vector concatenation operator

Part selection was used to select portions of a vector. The concatenation operator {a,b,c} is used to create larger vectors by concatenating smaller portions of a vector together.

{3'b111, 3'b000} => 6'b111000
{1'b1, 1'b0, 3'b101} => 5'b10101
{4'ha, 4'd10} => 8'b10101010     // 4'ha and 4'd10 are both 4'b1010 in binary
Concatenation needs to know the width of every component (or how would you know the length of the result?). Thus, {1, 2, 3} is illegal and results in the error message: unsized constants are not allowed in concatenations.

The concatenation operator can be used on both the left and right sides of assignments.

input [15:0] in;
output [23:0] out;
assign {out[7:0], out[15:8]} = in;         // Swap two bytes. Right side and left side are both 16-bit vectors.
assign out[15:0] = {in[7:0], in[15:8]};    // This is the same thing.
assign out = {in[7:0], in[15:8]};       // This is different. The 16-bit vector on the right is extended to
                                        // match the 24-bit vector on the left, so out[23:16] are zero.
                                        // In the first two examples, out[23:16] are not assigned.
A Bit of Practice
Given several input vectors, concatenate them together then split them up into several output vectors. There are six 5-bit input vectors: a, b, c, d, e, and f, for a total of 30 bits of input. There are four 8-bit output vectors: w, x, y, and z, for 32 bits of output. The output should be a concatenation of the input vectors followed by two 1 bits:

module top_module (
    input [4:0] a, b, c, d, e, f,
    output [7:0] w, x, y, z );
    assign z={e[0],f,2'b11};   //这里的2'b11自己原先没有想出来，写成了1,1或者[0],[0]，都是不对的；
    //见实例代码的第一个
    //这里的e[0]也可以写作e[0:0]
    assign y={d[3:0],e[4:1]};
    assign x={b[1:0],c,d[4]};
    assign w={a,b[4:2]};
endmodule

 如下的写法也是成立的

module top_module (
    input [4:0] a, b, c, d, e, f,
    output [7:0] w, x, y, z );//
    assign z[7:0]={e[0],f[4:0],1'b1,1'b1};
    assign y[7:0]={d[3:0],e[4:1]};
    assign x[7:0]={b[1:0],c[4:0],d[4]};
    assign w[7:0]={a,b[4:2]};
 
endmodule