Abstract:
A method and apparatus for overflow detection and clamping with parallel operand processing for fixed-point multipliers is disclosed. The invention predicts when a multiplication of a number of operands will exceed a pre-determined number of bits based upon the fixed-point format of the operands. The prediction is performed in parallel with the multiplication of the operands. The multiplication need not be completed in full, but only to the extent to determine whether overflow exists. If an overflow detection occurs, clamping is instituted. The parallel operation of the overflow detection and the multiplication provides a faster clamping circuit than would otherwise be available from a serial multiplication followed by a clamping analysis.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to the multiplication and clamping prediction of fixed-point multipliers. More particularly, the invention relates to a method and apparatus for increasing the speed of fixed-point data paths that involve multiplication of operands and parallel overflow detection and clamping based upon the magnitude of those operands. 
   Electrical circuits are routinely employed to perform arithmetic operations of operands represented by logical representations. Generally, it is desirable for arithmetic circuitry, and in particular multiplication circuitry, to have the fewest number of bits in order to perform the required calculations. Minimization of the required number of bits facilitates speed of the calculating circuit. However, counter-balancing the desire to have a limited number of bits to perform a given calculation is to avoid answer overflow. Overflow, or the situation in which an answer will exceed the number of bits designed for the answer, is not acceptable, as valuable valid data may be lost in performing the calculations. In circuits that perform calculations with a number of bits that may yield answers that overflow the set number of bits, clamping can be used to ensure that a result that overflows is clamped to a given acceptable value. Normally, the largest magnitude positive or negative representable number is employed as the overflow value. While the employment of a clamping operation is not always desirable, it is generally considered to be better than an overflow which may cause wrapping or undesired bits stored in the particular multiplication circuitry. 
   Typically, when clamping is desired, it is performed in a sequential fashion. In other words, the arithmetic operation is performed first, and when the result is available, it is then analyzed for overflow. If clamping is required, a clamping value replaces the computed value. 
   Referring now to  FIG. 1 , a prior system illustrates the serial processing in which the clamping analysis follows multiplication of particular operands. In this instance, operand  1  (reference numeral  2 ) and operand  2  (reference numeral  4 ) are input into an arithmetic operator  6 . The output  8  from the arithmetic operator  6  is then directed into the overflow detection scheme  10 . Any delays between arithmetic operator  6  and the overflow detection  10  are compounded, with each of their respective delays adding to the overall circuit delay. The clamp value  12  as determined by the overflow detection  10  is input along with the result  14  of the arithmetic operation  6 , and a preset clamp value  16  into multiplexer  18 . Multiplexer  18  selects the operation result  14  or the preset clamp value  16  in the instance of overflow detection based upon the logical level of clamp bit  12 . The result is then output into register  20 . 
   A major problem with sequential operation is that any delays, for both the arithmetic operation and the overflow analysis, tend to compound and therefore yield a relatively slow circuit to the extent that all delays are combined and added together to determine the total circuit delay. In any arithmetic operation, multiplication delays tend to be the largest. Therefore, there exists a need to implement a circuit and method in which arithmetic operations such as multiplication may be performed along with overflow detection. The result is a circuit that eliminates the compounded delay made up of the accumulation of each individual delay associated with sequential overflow analysis and operand processing. 
   SUMMARY OF THE INVENTION 
   The present invention provides overflow detection and clamping in parallel with multiplication of fixed-point multiplier operands that overcome the aforementioned problems, and provides a faster circuit than would otherwise be available from serial clamping analysis and arithmetic operation. 
   In accordance with one aspect of the invention, a method of detecting overflow in a clamping circuit includes inputting a first operand having a fixed-point format (i.e. represented by a fixed number of bits) into the clamping circuit and inputting a second operand having a second fixed-point format into the clamping circuit. A product overflow output is determined based upon the first and second fixed-point format. The method includes predicting whether multiplication of the first operand with the second operand yields a result that exceeds the product overflow output, and performing at least partially the multiplication of the first and second operands. The determining step occurs substantially in parallel with the performing step. 
   In accordance with another aspect of the invention, a method of clamp detection is disclosed, and includes inputting a first and second operand to both a multiplier and an overflow detection circuit. The method includes multiplying the first and second operands to generate a result not to exceed a pre-determined number of bits, and determining an initial clamping predictor bit based upon the first operand and the second operand. The initial clamping predictor bit is logically ORed with a most significant bit of the result to produce a final clamping predictor bit. 
   Again, the multiplying and determining steps occur substantially in parallel. 
   In yet another aspect of the invention, a multiplication overflow detection circuit is disclosed. The circuit includes multiplication circuitry for at least partially multiplying a first and a second operand, overflow detection circuitry receiving the first and second operands that detects whether a result of the multiplication of the first and second operands exceed a maximum representable positive or negative value. The multiplication circuitry and the overflow detection circuitry operate substantially in parallel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic of a prior detection overflow scheme showing serial operand operation and overflow detection. 
       FIG. 2  is a schematic of parallel operation of the arithmetic operator and the overflow detection in accordance with the present invention. 
       FIG. 3  is a table illustrating necessary information in determining whether to clamp multiplication of the two operands in accordance with one aspect of the present invention. 
       FIG. 4  is a graph illustrating the simple overflow predictor and the regions necessary for additional overflow prediction calculations. 
       FIG. 5  shows a schematic of another aspect of the present invention. 
       FIG. 6  shows a schematic of another aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 2 , a clamping system  22  in accordance with the present invention is illustrated. Operand  1  is loaded into register  24  and operand  2  is similarly loaded into register  26 . Registers  24 ,  26  are preferably flip-flops, but any suitable register capable of storing the operands are contemplated. Also, the operands may be passed directly from other arithmetic operators or other related logic, if suitable for the application. Operand  1  and operand  2  are loaded in a fixed-point format. 
   In fixed-point arithmetic, numbers are represented by zero or one sign bit, zero or more integer bits, and zero or more fractional bits. The integer and fractional bits can be a magnitude, a 1&#39;s complement, or a 2&#39;s complement value. The most common case is the 2&#39;s complement case. A short-hand description is used to denote how many bits there are in each category. This description is typically: &lt;sign-bits&gt;.&lt;integer-bits&gt;.&lt;fractional-bits&gt;or s.i.f. For a signed number with 4 integer bits and 5 fractional bits the notation would be. 1.4.5. For an unsigned number with 6 integer bits and 3 fractional bits the notation would be 0.6.3, and often the leading zero is omitted to yield 6.3. 
   For a given fixed-point representation s.i.f there are associated numerical properties, (and assuming that the 2&#39;s complement is used):
         Largest positive number representable: 2 i −2 −f      Largest (magnitude) negative number representable: −2i   Smallest positive number representable: 2 −f      Smallest (magnitude) negative number representable: −2 −f          

