Dynamic Arrays, Queues, and Associative Arrays

Choose a testbench collection for variable batches, transaction queues, and indexed results.

Collections for the testbench

Dynamic collections manage a number of transactions that is unknown at compile time. They belong mainly in verification and high-level models rather than synthesizable RTL.

Three forms serve different needs:

a dynamic array for a size allocated at run time;
a queue for easy insertion and removal at its ends;
an associative array for elements indexed by sparse keys.

Dynamic array

int samples[];
 
samples = new[128];
foreach (samples[i])
  samples[i] = i * 2;
 
$display("count=%0d", samples.size());
samples.delete();

new[128] allocates 128 elements. A new allocation can preserve part of the old content with new[new_size](samples). delete() releases the array.

This format fits a data block whose size is known at the beginning of an operation and which needs frequent direct indexing.

Queue

typedef struct {
  int unsigned id;
  logic [31:0]  data;
} expected_t;
 
expected_t pending_q[$];
int matches[$];
 
matches = pending_q.find_index(item)
          with (item.id == response_id);
 
case (matches.size())
  0: $error("No expected item for id=%0d", response_id);
  1: pending_q.delete(matches[0]);
  default: $error("Multiple expected items for id=%0d", response_id);
endcase

logic [31:0] expected_by_id[int unsigned];
 
expected_by_id[transaction_id] = predicted_value;
 
if (expected_by_id.exists(response_id)) begin
  assert (observed == expected_by_id[response_id]);
  expected_by_id.delete(response_id);
end else begin
  $error("Unexpected response id %0d", response_id);
end

Dynamic Arrays, Queues, and Associative Arrays

Collections for the testbench

Dynamic array

Queue

Associative array

Choose without adding needless complexity

Key takeaways