Q092FreeDesign Verification

Verify a minimum energy-window engine

Difficulty: Medium
Topic: Reference Models
Language: SV

Interview prompt

Question

Implement an exhaustive oracle and four-entry tagged scoreboard core for the shortest contiguous nonnegative energy window whose sum reaches a threshold. N is positive and defaults to 32. Samples are known unsigned 32-bit values, the threshold is unsigned 64-bit, and IDs are known 4-bit values. The maximum possible sum fits in 64 bits for the positive signed-int parameter range of N. Pass the value returned by predict into accept_request only for an accepted transaction. Invoke callbacks serially and process a retiring response before same-edge ID reuse. Observed response fields retain four-state logic so X/Z corruption is compared rather than coerced to zero. Reset clears pending IDs; distinguishing a delayed old response after ID reuse requires an external generation or quarantine rule.

Candidate starting point

Implementation scaffold

class energy_window_model #(int N = 32);
  typedef struct {
    logic        bad_request;
    logic        found;
    logic [31:0] start;
    logic [31:0] length;
  } prediction_t;

  prediction_t expected[bit [3:0]];
  function new();
    if(N<1)$fatal(1,"N must be positive");
  endfunction

  function automatic prediction_t predict(
      int unsigned sample[$], longint unsigned threshold);
  // TODO: Implement this oracle or scoreboard method.
endfunction

  function void accept_request(logic [3:0] id, prediction_t p);
  // TODO: Implement this oracle or scoreboard method.
endfunction

  function void reset_epoch();
  // TODO: Implement this oracle or scoreboard method.
endfunction

  function void accept_response(logic [3:0] id, prediction_t actual);
  // TODO: Implement this oracle or scoreboard method.
endfunction

  function void check_empty();
  // TODO: Implement this oracle or scoreboard method.
endfunction
endclass

// TODO: Identify exact hits, equal-length ties, zero density, response order, stalls, errors and reset coverage.

Reviewed example

Trace one case

Input

energy=[2,1,4,2]; threshold=6

Expected output

found=1; start=2; length=2

Window [4,2] reaches six in two elements, shorter than [2,1,4], and no one-element window qualifies.

What to cover

Requirements

Build an exhaustive widened-arithmetic oracle and treat zero threshold or count above the configured maximum as a bad request.
Return found=0 with zero start and length when no window qualifies; otherwise compare shortest length before lowest start.
Store at most four predictions by transaction ID, compare and retire each response exactly once, and reject duplicate or unknown IDs.
Flush every pending prediction on reset and provide an end-of-test empty check for missing responses.
Identify exact-threshold hits, equal-length ties, zero density, response order, stalls, errors, and reset as follow-up coverage targets.

Continue practicing

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class energy_window_model #(int N = 32); typedef struct { logic bad_request; logic found; logic [31:0] start; logic [31:0] length; } prediction_t; prediction_t expected[bit [3:0]]; function new(); if(N<1)$fatal(1,"N must be positive"); endfunction function automatic prediction_t predict( int unsigned sample[$], longint unsigned threshold); // TODO: Implement this oracle or scoreboard method. endfunction function void accept_request(logic [3:0] id, prediction_t p); // TODO: Implement this oracle or scoreboard method. endfunction function void reset_epoch(); // TODO: Implement this oracle or scoreboard method. endfunction function void accept_response(logic [3:0] id, prediction_t actual); // TODO: Implement this oracle or scoreboard method. endfunction function void check_empty(); // TODO: Implement this oracle or scoreboard method. endfunction endclass // TODO: Identify exact hits, equal-length ties, zero density, response order, stalls, errors and reset coverage.

Question

Implementation scaffold

Trace one case

Requirements

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