1,000+ ASIC Interview Questions: SystemVerilog, UVM & RTL

DescriptionQ216

Page ↗

Q216DesignArchFollow-up to Q292ASIC interview problem

Predict weighted round-robin grants

TechniquesDesignArchWeighted round robinSchedulerEpoch state

DifficultyHard

TopicArbiters

LanguageSystemVerilog

Requirements6 checkpoints

Problem

Extend the arbiter so port i has configurable weight W[i] and may receive up to that many consecutive grants before priority moves to the next active requester.

Class declarationSystemVerilog

class WRRPredictor #(int N = 8);
  function void set_weights(int values[N]);
  function int predict_next(bit [N-1:0] reqs);
  function void update_state(int grant_idx, bit [N-1:0] reqs);
  function void reset();
endclass

Example input and output

Use this case to check your interpretation

Input

weights=[2,1,3]; all three requesters remain asserted for six accepted grants

Output

grant sequence=[0,0,1,2,2,2]

Explanation

Each port consumes its complete positive epoch before service rotates, exactly matching the configured weights.

Requirements (6)

Require a positive effective weight for every port.
Track the current priority port and its remaining grants in the current epoch.
Skip inactive requesters and return -1 when all requests are inactive.
Begin a fresh weight epoch when service moves to another port.
Consume one unit only after an accepted grant.
Define weight reconfiguration as starting a fresh epoch at the current priority.