Abstract:
A new technique for the accumulation of partial product terms in a monolithic VLSI multiplier is disclosed. The method requires fewer than a 5% increase in transistors over older techniques yet provides more than three times the performance of the prior art when used to implement a 64×64 multiplier. The accumulator is implemented with one-bit cells to facilitate the VLSI mask design and is expandable to any desired precision.

Description:
BACKGROUND OF THE INVENTION 
     The multiplication of two N-bit operands is a fundamental operation in general purpose computer processors. To perform a longhand multiplication the first operand A is successively multiplied by each bit of the second operand B to create a partial product. The partial product is then shifted to assign the appropriate weight based on the weight of the corresponding digit of the second operand B. Finally, the shifted partial products are added together, i.e., accumulated, to form the final product D. 
     Several techniques have been devised to speed up and/or simplify the longhand multiplication described above. Usually, as explained by A. Booth, in &#34;A Signed Binary Multiplication Technique&#34;, Quarterly Journal of Mechanics and Applied Mathematics, Vol. IV, pt. 2, pp. 236-240 (1951), an encoding scheme is performed on the bits of the B operand by means of a signed-digit-carry set (SDC) of, for example, +2, +1, 0, -1, and -2, to reduce the number of partial products to be accumulated by one-half. The accumulation is then performed in a ripple fashion through the use of full adders arranged in a carry save format. Each partial product bit still requires one full adder. 
     A complete 8×8 encoded binary multiplier is shown in FIG. 1 with the related Boolean equations in Table 1. FIG. 1 consists of four sections; encoder logic 10, ripple accumulator 20, negative B operand correction 30, and a carry propagate adder 40. 
     
                       TABLE 1______________________________________L1 LOGICL1(K,J)    = [XP2(J) * A(K-1)] + [XP1(J) * A(K)] +       ##STR1##L2 LOGIC L2(2J)       ##STR2##L3 LOGICNEGA       = TCA * A(7) P       ##STR3## Q       ##STR4## BIT(2J+1)       ##STR5##BIT(2J)    = MINUS(J) XDR QMINUS(J+1) = MINUS(J) + P + QL4,L5,L6,L7 LOGICNEGB       =  TCB * B 7L4(K)      = [NEGB XDR SDC(B)] *      [NEGB XDR A(K)] L5       ##STR6##L5(J)      = XM1(J) + XM2(J)L7(J)      = A(-1) = .0.______________________________________ 
    
     The operands are A(0-7) and B(0-7) and the product is D(0-15). HADD and FADD are conventional half adders and full adders respectively with carry (C) and sum (S) ouputs. 
     The inputs TCA and TCB indicate whether the A and B operands are two&#39;s complement (=1) or unsigned (=0). The symbols XP2,  XP1, X0, XM1, and XM2 are used for the X(+2), X(+1), X(0), X(-1), and X(-2) encoded digits for clarity. &#34;*&#34; is the Boolean &#34;AND&#34;, &#34;+&#34; is the Boolean &#34;OR&#34;, and &#34;XOR&#34; is the Boolean &#34;Exclusive-OR&#34;. 
     In Table 1, logic block L1 consists of logic to select X(+2), X(+1), X(0), X(-2) multiples of the A operand. The multiples of B are generated in the array by simple shifting, complementing, or masking operations. The L2 block generates the lost bit that results from the single shift used to generate an X(+2) or X(-2) multiple. 
     
                       TABLE 2______________________________________B(2J+1) B(2J)   SDC(2J)   MULTIPLIER                               SDC(2J+2)______________________________________.0.     .0.     .0.       X.0.      .0..0.     .0.     1         XP1       .0..0.     1       .0.       XP1       .0..0.     1       1         XP2       .0.1       .0.     .0.       XM2       11       .0.     1         XM1       11       1       .0.       XM1       11       1       1         X.0.      1______________________________________ 
    
