256-to-1 4-bit multiplexer

Create a 4-bit wide, 256-to-1 multiplexer. The 256 4-bit inputs are all packed into a single 1024-bit input vector. sel=0 should select bits in[3:0], sel=1 selects bits in[7:4], sel=2 selects bits in[11:8], etc.

Mux256to1v - HDLBits (01xz.net)

module top_module( 
    input [1023:0] in,
    input [7:0] sel,
    output [3:0] out );
 assign out = {in[sel*4+3],in[sel*4+2],in[sel*4+1],in[sel*4]};
    /*题5中不能使用assign out = in[sel*4+3:sel*4]，此时会报错，
    因为该等式不能证明选择的这个位宽是个常数。所以我们继续按bit进行选取然后进行拼接。*/
endmodule