Interview Question · Code Reading

Wildcard Bins

L0 FresherL1 Juniorfunctional coveragebins

The interviewer asks

What does this print? How would you change it so every odd value must be seen?

systemverilog

bit [3:0] d;
covergroup cg;
  cp: coverpoint d { wildcard bins odd = {4'b???1}; }
endgroup
cg c = new();
initial begin
  d = 4'b0001; c.sample();
  $display("coverage=%0.2f", c.get_coverage());
end

Reveal the Model Answer

It prints coverage=100.00. With the wildcard keyword, each ? bit matches 0 or 1, so the bin collects all eight odd values. It is still one bin, and one hit on any odd value covers it. To need each odd value separately, write wildcard bins odd[] = {4'b???1};. That makes eight bins, and one sample covers 12.5%.

One Wildcard Bin or EightThe twin and its fix. Arrows show where the one sampled value lands.

Read the diagram as text

One sample: d = 4'b0001
wildcard bins odd = {4'b???1}: One bin for all eight odd values
wildcard bins odd[] = {4'b???1}: Eight bins, one per odd value
100.00%: One hit covers the only bin
12.50%: 1 of 8 bins covered

One sample → wildcard bins odd = {4'b???1}: matches
One sample → wildcard bins odd[] = {4'b???1}: matches one bin
wildcard bins odd = {4'b???1} → 100.00%
wildcard bins odd[] = {4'b???1} → 12.50%

Measured: the twin prints 100.00 and the fixed version prints 12.50. The brackets decide how many bins one line makes.

Self-Check

Tick each key point you covered in your answer.

Key point 1 It prints coverage=100.00. Key point 2 With the wildcard keyword, each ? bit matches 0 or 1, so the bin collects all eight odd values. Key point 3 It is still one bin, and one hit on any odd value covers it. Key point 4 To need each odd value separately, write wildcard bins odd[] = {4'b???1};. Key point 5 That makes eight bins, and one sample covers 12.5%.

What Interviewers Listen For (Fresher)

A good fresher answer says it is one bin, so one hit gives 100%. It then writes the [] form that makes eight bins.

Cross-Questions

Where the interviewer goes next. Open a node to practise that follow-up.

What IfProve ItCompareScale It

Wildcard Bins
CompareWildcard Bins vs Filtered Bin ArraysL1 JuniorL2 Mid-Level
Compare these two ways to cover word-aligned addresses. What does the covergroup print?
systemverilog
```
module tb;
  bit [7:0] addr;
  covergroup cg;
    wild: coverpoint addr { wildcard bins aligned = {8'b??????00}; }
    each: coverpoint addr { bins aligned[] = {[0:15]} with (item % 4 == 0); }
  endgroup
  cg c = new();
  initial begin
    addr = 0; c.sample();
    addr = 4; c.sample();
    addr = 5; c.sample();   // not aligned: neither coverpoint counts it
    $display("covergroup=%0.2f (wild 100, each 2 of 4)", c.get_coverage());
  end
endmodule
```
2:30

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Reveal the Model Answer

It prints covergroup=75.00. The wildcard bin is one bin for every address ending in 00, so two aligned samples give 100%. The with filter keeps the aligned values in 0 to 15 and makes one bin each, so two of four are covered. The covergroup averages 100 and 50. Use the wildcard to ask "was any aligned access seen" and the filter when each aligned address matters.

RUNVerilator (coverage build) · simulator service

Edit the code, then run it

Expected: covergroup=75.00 (wild 100, each 2 of 4)

Self-Check
Tick each key point you covered in your answer.
Key point 1 It prints covergroup=75.00. Key point 2 The wildcard bin is one bin for every address ending in 00, so two aligned samples give 100%. Key point 3 The with filter keeps the aligned values in 0 to 15 and makes one bin each, so two of four are covered. Key point 4 The covergroup averages 100 and 50. Key point 5 Use the wildcard to ask "was any aligned access seen" and the filter when each aligned address matters.
Misconception

"Both forms build the same bins."

Without [] the wildcard is one bin. The filter with [] builds one bin per value that passes, which the run shows as 2 of 4.

Source IEEE 1800-2017 §19.5.1.1
- Lesson C3Choosing Bins
Prove ItOpcode Classes Using the Top Two BitsL1 Junior
Prove it: write bins that split an 8-bit opcode into four classes by its top two bits.
2:30

You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.

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Reveal the Model Answer

Write one wildcard bin per class, with the top two bits fixed and the rest as ?. For example, wildcard bins alu = {8'b00??????};. After an ALU, a load and a branch opcode, the run reports 75%, because no store was seen.

RUNVerilator (coverage build) · simulator service

Edit the code, then run it

Expected: class coverage=75.00 (no store yet)

Self-Check
Tick each key point you covered in your answer.
Key point 1 Write one wildcard bin per class, with the top two bits fixed and the rest as ?. Key point 2 For example, wildcard bins alu = {8'b00??????};. Key point 3 After an ALU, a load and a branch opcode, the run reports 75%, because no store was seen.
- Lesson C3Choosing Bins
Scale ItOpcode Coverage for a Whole Instruction SetL2 Mid-LevelL3 Senior
The instruction set has 200 opcodes in a dozen classes. How do you structure the opcode coverage?

Reveal the Model Answer

Use two levels. A class coverpoint with wildcard bins shows broad progress early. A per-opcode coverpoint, generated from the same table the decoder uses, shows the long tail. Cross the class, not each opcode, with operand corners, or the cross explodes. Generate the bins from the spec table so a new opcode cannot be forgotten.

