Source

Bitmask: Thermometer to Rightmost 0 Bit

Credit: Hacker's Delight, Section 2-1: Manipulating Rightmost Bits

Use the following formula to create a word with 1’s at the positions of the rightmost 0-bit and the trailing 1’s in the input, producing all 1’s if no 0-bit, and the integer 1 if no trailing 1’s (e.g., 01010111 -> 00001111)

`default_nettype none

module Bitmask_Thermometer_to_Rightmost_0_Bit
#(
    parameter WORD_WIDTH = 0
)
(
    input   wire    [WORD_WIDTH-1:0]    word_in,
    output  reg     [WORD_WIDTH-1:0]    word_out
);

    initial begin
        word_out = {WORD_WIDTH{1'b0}};
    end

    localparam ONE = {{WORD_WIDTH-1{1'b0}},1'b1};

    always @(*) begin
        word_out = word_in ^ (word_in + ONE);
    end

endmodule

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