required on the second level of shifters to accommodate the output of two aligned single-precision numbers. The outputs of shifters  270 ,  274 , and  278  are not the final aligned output. The result needs to be limited for the overflow and underflow cases, as it is well-known by those skilled in the art. 
   As has been described, the present invention provides an improved method and apparatus for performing alignment shifting in a floating-point unit. 
   While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.