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ASIC.FYI

Q293FreeSystemVerilog

Select round-robin winners with rotate and isolate

Difficulty
Hard
Topic
Arbiters
Language
SV
Interview prompt

Question

Optimize round-robin winner selection with vector operations rather than a procedural N-entry scan while preserving the predictor's priority semantics.

Starting point

Question code

function int predict_next(bit [N-1:0] reqs);
Reviewed example

Trace one case

Input
N=4,next_priority=2; request vector has ports {0,3} active
Expected output
rotated isolate selects original port3; next_priority becomes0

Rotating port2 to bit zero makes port3 the lowest active offset; one-hot isolation maps it back without a procedural scan.

What to cover

Requirements

  1. Rotate the current next-priority position to bit zero.
  2. Isolate the lowest set bit of the rotated request vector.
  3. Encode that one-hot offset and map it back to the original port domain.
  4. Return -1 before performing one-hot decode when no request is active.
  5. Keep state update behavior identical to the strict round-robin predictor.
Continue practicing
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