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

DescriptionQ121

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Q121DVDesignASIC interview problem

Monitor a credit-based link

TechniquesDVDesignArchCredit flow controlProtocol monitor

DifficultyMedium

TopicVerification Utilities

LanguageSystemVerilog

Requirements5 checkpoints

Problem

Monitor a producer-consumer link in which each transmission consumes one credit and each return restores one credit. Detect a transmission with no available credit and a return that would exceed the legal maximum.

Class declarationSystemVerilog

class CreditMonitor;
  function new(int initial_credits = 16, int max_credits = 16);
  function void on_tx();
  function void on_credit_return();
  function int get_current_credits();
  function int error_count();
endclass

Example input and output

Use this case to check your interpretation

Input

initial_credits=2, max_credits=4; transmit three times; return five times

Output

credits 1,0, then underflow rejected at 0; four legal returns restore 4; fifth return rejected; error_count=2

Explanation

Rejected underflow and overflow events leave state unchanged while legal sequential events preserve the configured maximum.

Requirements (5)

Validate that zero is less than or equal to initial_credits and initial_credits is less than or equal to max_credits.
Leave state unchanged when underflow or overflow is detected.
Track any legal sequential interleaving of transmit and return events.
Expose current credits and the cumulative error count.
Treat max_credits as the legal capacity even when initial_credits begins below it.