Self-Check
Tick each key point you covered in your answer.
Key point 1 Use two levels. Key point 2 A class coverpoint with wildcard bins shows broad progress early. Key point 3 A per-opcode coverpoint, generated from the same table the decoder uses, shows the long tail. Key point 4 Cross the class, not each opcode, with operand corners, or the cross explodes. Key point 5 Generate the bins from the spec table so a new opcode cannot be forgotten.

What Interviewers Listen For (Senior)
A good senior answer generates the bins from the instruction table, so a new opcode cannot be forgotten. It keeps crosses at the class level and says how many cells that saves.

Wrong Altitude

"Cross every opcode with every operand case; the tool will cope."

Two hundred opcodes times ten operand cases is 2000 cells. Most will stay empty for months and hide the holes that matter.

Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §26.20 (PDF p481)
- Lesson C5Cross Coverage
- Lesson C7Closing Holes
What IfAn Ignore Bin Inside a Wildcard BinL1 JuniorL2 Mid-Level
What if value 0 must be excluded, but it also matches the wildcard bin for even values? What does this print?
systemverilog
```
module tb;
  bit [3:0] d;
  covergroup cg;
    cp: coverpoint d {
      wildcard bins even = {4'b???0};
      ignore_bins zero = {0};          // 0 matches 'even' too
    }
  endgroup
  cg c = new();
  initial begin
    d = 0; c.sample();
    $display("after 0: %0.2f", c.get_coverage());
    d = 6; c.sample();
    $display("after 6: %0.2f", c.get_coverage());
  end
endmodule
```
2:30

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Reveal the Model Answer

It prints after 0: 0.00, then after 6: 100.00. An ignored value is removed from every bin of the coverpoint, even a bin that also names it. So sampling 0 covers nothing, and the even bin needs some other even value.

RUNVerilator (coverage build) · simulator service

Edit the code, then run it

Expected: after 0: 0.00 after 6: 100.00

Self-Check
Tick each key point you covered in your answer.
Key point 1 It prints after 0: 0.00, then after 6: 100.00. Key point 2 An ignored value is removed from every bin of the coverpoint, even a bin that also names it. Key point 3 So sampling 0 covers nothing, and the even bin needs some other even value.
Misconception

"The value counts for whichever bin was declared first."

Declaration order does not matter. Ignore and illegal values are taken out of all the other bins.

Source IEEE 1800-2017 §19.5.5
- Lesson C3Choosing Bins

What People Get Wrong

Common wrong answers and why they fail. Run It shows the wrong belief failing on a real simulator.

Misconception

"A wildcard bin is covered only when every matching value has been seen."

A scalar bin is covered by any one of its values. The twin covers it with 4'b0001 alone; the [] form needs all eight.

Run It

The interview code with one odd sample.

RUNVerilator (coverage build) · simulator service

Edit the code, then run it

Expected: coverage=100.00 after one odd value FAIL: one wildcard bin is covered by a single matching value

module tb;
  bit [3:0] d;
  covergroup cg;
    cp: coverpoint d { wildcard bins odd = {4'b???1}; }   // the belief: needs all 8 odd values
  endgroup
  cg c = new();
  initial begin
    d = 4'b0001; c.sample();
    $display("coverage=%0.2f after one odd value", c.get_coverage());
    if (c.get_coverage() == 100.0) $display("FAIL: one wildcard bin is covered by a single matching value");
    else $display("PASS: the array form makes 8 bins, so one odd value covers 1 of 8");
  end
endmodule

See the Fix

module tb;
  bit [3:0] d;
  covergroup cg;
    cp: coverpoint d { wildcard bins odd[] = {4'b???1}; }   // the fix: one bin per odd value
  endgroup
  cg c = new();
  initial begin
    d = 4'b0001; c.sample();
    $display("coverage=%0.2f after one odd value", c.get_coverage());
    if (c.get_coverage() == 100.0) $display("FAIL: one wildcard bin is covered by a single matching value");
    else $display("PASS: the array form makes 8 bins, so one odd value covers 1 of 8");
  end
endmodule

Prints: coverage=12.50 after one odd value PASS: the array form makes 8 bins, so one odd value covers 1 of 8

Half-Truth

"Wildcard bins are the way to cover a range of values."

A range such as [0:7] says contiguous values more clearly. Wildcards fit bit patterns, where the values are spread out.

Source IEEE 1800-2017 §19.5.4

Buzzword Without Mechanism

"Wildcard means the bin matches any value."

Only the bits written as ?, x or z are free. The other bits must match exactly.

Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §26.22 (PDF p490)

Learn It, Then Prove It

Lesson C3Choosing Bins

Find Your Next Track

Put Your Skills to Work

Prepare to Explain Your Thinking

An Idea to Try? Pick a Tool.

Wildcard Bins

Your Answer

Self-Check

What Interviewers Listen For (Fresher)

What Interviewers Listen For (Junior)

Cross-Questions

Your Answer

Self-Check

Your Answer

Self-Check

Your Answer

Self-Check

What Interviewers Listen For (Senior)

Your Answer

Self-Check

What People Get Wrong

Learn It, Then Prove It

The Question

Your Answer

Self-Check

What Interviewers Listen For (Fresher)

What Interviewers Listen For (Junior)

Cross-Questions

Your Answer

Self-Check

Your Answer

Self-Check

Your Answer

Self-Check

What Interviewers Listen For (Senior)

Your Answer

Self-Check

What People Get Wrong

Learn It, Then Prove It