verilog设计一个2位BCD码减法器。

在Verilog HDL中，设计一个2位BCD（二进制编码十进制）码减法器通常会包含几个步骤。首先，你需要定义输入、输出以及中间状态变量，然后创建函数来处理BCD码的加法和移位操作。这里是一个简单的示例：

module bcd_subtractor(
    input [3:0] a,      // 4-bit BCD number A (MSB first)
    input [3:0] b,      // 4-bit BCD number B (MSB first)
    output reg [3:0] result, // Result of the subtraction
    output reg borrow_out   // Borrow flag for borrowing from the leftmost digit
);

// Function to perform BCD addition with carry propagation
function [3:0] add_bcd(input [3:0] x, input [3:0] y);
    wire [3:0] sum = x + y;
    if (sum[3] == 1) begin
        sum[3:0] <= {sum[2], sum[0], sum[1]};
        carry_out <= 1;
    else begin
        sum[3:0] <= sum;    // No carry out
        carry_out <= 0;
    end
endfunction

// Perform the actual subtraction using addition and borrow
always @(*) begin
    result[3] = a[3] ^ b[3];     // Subtract the most significant digits
    result[2] = add_bcd(a[2], b[2]);    // Handle second least significant digits
    result[1] = add_bcd(a[1], b[1]);    // Continue with remaining digits
    result[0] = add_bcd(a[0], b[0]);
    
    // If there's a borrow in the MSB, subtract 1 from all other digits
    if (borrow_out) begin
        result = result - 1'b1;
    end
end

endmodule