   When fixed-point numbers are used in arithmetic operations such as additions, subtractions, and multiplications, the results generally require more bits to avoid overflow. 
   The rules for the two types of operations are: 
   1. Addition and subtraction:
         If both inputs are of the same format s.i.f then the output requires s.(i+1).f to avoid overflow.   If the inputs are of different formats s.i1.f1 and s.i2.f2, then choose i=max(i1,i2) and f=max(f1,f2). Then the output requires s.(i+1).f bits to avoid overflow.       

   2. Multiplication:
         If the inputs are of the same format s.i.f then the output requires s.(2*i+1).(2*f) to avoid overflow.   If the inputs are of different formats s.i1.f1 and s.i2.f2, then the output requires s.(i1++i2+1).(f1+f2) to avoid overflow.       

   Since multiplication most often produces the need for overflow detection and clamping, it is preferred that the present invention be utilized in multiplication of operand  1  and operand  2 , with each operand in a fixed-point format. As  FIG. 2  demonstrates, operand  1  arid operand  2  are both supplied to the overflow detection  28  as well as the arithmetic operator or multiplier  30 . The multiplier  30  takes operand  1  and operand  2  and determines at least a partial product of the binary operands. It is important to note that the multiplier does not have to be full precision. It need only be twice the precision that is desired to be represented in the final result. For example, if it is desired to multiply two operands, each having eight bits, when the multiplication of eight bits and eight bits occurs, the answer will have sixteen bits of output. However, if it is only desired to have eight bits to come out in the final. output, when the multiplication could actually generate up to sixteen bits, the multiplier will be used to generate nine bits. The nine bits represent twice the desired output range. Therefore, in the above-identified example, the answer will have a nine-bit result instead of the full sixteen-bit result. The final seven bits of the multiplication are not necessarily calculated. 
   Operand  1  and operand  2  are also presented to the overflow detection logic. The overflow detection  28  occurs in parallel with the multiplier  30  operation. It is the substantially simultaneous processing of the overflow detection and the multiplication that produces the efficiencies in processing time for the circuit. Also, it is the fact that the full product need not necessarily be calculated that results in further efficiencies in processing time. 
   The overflow detection circuit  28  considers the two input operands and predicts whether or not they will cause the product to overflow. The overflow prediction circuit  28  predicts when the product is going to be greater than or equal to two times the maximum desired value. This handles most of the clamping cases, but it is not exact. To get exact clamping, the product is computed to the (io+1)&#39;th integer bit, and the most significant bit of the product  37  is used to finally determine whether or not to clamp. 
   The fixed-point format of the two inputs and the desired output must be known. These formats are denoted as: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Operand1: 
               s.i1.f1 
             
