Concatenation

keysymbols: {, }

The concatenation operator appends sized nets, registers, bit select, part select and constants.

        module concatTest;
           reg         a;
           reg [1:0] b;
           reg [5:0] c;

           initial begin
              a = 1'b1;
              b = 2'b00;
              c = 6'b101001;

              $displayb({a, b});      // produces a 3-bit number 3'b100
              $displayb({c[5:3], a}); // produces 4-bit number 4'b1011

           end

        endmodule // concatTest

Replication

Replication can be used along side concatenation to repeat a number as many times as specified, see example below.

        module replicTest;
           reg         a;
           reg [1:0] b;
           reg [5:0] c;

           initial begin
              a = 1'b1;
              b = 2'b00;

              $displayb({4{a}}); // evaluates as 1111
              c = {4{a}};
              $displayb(c);     // evaluates as 001111
           end

        endmodule // replicTest

According to the IEEE standard, replication and concatenation can be combined as in: c = {4{a}, b} however, the current software at Edinburgh University does not allow this.

EXERCISE
What does d evaluate to in the following ?

        module replicTest2;
           reg       a;
           reg [1:0] b;
           reg [3:0] c;
           reg [9:0] d;

           initial begin
              a = 1'b1;
              b = 2'b01;
              c = {4{a}};
              d = {b, c, b};

              $displayb(d);
           end

        endmodule // replicTest2

Answers