Unique Case vs Priority Case

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WhyProve ItCompareScale It

1. Unique Case vs Priority Case
2. WhyWhat the Unique Netlist Does with Two RequestsL2 Mid-Level

   Why do the gates and the RTL disagree when two requests arrive at a unique selector?

   Reveal the Model Answer

   The RTL runs the first matching arm, so it passes one data word. The netlist was built on the promise of one request at a time. Each data word is ANDed with its request and the results are ORed. With two requests, two words mix: 0010 | 0100 = 0110. The false promise became a functional bug.

   1. The chain

      req[0] is tested first, then req[1], and so on. A word passes only when every lower request is 0. Five levels of logic deep.

   2. The AND-OR

      Every word is gated by its own request, all at once, and the four results are ORed. Three levels deep, but seven cells more.

   3. Two requests

      With req = 0110 the chain passes d1, 0010. The AND-OR passes d1 and d2 together: 0110. unique told the synthesizer this input never happens.

      Follow the calls, values and waits

      1. data req = 0110: req[3:0] → priority chain
      2. data req = 0110: req[3:0] → AND-OR
      3. data 0010: priority chain → y (if chain)
      4. data 0110: AND-OR → y (unique case)

   Read the diagram as text

   • req[3:0]: requests
   • d0 .. d3: four 4-bit words
   • priority chain: 21 cells, 5 levels
   • AND-OR: 28 cells, 3 levels
   • y (if chain): lowest request wins
   • y (unique case): all requests mixed

   • req[3:0] → priority chain: tested in order
   • d0 .. d3 → priority chain: data
   • priority chain → y (if chain)
   • req[3:0] → AND-OR: one AND per word
   • d0 .. d3 → AND-OR: data
   • AND-OR → y (unique case)

   Bigger and shallower: the parallel form pays cells to save levels. Neither shape is smaller by rule; measure.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 The RTL runs the first matching arm, so it passes one data word. Key point 2 The netlist was built on the promise of one request at a time. Key point 3 Each data word is ANDed with its request and the results are ORed. Key point 4 With two requests, two words mix: 0010 | 0100 = 0110. Key point 5 The false promise became a functional bug.

   Misconception

   "Overlap just picks one of the two words at random."

   It is not random. The AND-OR passes both words together, giving a value neither source sent.

   Source Lesson R1.5 measurement: the Yosys 0.70 netlist of the unique selector, simulated with req = 0110, outputs 0110

   • Lesson R1.5Case Statements: Priority, Parallel, Unique
3. Prove ItCatching a False Unique Promise in RTLL2 Mid-Level

   Prove it: how do you catch that false promise in RTL, before the netlist shows it?

   Reveal the Model Answer

   Run RTL simulation with the unique checks on. The simulator reports a violation when two items match, or when none does. Also assert $onehot0(req) at the block boundary, so the caller's side is checked. Formal can then prove that assertion from the requesters' logic.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Run RTL simulation with the unique checks on. Key point 2 The simulator reports a violation when two items match, or when none does. Key point 3 Also assert $onehot0(req) at the block boundary, so the caller's side is checked. Key point 4 Formal can then prove that assertion from the requesters' logic.

   Tool as Truth

   "Synthesis would warn about overlapping items."

   Synthesis takes unique as a promise and builds smaller logic from it. The check lives in simulation, not in synthesis.

   Source Sutherland, RTL Modeling with SystemVerilog (2017) ch. 7.4.2 The unique and unique0 decision modifiers

   • Lesson R1.5Case Statements: Priority, Parallel, Unique
   • Problem R1.5.1A Priority Encoder with a Collision Flag
4. CompareUnique, Unique0 and PriorityL2 Mid-Level

   Compare unique, unique0 and priority. When would you choose each?

   Reveal the Model Answer

   priority keeps the order and only promises that something matches. Use it when overlaps are legal and the spec says who wins. unique promises exactly one match without a default. unique0 promises at most one and allows no match. Use unique0 for a decode where "nothing selected" is normal.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 priority keeps the order and only promises that something matches. Key point 2 Use it when overlaps are legal and the spec says who wins. Key point 3 unique promises exactly one match without a default. Key point 4 unique0 promises at most one and allows no match. Key point 5 Use unique0 for a decode where "nothing selected" is normal.

   Half-Truth

   "priority case is the same as a plain case."

   The order is the same, but priority adds a check. With no default, a value that matches no item is reported.

   Source IEEE 1800-2017 §12.5.3

   • Lesson R1.5Case Statements: Priority, Parallel, Unique
5. Scale ItA Policy for Unique in a Design TeamL3 SeniorL4 Staff

   As a design lead, when do you allow unique, and how do you make sure the promise holds?

   Reveal the Model Answer

   Allow it only where the inputs are exclusive by construction, such as a one-hot state register or a decoded address. Back each use with a one-hot assertion and a formal proof or a test that tries the overlap. Run regressions on a simulator that checks unique. Treat every violation as a design bug, not a warning.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Allow it only where the inputs are exclusive by construction, such as a one-hot state register or a decoded address. Key point 2 Back each use with a one-hot assertion and a formal proof or a test that tries the overlap. Key point 3 Run regressions on a simulator that checks unique. Key point 4 Treat every violation as a design bug, not a warning.

   What Interviewers Listen For (Staff)

   A good staff answer ties the language feature to sign-off. Each use gets an assertion and a proof, regressions use a simulator that checks unique, and any violation blocks the release.

   Wrong Altitude

   "Add unique everywhere for faster logic."

   Every false promise is a hidden RTL-versus-gates mismatch. The speed gain is not worth an unchecked assumption.

   • Lesson R1.5Case Statements: Priority, Parallel, Unique
   • Lesson R7.2Assertions Inside RTL and the Synthesis Guards
6. Prove ItSolve R1.5.1: A Priority Encoder with a Collision FlagSimulator-graded

   Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

   Open Problem R1.5.1