Verilog code to implement a 4-bit subtractor circuit.

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

module subtractor (output reg [3:0] difference, input [3:0] minuend, subtrahend, input borrow_in);

    always @(minuend, subtrahend, borrow_in) begin
        reg borrow;
        difference[0] = minuend[0] ^ subtrahend[0] ^ borrow_in;
        borrow = (

minuend[0] & subtrahend[0]) | ((

minuend[0] | subtrahend[0]) & borrow_in);
        difference[1] = minuend[1] ^ subtrahend[1] ^ borrow;
        borrow = (

minuend[1] & subtrahend[1]) | ((

minuend[1] | subtrahend[1]) & borrow);
        difference[2] = minuend[2] ^ subtrahend[2] ^ borrow;
        borrow = (

minuend[2] & subtrahend[2]) | ((

minuend[2] | subtrahend[2]) & borrow);
        difference[3] = minuend[3] ^ subtrahend[3] ^ borrow;
    end

endmodule

This code defines a module called “subtractor” that implements a 4-bit subtractor. The output “difference” is a registered output that represents the difference between the two 4-bit inputs “minuend” and “subtrahend”. The input “borrow_in” is the borrow-in to the subtraction operation.

The “always @(minuend, subtrahend, borrow_in)” block is a combinational logic block that updates the “difference” output based on the inputs. The subtractor operates by performing a full subtractor operation on each bit of the two input vectors, starting with the least significant bit and propagating the borrow to the next bit.

The “borrow” variable is a temporary storage for the borrow-out of each full subtractor operation. The first full subtractor operation takes the least significant bits of “minuend” and “subtrahend” and the borrow-in, and calculates the difference and borrow-out. The difference is then assigned to the least significant bit of “difference”, and the borrow-out is stored in the “borrow” variable. The process is repeated for the next three bits, with the “borrow” variable holding the borrow-out from the previous bit.

Note that this Verilog code assumes that the “minuend” and “subtrahend” inputs, “difference” output, and the “borrow_in” signal are all 1-bit signals. If you want to use multi-bit signals, you would need to modify the code accordingly.