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/* A 4-bit multiplier, following the algorithm from

 * Pg. 179, Figure 3.7. */

module MUL4_CONTROL (prod_in, alu_op, write_op, shift_op, clk, ctr);

    input prod_in, clk;

    input[0:3] ctr;

    output[0:1] alu_op;

    output write_op, shift_op;

    always @(negedge clk) begin

        if (ctr <= 4) begin

            force write_op = prod_in;

            force shift_op = 'b1;

            force alu_op = 'b10;

        end else begin

            release write_op;

            release shift_op;

            release alu_op;

        end

    end

endmodule

module MUL4_MULTIPLICAND (in, out, clk);

    input[0:3] in;

    output[0:3] out;

    input clk;

    reg[0:3] value;

    always @(in) begin

        value = in;

    end

    assign out = value;

endmodule

module MUL4_PRODUCT (left4, right4, shift_op, write_op, out, clk);

    input[0:3] left4, right4;

    input shift_op, write_op, clk;

    output[0:7] out;

    reg[0:7] value;

    always @(right4) begin

        value[0:3] = 'b0000;

        value[4:7] = right4;

    end

    always @(negedge clk) begin

        if (shift_op == 1) begin

            if (write_op == 1) begin

                value[0:3] = left4[0:3];

            end

            value = value >> 1;

        end

    end

    assign out = value;

endmodule

module testme;

    reg[0:3] a, b;

    wire[0:7] w_out;

    MUL4 mult (a, b, w_out);

    initial begin

        $monitor ("time=%0d a=%b b=%b prod=%b", $time, a, b, w_out);

    end

    initial begin

            a = 'b0011; b = 'b0011;

        #10 a = 'b0001; b = 'b1100;

        #10 a = 'b1100; b = 'b0011;

        #10 a = 'b0101; b = 'b1010;

        #8

        #2 $finish;

    end

endmodule

module MUL4 (a, b, out);

    input[0:3] a, b;

    output[0:7] out;

    reg[0:3] counter;

    reg clk;

    initial begin

        clk = 'b0;

        counter ='b0000;

        cin = 'b0;

    end

    always begin

        #1 clk = ~clk;

    end

    always @(negedge clk) begin

        counter = counter + 'b1;

    end

/*

    initial begin

        $monitor ("time=%0d counter=%b clk=%b w_mout=%b w_prod=%b w_aop=%b w_sop=%b w_wop=%b out=%b",

               $time, counter, clk, w_mout, w_prod, w_aop, w_sop, w_wop, out);

    end

*/

    wire[0:7] w_prod;

    wire[0:3] w_mout, w_alu_out;

    wire[0:1] w_aop;

    wire w_sop, w_wop, w_cout;

    reg cin;

    MUL4_PRODUCT mp (w_alu_out, a, w_sop, w_wop, w_prod, clk);

    MUL4_MULTIPLICAND mm (b, w_mout, clk);

    MUL4_CONTROL mc (w_prod[7], w_aop, w_wop, w_sop, clk, counter);

    ALU4 ma (w_prod[0:3], w_mout, cin, w_aop, w_cout, w_alu_out);

    always @(counter) begin

        if (counter == 4) begin

            force out = w_prod;

            counter = 'b1111; //reset counter

        end else begin

            release out;

        end

    end

endmodule