Verilog code to implement a 4-bit adder-subtractor circuit.

Here’s an example Verilog code to implement a 4-bit adder-subtractor circuit:

module add_sub (output reg [3:0] result, input [3:0] a, input [3:0] b, input sub);

    always @(a, b, sub) begin
        if (sub)
            result = a - b;
        else
            result = a + b;
    end

endmodule

This code defines a module called “add_sub” that implements a 4-bit adder-subtractor. The output “result” is a registered output that represents the result of the addition or subtraction operation. The inputs “a” and “b” are two 4-bit inputs that represent the operands. The input “sub” is a 1-bit input that controls the operation of the circuit. If “sub” is 1, the circuit performs subtraction; otherwise, it performs addition.

The “always” block is a combinational logic block that performs addition or subtraction based on the value of the “sub” input. If “sub” is 1, the output is set to the difference between “a” and “b”. If “sub” is 0, the output is set to the sum of “a” and “b”.

Note that this Verilog code assumes that the “result”, “a”, and “b” signals are all 4-bit signals, and the “sub” input is a 1-bit signal. If you want to use different signal widths, you would need to modify the code accordingly. Additionally, this code implements a combinational circuit, which means that the output is updated immediately whenever there is a change in the input signals. If you wanted to implement a registered circuit, you would need to modify the code accordingly.