UVM Coding Practice · Senior

Ordered FIFO Compare

In-order expected queue, compare on match, mismatch reporting.

Interview prompt

Sketch a scoreboard with expected and actual FIFOs for an in-order APB peripheral response stream.

diagram

WHITEBOARD CHAIN

1. DECLARE    interfaces / types / ports you need
2. SKELETON   class extends + utils macro + key methods
3. MECHANISM  fill one critical method while narrating
4. PITFALL    name one bug juniors make on this pattern
5. TEST       how you would smoke-test the component

Reference sketch (≤40 lines)

systemverilog

class apb_sb extends uvm_scoreboard;
  `uvm_component_utils(apb_sb)
  uvm_analysis_imp_exp #(apb_txn, apb_sb) exp_imp;
  uvm_analysis_imp_act #(apb_txn, apb_sb) act_imp;
  apb_txn exp_q[$], act_q[$];

  function void write_exp(apb_txn t); exp_q.push_back(t); endfunction
  function void write_act(apb_txn t);
    act_q.push_back(t);
    if (exp_q.size() && act_q.size())
      compare_pair(exp_q[0], act_q[0]);
  endfunction
  function void compare_pair(apb_txn e, apb_txn a);
    if (!e.compare(a)) `uvm_error("SB", "mismatch")
    else begin exp_q.delete(0); act_q.delete(0); end
  endfunction
endclass

Buggy snippet — critique verbally

systemverilog

function void write_act(apb_txn t);
  compare_pair(exp_q[0], t); // always compare — exp_q may be empty
endfunction

Smoke test (5 minutes)

One directed read/write sequence.
Inject single wrong data — expect one UVM_ERROR.
Check queues drain to zero after match.

Code lab drill

After reading, try Code lab exercise uvm-l8-int-scoreboard-fifo — topic reference: /topics/uvm/coding-practice/scoreboard/ordered-fifo-compare

Common pitfalls

Compare when exp_q empty — index error or bogus match.
No txn.compare() — field-wise typos missed.
Not deleting matched entries — duplicate compares.

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