     Logic block L3 incorporates the signed digit encoding set of Table 2 as proposed by Booth. Block L3 also performs an effective sign extension of each partial product to 16 bits. The signal Minus(J) indicates that at least one previous bit pair generated a negative partial product. This signal is combined with the signed digit multiplier of a bit pair to generate bit (2J) and bit (2J+1). These two signals are the effective sign extension for each partial product fully merged with the sign extensions of all previously generated partial products. Logic blocks L4 and L5 perform a correction for a negative B operand and for a signed digit carry out of the last multiplier bit pair. Blocks L6 and L7 perform a two&#39;s complement operation on the A operand for X(-1) and X(-2) signed digits. 
     The advantage of this ripple adder technique is that it is usually convenient to lay out the required circuitry in monolithic single chip form. However, this technique is generally quite slow because the partial product accumulation takes place in ripple fashion, and the worst case delay path must pass through all of the rows of full adders. For example, in the case of the 8×8 multiplier with encoding as shown in FIG. 1 which will have four 8-bit partial products, the worst case delay will be equivalent to four full adder delays. Without encoding, the worst case delay would be eight full adder delays. In the case of a 64×64 multiplier with encoding, the worst case delay would be thirty-two, and without encoding the maximum delay would be sixty-four full adder delays. 
     SUMMARY OF THE INVENTION 
     The present invention replaces the standard ripple accumulator 20 described in FIG. 1. This new technique requires fewer than 5% more transistors to implement than the prior art shown in FIG. 1 yet is able to provide more than three times the performance of the prior technique for a 64×64 multiplier. Thus, rather than 32 full adder delays, the present invention can perform a 64×64 multiply with only 10 full adder delays. This compares favorably with the theoretical minimum delay of 8 full adders as explained by C. S. Wallace in &#34;A Suggestion for a Fast Multiplier&#34;, IEEE Transactions on Electronic Computers, pp. 14-17  (February 1964). 
     The new technique involves breaking the rows of the accumulator array into blocks. The blocks themselves are of varying length and are implemented as a series of one-bit ripple cells. To implement a multiplier of any desired precision only eight different cell types are required for the entire accumulator. The accumulation of the blocks still occurs in ripple fashion, but the ripple of the blocks occurs in parallel. The number of cells per block is arranged as an arithmetic progression to match the desired precision of the multiplication with the result that after the local ripple of the first block, each additional block requires only two more full adder delays. 
     Thus, the present technique can be used to provide for the accumulation of partial product terms in a high precision, high performance multiplier. The method is easily expandable to any desired precision. It is especially suited for utilization in a monolithic VLSI multiplier because of its high speed, low complexity, and straight forward mask design. 
     Generally, the present technique will be implemented in a multiplier which employs some form of encoding, as described earlier, since encoding can reduce the size of the accumulator by one-half, while the encoding itself usually requires only simple logic gates or multiplexors. However, the disclosed technique is a general method of accumulation and will function in a system which does not utilize encoding at all. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIGS. 1&#39; and 1A-1C show a complete 8×8 bit encoded binary multiplier according to the prior art. FIGS. 2A-2H show the set of cells required to implement the accumulation technique of the present invention. 
     FIG. 3 shows the organization of the cells of FIGS. 2A-2H into blocks to form a 64×64 bit accumulator with encoded B inputs. 
     FIG. 4 shows a modified version of the cell of FIG. 2D to attain the minimum circuit delay. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2A-2H show the complete set of cells necessary to implement the present digital accumulator. Shown are cells 1-8, respectively, which are organized, for example, as shown in FIG. 3 to form a 64×64 bit memory array which can replace the ripple accumulator array 20 of the type shown in FIG. 1. Cells 1-4 are used in the main array 310 of FIG. 3. Each cell is one bit by one bit in area, thus requiring a main array 310 of 64×32 cells in the example shown in FIG. 3 where encoding is assumed. The logic elements surrounding the main array 310 (encoder logic 10, negative operand correction 30, and the carry propagate adder 40 shown in FIG. 1) are unchanged except for a single column 320 of end cells 5-8 which are also one bit in area as shown in FIGS. 2E-2H respectively, on the right side of array 310. The logic element L1 used in cells 1-4 and logic element L6 used in cells 5-8 are the same elements as used in the prior art as illustrated in Table 1. The full adders FADD are also of convention design as used in the prior art. 
     As shown in FIG. 3, the accumulation technique involves breaking the rows of the matrix array 310 with end slice 310 into blocks. In the 64 bit example shown, the 32 rows of partial product blocks are broken into five blocks 1-5 of height 4, 4, 6, 8, and 10 bit rows. Each row is coupled to one pair of encoded B operand bits. In addition the matrix array 310 is composed of 64 columns of cells as shown by bit slice 330. Each slice 330 is coupled to one bit of the A operand. 
     Each slice 330 of blocks 1-5 in the main array 310 is implemented as a cell 1, followed by a cell 2, followed by one or more of continue cells 3, and finished by a cell 4, Similarly, the end cell slice 320 of each block 1-5 is implemented as a cell 5, followed by a cell 6, followed by one or more of cells 7, and finished by a cell 8. Each block 1-5 performs a local ripple accumulation of its partial product terms, labeled as LC a , and LC b , with a relative weight depending on the corresponding row position of the cell involved. For example, LC a5  is a local ripple term with a relative bit weight of 5. 
     The ripple accumulation of the blocks 1-5 is accomplished by the global ripple terms G a  and G b , and occurs in parallel. The length of the blocks 1-5 is selected to form an arithmetic progression, for example, 4-4-6-8-10 as illustrated in FIG. 3, which matches the number of partial products required. An ideal progression would be of the form 1-2-3-4-5, but the sum of the lengths must match the number of partial products and the technique requires four rows in the first block as explained above. Thus a progression of 4-6-6 would best match an accumulator with sixteen partial products and 4-4-6-8-10 effectively matches a system with 32 partial products. The result of arranging the length of blocks to form an arithmetic progression is that as block 1 finishes rippling, its terms are ready to be accumulated into the two global bits G a  and G b  along with the output terms of block 2. The output terms of block 2 then pass down the array 310 and end slice 320 and are accumulated with the local bits LC a  and LC b  of block 3, and so on. The result is that after block 1, each additional block requires only two more full adder delays. 
     As each local bit LC a  or LC b  or global bit G a  and G b  is passed down the array 310 with 320 from one row to the next, it must be shifted two bits to the right in order to be assigned the proper bit weight. This is the reason for the additional full adders in cells 5-8 in the end slice 320. The result is that the total number of intermediate product terms P a  and P b  is reduced to two terms per bit so that the intermediate product terms P a  and P b  may then pass on to a conventional carry propagate adder 40 (FIG. 1), which adder 40 produces the final multiplier product bits D. 
     Since the first global bits G a  and G b  on the top row of the main array 310 are all zero, as shown in FIG. 1, cell 4 of block 1 may be modified as shown in FIG. 4 to remove the logically redundant full adders in that cell to attain the minimum overall delay in the entire array 310.