concatenation - What do curly braces mean in Verilog?

Question

I am having a hard time understanding the following syntax in Verilog:

input  [15:0] a;      // 16-bit input
output [31:0] result; // 32-bit output
assign result = {{16{a[15]}}, {a[15:0]}};

I know the assign statement will wire something up to the result bus using wires and combinational logic, but what's up with the curly braces and 16{a[15]}?

Answer 1

The curly braces mean concatenation, from most significant bit (MSB) on the left down to the least significant bit (LSB) on the right. You are creating a 32-bit bus (result) whose 16 most significant bits consist of 16 copies of bit 15 (the MSB) of the a bus, and whose 16 least significant bits consist of just the a bus (this particular construction is known as sign extension, which is needed e.g. to right-shift a negative number in two's complement form and keep it negative rather than introduce zeros into the MSBits).

For what it's worth, the nested curly braces around a[15:0] are superfluous.