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{{Unreferenced stub|auto=yes|date=December 2009}}

A '''carry-skip adder''' (also known as a '''carry-bypass adder''') is an [[adder (electronics)|adder]] implementation that improves on the delay of a [[Ripple_carry_adder#Ripple_carry_adder|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 <math>\mathrm{Cout}_{i+n-1}</math>, for the block corresponding to bits ''i'' to ''i+n-1'' is obtained from a multiplexer, wired as follows:

* <math>SEL = (A_i + B_i) (A_{i+1} + B_{i+1}) ... (A_{i+n-1} + B_{i+n-1})</math>
* <math>A = \mathrm{C}_{ripple,i+n-1}</math> (the carry output for the ripple adder summing bits ''i'' to ''i+n-1'')
* <math>B = \mathrm{C}_{out,i-1}</math>

This greatly reduces the latency of the adder through its critical path, since the carry bit for each block can now "skip" over blocks with a ''group'' propagate signal set to logic 1 (as opposed to a long ripple-carry chain, which would require the carry to ripple through each bit in the adder).



==Implementation overview==

Breaking this down into more specific terms, in order to build a 4-bit carry-bypass adder, 6 [[Full_adder#Full_adder|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 (<math>C_1</math>)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 <math>C_1</math> is a logical 0. And the final set of full adders would assume that <math>C_1</math> is a logical 1.

The multiplexers then control which output signal is used for ''COUT'', <math>X_2</math> and <math>X_3</math>.

===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

{{DEFAULTSORT:Carry-bypass adder}};
[[Category:Adders]]


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