priority_select_recurse.v

module priority_select #(
	parameter N = 16 // any positive integer
) (
	input  wire [N-1:0] req,
	output wire [N-1:0] gnt
);

generate
if (N == 1) begin: base_case0
	assign gnt = req;
end else if (N == 2) begin: base_case1
	assign gnt = {req[1] && !req[0], req[0]};
end else begin: recurse
	wire [1:0] coarse;
	wire [N-1:0] fine;
	priority_select #(
		.N (N / 2)
	) recurse_lower (
		.req (req [N/2-1:0]),
		.gnt (fine[N/2-1:0])
	);
	priority_select #(
		.N (N - N / 2)
	) recurse_upper (
		.req (req [N-1:N/2]),
		.gnt (fine[N-1:N/2])
	);

	assign coarse = {|req[N-1:N/2], |req[N/2-1:0]};
	assign gnt = fine & {
		{N - N / 2{coarse[1] && !coarse[0]}},
		{    N / 2{              coarse[0]}}
	};
end
endgenerate

endmodule

priority_select_ripple.v

module priority_select #(
	parameter N = 16 // any positive integer
) (
	input  wire [N-1:0] req,
	output wire [N-1:0] gnt
);

reg [N-1:0] deny;
integer i;

always @ (*) begin
	deny[0] = 1'b0;
	for (i = 1; i < N; i = i + 1)
		deny[i] = deny[i - 1] || req[i - 1];
end

assign gnt = req & ~deny;

endmodule