-- Copyright © 1993 by  McGraw-Hill, Inc. and Zainalabedin Navabi

-- FIGURE 5.22 




-- ENTITY DECLERATION OF BIT COMPARATOR :

ENTITY bit_comparator IS
  PORT (a, b,                -- data inputs
        gt,                  -- previous greater than
        eq,                  -- previous equal
        lt : IN BIT;         -- previous less than
        a_gt_b,              -- greater
        a_eq_b,              -- equal 
        a_lt_b : OUT BIT);   -- less than 
END bit_comparator;
--



-- ASSOCIATING FIXED VALUES WIHT THE GENERICS OF THE LOGIC GATES :

ARCHITECTURE fixed_delay OF bit_comparator IS
   COMPONENT n1 
     GENERIC (tplh, tphl : TIME);
     PORT (i1: IN BIT; o1: OUT BIT); 
   END COMPONENT;
   COMPONENT n2 
     GENERIC (tplh, tphl : TIME);
     PORT (i1, i2: IN BIT; o1: OUT BIT); 
   END COMPONENT;
   COMPONENT n3 
     GENERIC (tplh, tphl : TIME);
     PORT (i1, i2, i3: IN BIT; o1: OUT BIT); 
   END COMPONENT;
   FOR ALL : n1 USE ENTITY WORK.inv_t (average_delay);
   FOR ALL : n2 USE ENTITY WORK.nand2_t (average_delay);
   FOR ALL : n3 USE ENTITY WORK.nand3_t (average_delay);
-- Intermediate signals
   SIGNAL im1,im2, im3, im4, im5, im6, im7, im8, im9, im10 : BIT;
BEGIN  
-- a_gt_b output
   g0 : n1 GENERIC MAP (2 NS, 4 NS) PORT MAP (a, im1);
   g1 : n1 GENERIC MAP (2 NS, 4 NS) PORT MAP (b, im2);
   g2 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (a, im2, im3); 
   g3 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (a, gt, im4); 
   g4 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (im2, gt, im5);
   g5 : n3 GENERIC MAP (4 NS, 6 NS) PORT MAP (im3, im4, im5, a_gt_b);
-- a_eq_b output
   g6 : n3 GENERIC MAP (4 NS, 6 NS) PORT MAP (im1, im2, eq, im6);
   g7 : n3 GENERIC MAP (4 NS, 6 NS) PORT MAP (a, b, eq, im7);
   g8 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (im6, im7, a_eq_b);
-- a_lt_b output
   g9 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (im1, b, im8); 
   g10 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (im1, lt, im9); 
   g11 : n2 GENERIC MAP (3 NS, 5 NS) PORT MAP (b, lt, im10);
   g12 : n3 GENERIC MAP (4 NS, 6 NS) PORT MAP (im8, im9, im10, a_lt_b);
END fixed_delay;
--