1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU1 Project Step 4 Step 1 in transitioning to behavioral modeling. We will wire behavioral code 1-to-1 with our gate level model.

1/8/ L7 Project Step 3 Copyright Joanne DeGroat, ECE, OSU2 Project Step 4  This is the start of the transition to behavioral modeling.  At the gate level you are near 1-to-1 with the hardware that is implemented  At the behavioral level you have a physical interface and then the function between.

1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU3 VHDL view of the world  The VHDL view is one that starts with the interface. How do you connect to the world.  After this, you have the ARCHITECTURE which tell you what actions, responses, operations and transformations happen within the interface.  Many designers first think about what the function is, and, after the function is described what the interface is.  In the VHDL world you must first define the format, timing, frequency, etc. of how the data arrives and is output first.

1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU4 Thus Far  Have described the ALU architecture structurally using 2 basic components described at the gate level. Very close to a 1- to-1 representation of the gates.  Now will start a transition to a behavioral description where we raise the level of abstraction each time.

1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU5 Straight 1-to-1 algorithmic representation of the slice  Will use a process with a loop. Each iteration of the loop will move to the next most significant slice.  On the first iteration will compute the P operation, then the K operation, do the logic function of the carry block, and then finally the results block. This is done iteratively from bit position 0 to the msb.  Naturally fits into a loop.

Example of what I am talking about  Consider the bit-sliced comparator  Now lets do it slice-by-slice algorithmically Start by writing the multi-bit entity 1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU6

The Entity and start of Architecture ENTITY byte_comparator IS PORT (a,b : IN bit_vector (7 downto 0); --a & b data gt,eq,lt: IN bit; --previous slice results a_gt_b, a_eq_b, a_lt_b : OUT bit); --outputs END byte_comparator; --note that one of the gt,eq,lt inputs must be tied high ARCHITECTURE algorithmic OF byte_comparator IS 1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU7

The algorithm – the comparator BEGIN PROCESS (a,b,gt,eq,lt) VARIABLE igt,ieq,ilt : bit_vector (0 to 8); BEGIN igt(0) := gt; --connect to external inputs ieq(0) := eq; ilt(0) := lt; FOR i in 0 to 7 loop igt(i+1) := a (i)AND NOT b(i) OR a(i) AND igt(i) OR NOT b(i) AND igt(i); ieq(i+1):=ieq(i) AND (NOT a(i) AND NOT b(i) OR a(i) AND b(i)); ilt(i+1) :=NOT a(i) AND b(i) OR ilt(i) AND NOT a(i) OR ilt(i) AND b(i); END LOOP; a_gt_b <= igt(8); a_eq_b <= ieq(8); a_lt_b <= ilt(8); END PROCESS; END byte_comparator; 1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU8

An alternative BEGIN PROCESS (a,b,gt,eq,lt) VARIABLE igti,ieqi,ilti,igto,ieqo,ilto : bit; BEGIN igti := gt; --connect to external inputs ieqi := eq; ilti := lt; FOR i in 0 to 7 loop igto := a (i)AND NOT b(i) OR a(i) AND igti OR NOT b(i) AND igti; ieqo:=ieqi AND (NOT a(i) AND NOT b(i) OR a(i) AND b(i)); ilto :=NOT a(i) AND b(i) OR ilt(i) AND NOT a(i) OR ilti AND b(i); igti := igto; ieqi := ieq0; ilti := ilto; END LOOP; a_gt_b <= igto; a_eq_b <= ieqo; a_lt_b <= ilto; END PROCESS; END byte_comparator; 1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU9

1/8/ L7 Project Step 3 Copyright Joanne DeGroat, ECE, OSU10 Iteration across the slices -alu For I in 0 to 7 loop -- do the p operation -- do the k operation -- carry out the carry chain unit operations -- do an r operation -- will have inputs of a(i),b(i),icin, p,k,r to each slice -- produce sum(i) and icout End loop;

1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU11 Internal Connections  Before a signal was used to connect the output of the P unit and the K unit to the carry chain and R unit.  Now time will not advance while in the processes iterations. What to USE?????? How about a variable What effect does using a variable have?

The structure  The structure is very similar to that shown for the comparator.  Have a process Start with boundary Iterate the slices – the loop Final slice output 1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU12

1/8/ L7 Project Step 3Copyright Joanne DeGroat, ECE, OSU13 The test file  Be sure to properly configure your design into the testbench.  We will start looking at previous steps results, i.e., solutions.