算术逻辑单元(arithmetic and logic unit) 是能实现多组算术运算和逻辑运算的组合逻辑电路，简称ALU。

module ALU(A, B, Cin, Sum, Cout, Operate, Mode); 

input [3:0] A, B; // two operands of ALU 
input Cin; //carry in at the LSB 
input [3:0] Operate; //determine f(.) of sum = f(a, b) 
input Mode; //arithmetic(mode = 1'b1) or logic operation(mode = 1'b0) 
output [3:0] Sum; //result of ALU 
output Cout; //carry produced by ALU operation 
// carry generation bits and propogation bits. 
wire [3:0] G, P; 

// carry bits; 
reg [2:0] C; 
reg Cout;

// function for carry generation: 
function gen; 
input A, B; 
input [1:0] Oper; 
begin 
case(Oper) 
2'b00: gen = A; 
2'b01: gen = A & B; 
2'b10: gen = A & (~B); 
2'b11: gen = 1'b0; 
endcase
end 
endfunction
// function for carry propergation: 
function prop; 
input A, B; 
input [1:0] Oper; 

begin 
case(Oper) 
2'b00: prop = 1; 
2'b01: prop = A | (~B); 
2'b10: prop = A | B; 
2'b11: prop = A; 
endcase
end 
endfunction 

// producing carry generation bits; 
assign G[0] = gen(A[0], B[0], Operate[1:0]); 
assign G[1] = gen(A[1], B[1], Operate[1:0]); 
assign G[2] = gen(A[2], B[2], Operate[1:0]); 
assign G[3] = gen(A[3], B[3], Operate[1:0]); 

// producing carry propogation bits; 
assign P[0] = prop(A[0], B[0], Operate[3:2]); 
assign P[1] = prop(A[1], B[1], Operate[3:2]); 
assign P[2] = prop(A[2], B[2], Operate[3:2]); 
assign P[3] = prop(A[3], B[3], Operate[3:2]); 

// producing carry bits with carry-look-ahead; 
always @(G or P or Cin, Mode) 
begin 
if (Mode) begin 
C[0] = G[0] | P[0] & Cin; 
C[1] = G[1] | P[1] & G[0] | P[1] & P[0] & Cin;
C[2] = G[2] | P[2] & G[1] | P[2] & P[1] & G[0] | P[2] & P[1] & P[0] & Cin; 
Cout = G[3] | P[3] & G[2] | P[3] & P[2] & G[1] | P[3] & P[2] & P[1] & G[0] | P[3] & 
P[2] & P[1] & P[0] & Cin; 
end 
else begin 
C[0] = 1'b0; 
C[1] = 1'b0; 
C[2] = 1'b0; 
Cout = 1'b0; 
end 
end 

// calculate the operation results; 
assign Sum[0] = (~G[0] & P[0]) ^ Cin; 
assign Sum[1] = (~G[1] & P[1]) ^ C[0]; 
assign Sum[2] = (~G[2] & P[2]) ^ C[1]; 
assign Sum[3] = (~G[3] & P[3]) ^ C[2]; 

endmodule