超前进位４位加法器74LS283的VHDL程序实现

　　由于串行多位加法器的高位相加时要等待低位的进位，所以速度受到进位信号的限制而变慢，人们又设计了一种多位数超前进位加法器逻辑电路，使每位求和结果直接接受加数和被加数而不必等待地位进位，而与低位的进位信号无关，这就大大的提高了运算速度。现在简单介绍超前进位的运算方法，以及VHDL可编程逻辑编程。

 Library   ieee;
Use    ieee.std_logic_1164.all;
entity     CQadd_Co    is
    port(C0,A1,B1,A2,B2,A3,B3,A4,B4:In std_logic;
          S1,S2,S3,S4,Co:out std_logic);
end CQadd_Co;
architecture    adder    of      CQadd_Co      is
    signal    c1,c2,c3,t1,t2,t3,t4,m1,m2,m3,m4:std_logic;
    signal    P1,P2,P3,P4:std_logic;
       begin
          t1<=not(A1 AND B1);m1<=not(A1 or B1);
          t2<=not(A2 AND B2);m2<=not(A2 or B2);
          t3<=not(A3 AND B3);m3<=not(A3 OR B3);
          t4<=not(A4 AND B4);m4<=not(A4 OR B4);
          P1<=(not m1)and t1;P2<=(not m2)and t2;
          P3<=(not m3)and t3;P4<=(not m4)and t4;
          c1<=not(((not C0)and t1)or m1);
          c2<=not(((not C0)and t1 and t2)or (t2 and m1) or m2);
    c3<=not(((not C0)and t1 and t2 and t3)or (t2 and t3 and m1) or (m2 and t3)or m3);
    Co<=not(((not C0)and t1 and t2 and t3 and t4)or(m1 and t4 and t3 and t2)or(t2 and t3 and t4)or(t4 and m3)or m4);
    S1<=P1 XOR C0;S2<=P2 XOR c1;S3<=P3 XOR c2;S4<=P4 XOR c3;
   end;