XOR Gate Using Basic Gates

November 24, 2025

Solving Approach

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Code

// ============================================================
// Basic Gates (given)
// ============================================================
module and_gate(input a, b, output y);
    assign y = a & b;
endmodule

module or_gate(input a, b, output y);
    assign y = a || b;
endmodule

module not_gate(input a, output y);
    assign y = ~a;
endmodule

// ============================================================
// XOR Gate
// ============================================================
module xor_gate (
    input  a, b,
    output y
);
    wire not_a, not_b;
    wire and_1, and_2;

    not_gate ng_a (
        .a(a), 
        .y(not_a)
    );
    not_gate ng_b (
        .a(b), 
        .y(not_b)
    );

    and_gate ag_1 (
        .a(a), 
        .b(not_b), 
        .y(and_1)
    );
    and_gate ag_2 (
        .a(b), 
        .b(not_a), 
        .y(and_2)
    );

    or_gate og (
        .a(and_1), 
        .b(and_2), 
        .y(y)
    );

endmodule

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Testbench Code

`timescale 1ns/1ps

module tb_xor_gate;
    // Inputs
    reg a, b;
    // DUT output
    wire y;
    // Expected output
    reg expected_y;

    // Mismatch flag
    wire mismatch = (y !== expected_y);

    // Counters
    integer TOTAL_TEST_CASES = 0;
    integer TOTAL_PASSED_TEST_CASES = 0;
    integer TOTAL_FAILED_TEST_CASES = 0;

    // DUT
    xor_gate dut (.a(a), .b(b), .y(y));

    // VCD: only inputs, output, expected, mismatch
    initial begin
        $dumpfile("tb_xor_gate.vcd");
        $dumpvars(0,
            tb_xor_gate.a, tb_xor_gate.b,
            tb_xor_gate.y,
            tb_xor_gate.expected_y,
            tb_xor_gate.mismatch
        );
    end

    // Apply, check, and print ONE row (no duplicate driving)
    task run_and_show;
        input ta, tb_;
        begin
            a = ta; b = tb_;
            expected_y = ta ^ tb_;
            #1; // settle

            TOTAL_TEST_CASES = TOTAL_TEST_CASES + 1;
            if (!mismatch)
                TOTAL_PASSED_TEST_CASES = TOTAL_PASSED_TEST_CASES + 1;
            else
                TOTAL_FAILED_TEST_CASES = TOTAL_FAILED_TEST_CASES + 1;

            $display("%b %b | %b | %b | %b", a, b, y, expected_y, mismatch);
        end
    endtask

    initial begin
        $display("a b | y | expected_y | mismatch");
        $display("--------------------------------");

        // Truth table — each case executed exactly once
        run_and_show(0,0);
        run_and_show(0,1);
        run_and_show(1,0);
        run_and_show(1,1);

        // Summary
        $display("======================================");
        $display("TOTAL_TEST_CASES=%0d",        TOTAL_TEST_CASES);
        $display("TOTAL_PASSED_TEST_CASES=%0d", TOTAL_PASSED_TEST_CASES);
        $display("TOTAL_FAILED_TEST_CASES=%0d", TOTAL_FAILED_TEST_CASES);
        $display("ALL_TEST_CASES_PASSED=%s",
                 (TOTAL_FAILED_TEST_CASES==0) ? "true" : "false");
        $display("======================================");

        $finish;
    end
endmodule