1. Modeling styles: 1. Structural Modeling: As a set of interconnected components (to represent structure), 2. Dataflow Modeling: As a set of concurrent assignment statements (to represent dataflow), 3. Behavioral Modeling: As a set of sequential assignment statements (to represent behavior), 4. Mixed Modeling: Any combination of the above three.

2. Structural Style of Modeling In the structural style of modeling, an entity is described as a set of interconnected components. entity HALF_ADDER is port (A, B: in BIT; SUM, CARRY: out BIT); end HALF_ADDER; -- This is a comment line.

3. model for the HALF_ADDER entity architecture HA_STRUCTURE of HALF_ADDER is component XOR2 port (X, Y: in BIT; Z: out BIT); end component; component AND2 port (L, M: in BIT; N: out BIT); end component; begin X1: XOR2 port map (A, B, SUM); A1: AND2 port map (A, B, CARRY); end HA_STRUCTURE;

4. Dataflow Style of Modeling The dataflow model for the HALF_ADDER is described using two concurrent signal assignment statements architecture HA_CONCURRENTof HALF_ADDER is begin SUM <= A xor B after 8 ns; CARRY <= A and B after 4 ns; end HA_CONCURRENT; Concurrent signal assignment statements are concurrent statements, and therefore, the ordering of these statements in an architecture body is not important.

5. Behavioral Style of Modeling The behavioral style of modeling specifies the behavior of an entity as a set of statements that are executed sequentially in the specified order. This set of sequential statements, that are specified inside a process statement, do not explicitly specify the structure of the entity but merely specifies its functionality. A process statement is a concurrent statement that can appear within an architecture

6. For example, consider the following behavioral model for the DECODER2x4 entity. architecture DEC_SEQUENTIAL of DECODER2x4 is begin process (A, B, ENABLE) variable ABAR, BBAR: BIT; begin ABAR := not A; -- statement 1 BBAR := not B; --statement 2 if (ENABLE = '1') then –statements Z(3) <= not (A and B): - statement 4 Z(0) <= not (ABAR and BBAR); ~ statement 5 Z(2) <= not (A and BBAR); - statement 6 Z(1 ) <= not (ABAR and B); - statement 7 else Z<= "1111"; -statements end if; end process; end;

7. A process statement, too, has a declarative part (between the keywords process and begin), and a statement part (between the keywords begin and end process). The statements appearing within the statement part are sequential statements and are executed sequentially. The list of signals specified within the parenthesis after the keyword process constitutes a sensitivity list and the process statement is invoked whenever there is an event on any signal in this list. In the previous example, when an event occurs on signals A, B, or ENABLE, the statements appearing within the process statement are executed sequentially.

8. Mixed Style of Modeling It is possible to mix the three modeling styles that we have seen so far in a single architecture body. That is, within an architecture body, we could use component instantiation statements (that represent structure), concurrent signal assignment statements (that represent dataflow), and process statements (that represent behavior).

9. Example of a mixed style model for a one-bit full-adder entity FULL_ADDER is port (A, B, CIN: in BIT; SUM, COUT: out BIT); end FULL_ADDER; architecture FA_MIXED of FULL_ADDER is component XOR2 port (A, B: in BIT; Z: out BIT); end component; signal S1: BIT; begin X1: XOR2 port map (A, B, S1 ); -- structure. process (A, B, CIN) -- behavior. variable T1, T2, T3: BIT; begin T1 :=A and B; T2 := B and CIN; T3:=A and CIN; COUT <= T1 or T2 or T3; end process; SUM <= S1 xor CIN; -- dataflow. end FA_M!XED;