Wire c0, c1, c2, c3 //temporary buses for cases of carriesĪssign ch = 1 //assign ch to high and cl to low Wire s0, s1, s2, s3 //temporary buses for cases of sums Wire P,G //buses for P and G outputs of fullAdder Reg ch, cl //temporary variables to define cases that previous carry is high or low Output sum //sum output of the adder, 4 bits wide Module carrySelect (sum, cout, a, b, cin)
Full Adder body, define structure and internal wiring Define all inputs and outputs for single bit Fulll Adder The second bit is calculated by XORing the propagate in second box from the right (a "0") with C0 (a "0"), producing a "0".Ĥ-bit Kogge-Stone adder, Radix-2, without Cin on Borland Turbo Basic 1.So I am trying to design a 4-bit carry select adder in verilog, and am using the following code for the design: module fullAdder (S,Cout,P,G,A,B,Cin)
E.g., the first (least-significant) sum bit is calculated by XORing the propagate in the farthest-right red box (a "1") with the carry-in (a "0"), producing a "1". The culminating generate bits (the carries) are produced in the last stage (vertically), and these bits are XOR'd with the initial propagate after the input (the red boxes) to produce the sum bits. Each vertical stage produces a "propagate" and a "generate" bit, as shown. Īn example of a 4-bit Kogge–Stone adder is shown in the diagram. The general problem of optimizing parallel prefix adders is identical to the variable block size, multi level, carry-skip adder optimization problem, a solution of which is found in Thomas Lynch's thesis of 1996. The Lynch–Swartzlander design is smaller, has lower fan-out, and does not suffer from wiring congestion however to be used the process node must support Manchester carry chain implementations.
However, wiring congestion is often a problem for Kogge–Stone adders. The Kogge–Stone adder takes more area to implement than the Brent–Kung adder, but has a lower fan-out at each stage, which increases performance for typical CMOS process nodes. Other parallel prefix adders (PPA) include the Sklansky adder (SA), Brent–Kung adder (BKA), the Han–Carlson adder (HCA), the fastest known variation, the Lynch–Swartzlander spanning tree adder (STA), Knowles adder (KNA) and Beaumont-Smith adder (BSA). In computing, the Kogge–Stone adder ( KSA or KS) is a parallel prefix form carry look-ahead adder. Graph of carry generator of a 4-bit Kogge–Stone adder with zero carry-in, Radix-2, valency-2.
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