2D arrays: A net is not a legal lvalue in this context [duplicate]

Question

I am looking to create a FIFO using a 2D array (P_MDIM is the width of each vector, P_NDIM is the depth of the array/memory to store these vectors).

I am having trouble with the above error message on many spots in my code (assigning to out, full, and empty).

I don't know why it occurs. I thought my array ("memory") would be a 2D array of reg (not nets). Below is my full code:

module fifo( signal, enqueue, dequeue, clk, out, full, empty );

//depth of the memory
parameter P_NDIM = 16;
//width of each memory cell(in bits)
parameter P_MDIM = 16;

input [P_MDIM - 1:0] signal;
input enqueue;
input dequeue;
input clk;

output [P_MDIM - 1:0] out;
output full;
output empty;

logic [P_MDIM - 1:0] [P_NDIM - 1:0] memory = 0;
integer enqueue_pos = 0;
integer i = 0;


always @(posedge clk) begin

  //enqueue data
  if(enqueue) begin
    //checking if memory is full
     if(enqueue_pos == P_NDIM - 1) begin
      $display("data not entered, memory full");
    end
    //add signal into memory
    else begin
      memory[enqueue_pos][P_MDIM-1:0] = signal;
      enqueue_pos <= enqueue_pos + 1;
    end//else
  end//if enqueue

  //dequeue data
  if(dequeue) begin
    //checking if memory is empty
    if(enqueue_pos == 0) begin
      $display("no data to dequeue, memory empty");
      out[P_MDIM-1:0] = 0;
    end
    //output signal from memory, shift memory
    else begin
      out = memory[0][P_MDIM - 1:0];
      enqueue_pos <= enqueue_pos - 1;
      for(i = 0; i <= P_NDIM - 2; i = i + 1) begin
        memory[i][P_MDIM-1:0] = memory[i + 1][P_NDIM-1:0];
      end//for
    end//else
  end//if dequeue

  //check full and empty
     full = (enqueue_pos == P_NDIM-1 ? 1'b0 : 1'b1);
     empty = (enqueue_pos == 0 ? 1'b0 : 1'b1);

end//always

endmodule

Answer 1

You declared out as a net type, but then you assigned to it inside a procedural always block. Signals assigned in a procedural block should be declared as logic:

Change:

output [P_MDIM - 1:0] out;
output full;
output empty;

to:

output logic [P_MDIM - 1:0] out;
output logic full;
output logic empty;

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Unrelated to the error, in your always block, you are using a mixture of blocking (=) and nonblocking (<=) assignments. They should all be nonblocking.