7420 chip

题目如下：

The 7400-series integrated circuits are a series of digital chips with a few gates each. The 7420 is a chip with two 4-input NAND gates.

代码如下：

module top_module ( 
    input p1a, p1b, p1c, p1d,
    output p1y,
    input p2a, p2b, p2c, p2d,
    output p2y );
    assign p1y=~(p1a&p1b&p1c&p1d);
    assign p2y=~(p2a&p2b&p2c&p2d);

endmodule