ch2-Packed array

2019-10-24

SystemVerilog Packed Arrays

There are two types of arrays in SystemVerilog - packed and unpacked arrays.

​ A packed array is used to refer to dimensions declared before the variable name.

bit [3:0]   data;       // Packed array or vector
logic     queue [9:0];   // Unpacked array
 

​ A packed array is guaranteed to be represented as a contiguous set of bits. They can be made of only the single bit data types like bit, logic, and other recursively packed arrays.

Single Dimensional Packed Arrays

A one-dimensional packed array is also called as a vector.

module tb;
  bit [7:0]   m_data;   // A vector or 1D packed array
 
  initial begin
    // 1. Assign a value to the vector
    m_data = 8'hA2; 
 
    // 2. Iterate through each bit of the vector and print value
    for (int i = 0; i < $size(m_data); i++) begin
      $display ("m_data[%0d] = %b", i, m_data[i]);
    end
  end
endmodule

Multidimensional Packed Arrays

​ A multidimensional packed array is still a set of contiguous bits but are also segmented into smaller groups.

Example #1

The code shown below declares a 2D packed array that occupies 32-bits or 4 bytes and iterates through the segments and prints its value.

module tb;
  bit [3:0][7:0]   m_data;   // A MDA, 4 bytes
 
  initial begin
    // 1. Assign a value to the MDA
    m_data = 32'hface_cafe;
 
      $display ("m_data = 0x%0h", m_data);
 
    // 2. Iterate through each segment of the MDA and print value
      for (int i = 0; i < $size(m_data); i++) begin
        $display ("m_data[%0d] = %b (0x%0h)", i, m_data[i], m_data[i]);
    end
  end
endmodule
 

Simulation Log

注意上面遍历的索引范围是4，代表4个byte。

ncsim> run
m_data = 0xfacecafe
m_data[0] = 11111110 (0xfe)
m_data[1] = 11001010 (0xca)
m_data[2] = 11001110 (0xce)
m_data[3] = 11111010 (0xfa)
ncsim: *W,RNQUIE: Simulation is complete.

Example #2

Let us see a 3D packed array now.

module tb;
  bit [2:0][3:0][7:0]   m_data;   // An MDA, 12 bytes
 
  initial begin
    // 1. Assign a value to the MDA
      m_data[0] = 32'hface_cafe;
      m_data[1] = 32'h1234_5678;
      m_data[2] = 32'hc0de_fade;
 
      // m_data gets a packed value
      $display ("m_data = 0x%0h", m_data);
 
    // 2. Iterate through each segment of the MDA and print value
      foreach (m_data[i]) begin
        $display ("m_data[%0d] = 0x%0h", i, m_data[i]);
        foreach (m_data[i][j]) begin
          $display ("m_data[%0d][%0d] = 0x%0h", i, j, m_data[i][j]);
        end
      end
  end
endmodule
 

Simulation Log

ncsim> run
m_data = 0xc0defade12345678facecafe
m_data[2] = 0xc0defade
m_data[2][3] = 0xc0
m_data[2][2] = 0xde
m_data[2][1] = 0xfa
m_data[2][0] = 0xde
m_data[1] = 0x12345678
m_data[1][3] = 0x12
m_data[1][2] = 0x34
m_data[1][1] = 0x56
m_data[1][0] = 0x78
m_data[0] = 0xfacecafe
m_data[0][3] = 0xfa
m_data[0][2] = 0xce
m_data[0][1] = 0xca
m_data[0][0] = 0xfe
ncsim: *W,RNQUIE: Simulation is complete.

Click to try this example in a simulator!