A carry-bypass adder is an adder

Adder (electronics)

In electronics, an adder or summer is a digital circuit that performs addition of numbers.In many computers and other kinds of processors, adders are used not only in the arithmetic logic unit, but also in other parts of the processor, where they are used to calculate addresses, table indices, and...

 implementatin that improves on the delay of a ripple-carry adder.

The two addends are split in blocks of n bits. The output carry of each block is dependent on the input carry only if, for each of the n bits in the block, at least one addend has a 1 bit (see the definition of carry propagation at Carry-lookahead adder). The output carry , for the block corresponding to bits i to i+n-1 is obtained from a multiplexer, wired as follows:

• 
• (the carry output for the ripple adder summing bits i to i+n-1)
• 

Implementation overview

Breaking this down into more specific terms, in order to build a 4-bit carry-bypass adder, 6 full adders would be needed. The input buses would be a 4-bit A and a 4-bit B, with a carry-in (CIN) signal. The output would be a 4-bit bus X and a carry-out signal (COUT).

The first two full adders would add the first two bits together. The carry-out signal from the second full adder ()would drive the select signal for three 2 to 1 multiplexers. The second set of 2 full adders would add the last two bits assuming is a logical 0. And the final set of full adders would assume that is a logical 1.

The multiplexers then control which output signal is used for COUT, and .

Verilog

module cba_4(A, B, X, CIN, COUT);
input [3:0]A, B;
input CIN;
output [3:0]X;
output COUT;

reg [3:0]X;
reg [2:0]base;
reg [2:0]ifzero;
reg [2:0]ifone;
reg COUT;

always @(A or B or CIN)
begin
base = A[1:0] + B[1:0] + {1'b0, CIN};
ifzero = A[3:2] + B[3:2];
ifone = A[3:2] + B[3:2] + 2'b01;

if(base[2])
begin
X = {ifone[1:0], base[1:0]};
COUT = ifone[2];
end
else
begin
X = {ifzero[1:0], base[1:0]};
COUT = ifzero[2];
end
end
endmodule