             
                 
               Operand2: 
               s.i2.f2 
             
             
                 
               Output: 
               s.io.fo 
             
             
                 
                 
             
           
        
       
     
   
   The product of the two input operands will require s.(i1+i2+1).(f1+f2) to avoid overflow. The assumption is that io&lt;i1+i2+1, (because, if io&gt;i1+i2+1 then an overflow can not occur). 
   EXAMPLE 
   Consider the simple case of two positive input operands that yield a positive product. The maximum positive number that can be represented by the desired output representation is:
 
2 io −2 −fo  
 
   Therefore, any combination of input operands that yields a product greater than or equal to the maximum value should be clamped to the maximum positive value:
 
Output= op 1 *op 2 if ( op 1 *op 2)&lt;2 io −2 −fo  
 
Output=2 io −2 −fo  if ( op 1 *op 2)≧2 io −2 −fo  
 
   The result  32  of multiplier  30  (which will in a preferred embodiment be a partial multiplication) is output from the multiplier  30 . The detection overflow  28  outputs result  34  from its overflow detection circuitry, and as a result of a logical level of the clamp bit  34  it is determined whether clamping will occur. The most significant bit  37  (on line  36 ) of result  32  is logically ORed with clamp bit  34  through OR gate  38 . Therefore, if any of clamp bit  34  or the most significant bit  37  is logically high, clamping will occur and a clamping signal  40  is output from OR gate  38 . The clamping signal  40  is input into multiplexer  42 . Result  32  of the multiplication is also input into multiplexer  42 . Clamp value  44  is preferably hard-wired into multiplexer  42 . The value of clamp value  44  is predetermined depending on the number of bits in operand  1  and operand  2 . Preferably, the clamp value is the maximum representable positive or negative value. It is contemplated that in selecting a positive or negative value for operand  1  or operand  2 , the MSB (most significant bit) of each register  24 ,  26  could be exclusively ORed (XOR) together such that if the operands are of a different sign, the negative clamping value will be used for clamp value  44 . Consequently, if both operands are of the same sign, the positive value for clamp value  44  will be used. Multiplexer  42  will select either result  32  or clamp value  44  depending upon the logical level of clamp signal  40 , and will output the selection into output register  46 . 
   Referring now to  FIG. 3 , a binary representation of several scenarios are given. 
   Take the case of two  1 . 6 . 4  operands, multiply them and return the product clamped to a 1.6.4 number. The maximum representable value in the output is 2 6 −2 −4 =64− 1/16=63.9375. Some simple cases exist: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               If op1 ≧ 32 and op2 ≧ 2 then clamp 
               (45a) 
             
             
                 
               If op1 ≧ 16 and op2 ≧ 4 then clamp 
               (45b) 
             
             
                 
               If op1 ≧ 8 and op2 ≧ 8 then clamp 
               (45c) 
             
             
                 
               If op1 ≧ 4 and op2 ≧ 16 then clamp 
               (45d) 
             
             
                 
               If op1 ≧ 2 and op2 ≧ 32 then clamp 
               (45e) 
             
             
                 
                 
             
           
        
       
     
   
   The binary representation of these scenarios is given in  FIG. 3 . 
   It can be seen that the number of leading zeros in the integer portion of the operands is indicative of the magnitude of the operands, and by adding the number of leading zeros of both op 1  (47) and op 2  (49), there is a constant number of leading zeros  51 . Therefore, the fixed-point format of the operands will determine the constant number of leading zeros to determine whether clamping occurs. Clamping must occur when:
 
If (&lt; op 1 leading zeros&gt;+&lt; op 2 leading zeros&gt;)≦4 then clamp Else don&#39;t clamp
 
   This relatively simple predictor works for a substantial portion of products, but it is not completely accurate. If only this predictor were used, it would be possible to get products that could be as much as twice the desired max value, i.e.,
 
(max product given simple predictor)&lt;2*(desired max value)
 
