// add8c.e  a simple 8 bit adder with fast cout generation
//          this uses the components, below,  fadd  add4c 

// a one stage adder
define fadd(a, b, cin, s, cout)
circuits
  s <= a ^ b ^ cin after 1ns;
  cout <= (a&b)|(a&cin)|(b&cin) after 1ns;
end circuits;
end fadd;

// add4c   4 bit full adder component
// simple ripple carry adder with fast cout
// inputs a, b, cin
// outputs sum, cout

define add4c(a[4], b[4], cin, sum[4], cout)
  signal c[4];
circuits
  a0 use fadd(a[0], b[0], cin , sum[0], c[0]);
  a1 use fadd(a[1], b[1], c[0], sum[1], c[1]);
  a2 use fadd(a[2], b[2], c[1], sum[2], c[2]);
  a3 use fadd(a[3], b[3], c[2], sum[3], c[3]);
  cout <= (a[3]&b[3])|((a[3]|b[3])&((a[2]&b[2])|((a[2]|b[2])&
         ((a[1]&b[1])|((a[1]|b[1])&((a[0]&b[0])|((a[0]|b[0])&cin)))))))
         after 1ns;
end circuits;
end add4c;


// use two 4 bit adders to make an eight bit adder
define add8c(a[8], b[8], cin, sum[8], cout)
signal c;  // internal carry signal
circuits
  a0 use add4c(a[3:0], b[3:0], cin, sum[3:0], c);
  a1 use add4c(a[7:4], b[7:4], c  , sum[7:4], cout);
end circuits;
end add8c;