Rows remain under macros or blockages. Why is that dangerous?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Think of it like a painted parking space directly underneath a building — it still shows up as a marked space on the map, but no car can ever actually park there. Rows left under a macro or blockage are the placement-area equivalent. The danger is concrete: these rows inflate the nominal row-area / placeable-capacity number, which can mislead early utilisation and legality checks into looking healthier than the design actually is.
Technical Explanation
- g. reassigned to reflect the macro's actual footprint)?
- Check the macro's fixed/placed status, the purpose of the keepout, whether the site arrays under it are still active, which voltage area they belong to, and — critically — whether the floorplan was actually re-synced after the macro moved (a very common way stale rows appear).
- Leaving this unresolved doesn't just create a cosmetic inconsistency — it can let a placer attempt (and fail) to legalize cells into space that was never really available.
Formula Or Decision Rule
Decision rule: reported usable rows match the space where intended cells can actually be placed.
What To Check
- Warning sign: The team counts the hidden row area as whitespace.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Regenerate or trim rows and recompute usable capacity before rerunning placement/congestion.
Command Checks & Actions
report_design-floorplan: reports the floorplan state understood by the toolcheck_design-checks {dp_pre_macro_placement}: runs named design-planning check groups
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: They inflate nominal placeable area and can mislead early placement, utilisation, or legality. Determine whether the rows should be cut, made transparent by methodology, or associated differently.
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Rows remain under macros or blockages. ” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that the team counts the hidden row area as whitespace. ”
Physical Design & Planning Handbook
Master ASIC Physical Design Planning & Floorplanning
Dive into 14 comprehensive chapters covering netlist sanity, FinFET grids, macro placement, power grids, CTS, and timing budgeting.
Offline PDF Bundle
Want all 1109 questions offline?
Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.
Continue practising