题目：
A priority encoder is a combinational circuit that, when given an input bit vector, outputs the position of the first 1 bit in the vector. For example, a 8-bit priority encoder given the input 8’b10010000 would output 3’d4, because bit[4] is first bit that is high.

Build a 4-bit priority encoder. For this problem, if none of the input bits are high (i.e., input is zero), output zero. Note that a 4-bit number has 16 possible combinations.

解题：

module top_module (
    input [3:0] in,
    output reg [1:0] pos  );
    always@(*)
        begin
            case(in)
                4'b0000:pos = 2'd00;
                4'b0001:pos = 2'd00;
                4'b0010:pos = 2'd01;
                4'b0011:pos = 2'd00;
                4'b0100:pos = 2'd02;
                4'b0101:pos = 2'd00;
                4'b0110:pos = 2'd01;
                4'b0111:pos = 2'd00;
                4'b1000:pos = 2'd03;
                4'b1001:pos = 2'd00;
                4'b1010:pos = 2'd01;
                4'b1011:pos = 2'd00;
                4'b1100:pos = 2'd02;
                4'b1101:pos = 2'd00;
                4'b1110:pos = 2'd01;
                4'b1111:pos = 2'd00;
                default:pos = 2'd00;
            endcase
        end

endmodule

结果正确：

答案：

module top_module (
	input [3:0] in,
	output reg [1:0] pos
);

	always @(*) begin			// Combinational always block
		case (in)
			4'h0: pos = 2'h0;	// I like hexadecimal because it saves typing.
			4'h1: pos = 2'h0;
			4'h2: pos = 2'h1;
			4'h3: pos = 2'h0;
			4'h4: pos = 2'h2;
			4'h5: pos = 2'h0;
			4'h6: pos = 2'h1;
			4'h7: pos = 2'h0;
			4'h8: pos = 2'h3;
			4'h9: pos = 2'h0;
			4'ha: pos = 2'h1;
			4'hb: pos = 2'h0;
			4'hc: pos = 2'h2;
			4'hd: pos = 2'h0;
			4'he: pos = 2'h1;
			4'hf: pos = 2'h0;
			default: pos = 2'b0;	// Default case is not strictly necessary because all 16 combinations are covered.
		endcase
	end
	
	// There is an easier way to code this. See the next problem (always_casez).
	
endmodule

注意：
本题一开始没理解题目意思，比较迷，还是需要先把题目意思理解才会写。4位优先编码器