   Therefore, to get an accurate clamp predictor, the above simple predictor is used, and the multiplication generates a result that is of format s.(io+1).fo. Then, for positive operands, an overflow is detected by ORing together the MSB of the integer bits and the clamp-prediction. 
   Referring now to  FIG. 4 , a graph is shown showing the regions of products that are clamped by the initial clamp predictor. The broad region covered by the simple, initial clamp predictor is shown generally by the numeral  50 . Regions  52  represent those products that require the io+1 integer bits of the product to get accurate overflow detection and therefore require more precise clamp prediction. Region  51  represents the region where clamping is not needed because the product does not exceed the desired number of bits. However, this graph is representative of only one particular set of operands (both positive). Other predictors will produce different data depending upon the signs of the operands. 
   General Case 
   The general case consists of three subcases depending on the signs of the input operands: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Case 1: 
               (op1 &gt; 0 and op2 &gt; 0) 
             
             
                 
               Case 2: 
               (op1 &lt; 1 and op2 &lt; 1) 
             
             
                 
               Case 3: 
               (op1 &gt; 0 and op2 &lt; 1) or (op1 &lt; 0 and op2 &gt; 0) 
             
             
                 
                 
             
           
        
       
     
   
   Each of these cases will be examined in turn. 
   Case 1: Both operands are positive 
   The simple clamp predictor is: 
   If the sum of the input operands&#39; leading zeros is less than or equal to
 
( i 1 −io )+( i 2 −io )+( io −2)= i 1 +i 2 −io− 2
 
then the circuit must clamp.
 
   The accurate clamp prediction must use the (io+1) bit of the product, i.e., the product must be computed at least to (io+1) integer bits. This bit is ORed logically with the simple clamp predictor to yield the accurate clamp determination. 
   Case 2: Both operands are negative 
   When both operands are negative, their product is positive. Therefore, if an overflow case exists we clamp to the same value as mentioned before, namely:
 
2 io −2 −fo  
 
   When both operands are negative the simple clamp predictor must count leading ones in the input operands. If the sum of the input operands&#39; leading ones is less than or equal to
 
( i 1 −io )+( i 2 −io )+( io− 1)= i 1 +i 2 −io −1
 
then the circuit must clamp.
 
   However, there is an additional case, when both operands have only zeros after the leading ones (e.g., 111100:0000) then the simple clamp predictor should also clamp. This is like counting leading zeros on a bit-reversed version of each input. If the trailing zeros plus the leading ones equal (i+f) for both inputs then clamp. As before, the accurate clamp prediction must use the (io+1) bit of the product, i.e., the product must be computed at least to (io+1) integer bits. This bit is ORed logically with the simple clamp predictor to yield the accurate clamp determination. 
   Case 3: Only One of the Operands is Negative 
   When only one of the input operands is negative, their product will be negative. Therefore, if an overflow case exists we clamp to the largest (magnitude) negative value, namely, −2 io . 
   In this case, the simple clamp predictor must count leading ones for the negative input, and leading zeros for the positive input. Then, if the sum of the inputs&#39; leading ones and leading zeros is less than or equal to
 
( i 1 −io )+( i 2 −io )+( io− 2)= i 1 +i 2 −io− 2
 
   then the circuit must clamp. 
   As before, the accurate clamp prediction must use the (io+1) bit of the product, i.e., the product must be computed at least to (io+1) integer bits. As shown in  FIG. 6 . this bit is logically inverted, by inverter  70 , then ORed logically with the simple clamp predictor to yield the accurate clamp determination. 
   Referring now to  FIG. 5 , another embodiment of the present invention is shown. In this embodiment, the clamp bit may be stored in a clamp bit register  60  and the result of the multiplier operation may be stored in multiplier result register  62 . In this embodiment, after one of the result  62  and the clamp value  64  is selected, other logic  68  is introduced to process the output  66  as part of a pipelining stage. The pipelining stage enables further processing of a partial result if the computation is not completed in a single clock cycle. The clamp prediction along with a partial result is then saved for the following clock cycle where completion of the clamping prediction will occur in the second clock cycle. 
   The initial clamp predictor may be utilized as soon as the two operands are available. The final clamp predictor must occur after the result of the multiplier has completed its partial computation to the appropriate necessary bit. Therefore, the simple clamp prediction occurs at the same time as the multiplier as manipulating operand  1  and operand  2 . 
   Although two operands are shown, it is contemplated by the present invention that any number of operands may be used as inputs to the multiplier operation. In addition, the multiplication may occur in several stages with other multiplications coming before or after in a similar manner. The cloud of logic represents other multiplications, other additions or other logic operations on the result  66  of the multiplexer. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.