Interview Question · Concept
Legal Placement and Displacement
The Question
The interviewer asks
What makes a placement legal, and what does the legalizer's displacement report tell you?
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Reveal the Model Answer
A legal placement has every cell on a row, on a site, in the row's orientation or its mirror, and overlapping nothing. Legalization moves each cell from its rough global position to the nearest legal spot, and displacement measures those moves. A large maximum displacement points to an over-full region. The moves cost wirelength and slack, and detailed placement only partly wins them back.
Read the diagram as text
- Global placement: Rough spots; cells may overlap
- Legalizer: Moves each cell to the nearest free site
- Detailed placement: Swaps, shifts and mirrors neighbours
- Over-full region: No free sites nearby
- Displacement report: Total and maximum move
- Global placement → Legalizer
- Legalizer → Detailed placement
- Legalizer → Displacement report
- Over-full region → Displacement report: large maximum
In lesson L4.3, legalization added 1236 µm of wire, and detailed placement won back about 530 µm.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Fresher)
A good fresher answer lists the rules: on a row, on a site, in the right orientation and with no overlap. It says legalization moves each cell into such a legal spot.
What Interviewers Listen For (Mid-Level)
A good mid-level answer reads both the total and the maximum displacement. It links a large maximum to a region that is too full. It re-checks timing after legalization.
Cross-Questions
Where the interviewer goes next. Open a node to practise that follow-up.
WhyWhat IfProve ItCompare
- Legal Placement and Displacement
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WhyWhy Every Other Row Is FlippedL0 FresherL1 Junior
Why is every other standard-cell row flipped upside down?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Each cell has VDD along one edge and VSS along the other. Flipping alternate rows puts the same supply on the boundary between two rows, so neighbours share one rail instead of needing two. A cell placed in a row must take that row's orientation, or its mirror image left to right.
Self-Check
Tick each key point you covered in your answer.
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Prove ItSolve L1.4.1: Finish Legal Cell PlacementSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
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CompareLegalization vs Detailed PlacementL1 JuniorL2 Mid-Level
Compare legalization with detailed placement.
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Legalization only makes the placement legal, moving cells as little as possible. Detailed placement then improves a legal placement without breaking it, by swapping and shifting neighbours and mirroring cells to shorten nets. In lesson L4.3 it won back about 530 µm of the 1236 µm that legalization had added.
Self-Check
Tick each key point you covered in your answer.
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WhyWhy Legalization Can Cost SlackL1 JuniorL2 Mid-Level
Why can legalization cost slack, and not only wirelength?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Legalization moves each cell to a nearby free site without looking at timing. In an over-full region a cell on a critical path can move far. The longer wires add delay on that path. Detailed placement and repair then win part of it back.
Self-Check
Tick each key point you covered in your answer.
Half-Truth
"Legalization moves cells by a site or two, so timing cannot change."
Most moves are small, but the largest move shows over-full regions. A long move on a critical path costs real slack.
Source Lesson L4.3 key points
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What IfAdding a Cell Where the Fillers AreL1 JuniorL2 Mid-Level
What if you need to add a buffer after routing, when every gap in the rows already holds a filler cell?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Filler cells only keep the rails and wells continuous, so they can be taken out where the new cell goes. Remove the fillers around the edit, place and legalize the new cell, route the change, then insert fillers again and re-check. In the pinned flow fillers go in after detailed routing for exactly this reason.
Self-Check
Tick each key point you covered in your answer.
What People Get Wrong
Common wrong answers and why they fail.
Misconception
"Low overflow after global placement means the placement is legal."
Global placement gives continuous positions with some overlap allowed. Legality is a separate step, checked by check_placement.
Source ASIC Point lesson L4.1 (legality checked after legalization)
Half-Truth
"Legalization is free; it only nudges cells."
In lesson L4.3, legalizing a 70 % FIFO moved one cell 35 µm and raised HPWL from about 5585 to 6822 µm. Worst slack fell from 0.375 to 0.178 ns.
Source ASIC Point lesson L4.3 (measured)