How to declare and use 1D and 2D byte arrays in Verilog?

Question

How to declare and use 1D and 2D byte arrays in Verilog?

eg. how to do something like

byte a_2D[3][3]; byte a_1D[3];  // using 1D for (int i=0; i< 3; i++) {     a_1D[i] = (byte)i; }  // using 2D for (int i=0; i< 3; i++) {     for (int j=0; j< 3; j++)     {         a_2D[i][j] = (byte)i*j;     } } 

Answer 1

Verilog thinks in bits, so reg [7:0] a[0:3] will give you a 4x8 bit array (=4x1 byte array). You get the first byte out of this with a[0]. The third bit of the 2nd byte is a[1][2].

For a 2D array of bytes, first check your simulator/compiler. Older versions (pre '01, I believe) won't support this. Then reg [7:0] a [0:3] [0:3] will give you a 2D array of bytes. A single bit can be accessed with a[2][0][7] for example.

reg [7:0] a [0:3]; reg [7:0] b [0:3] [0:3];  reg [7:0] c; reg d;  initial begin     for (int i=0; i<=3; i++) begin       a[i] = i[7:0];    end     c = a[0];    d = a[1][2];       // using 2D    for (int i=0; i<=3; i++)       for (int j=0; j<=3; j++)           b[i][j] = i*j;  // watch this if you're building hardware  end 

Answer 2

In addition to Marty's excellent Answer, the SystemVerilog specification offers the byte data type. The following declares a 4x8-bit variable (4 bytes), assigns each byte a value, then displays all values:

module tb;  byte b [4];  initial begin     foreach (b[i]) b[i] = 1 << i;     foreach (b[i]) $display("Address = %0d, Data = %b", i, b[i]);     $finish; end  endmodule 

This prints out:

Address = 0, Data = 00000001 Address = 1, Data = 00000010 Address = 2, Data = 00000100 Address = 3, Data = 00001000 

This is similar in concept to Marty's reg [7:0] a [0:3];. However, byte is a 2-state data type (0 and 1), but reg is 4-state (01xz). Using byte also requires your tool chain (simulator, synthesizer, etc.) to support this SystemVerilog syntax. Note also the more compact foreach (b[i]) loop syntax.

The SystemVerilog specification supports a wide variety of multi-dimensional array types. The LRM can explain them better than I can; refer to IEEE Std 1800-2005, chapter 5.