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

DescriptionQ263

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Q263ArchFollow-up to Q212ASIC interview problem

Make the allocator thread-safe

TechniquesAllocatorSemaphoreConcurrency

DifficultyMedium

TopicMemory Systems

LanguageSystemVerilog

Requirements4 checkpoints

Problem

Protect allocator invariants when several SystemVerilog processes may allocate, deallocate, or inspect state concurrently.

Class declarationSystemVerilog

class LockedRangeAllocator;
  task alloc_exclusive(
    int unsigned size,
    output int unsigned addr,
    output bit success
  );
  task dealloc_exclusive(
    int unsigned addr,
    int unsigned size,
    output bit success
  );
  task fragmentation_metrics_exclusive(
    output longint unsigned total_free,
    output int unsigned largest_block,
    output real external_ratio
  );
  task compact_exclusive(
    ref int unsigned relocated_addr[int unsigned]
  );
endclass

Example input and output

Use this case to check your interpretation

Input

process A calls alloc_exclusive(16, addr, ok) while process B calls dealloc_exclusive(0x1000,16,ok)

Output

one complete operation runs before the other; observers see either whole pre-state or whole post-state

Explanation

A single lock spans every metadata update and is released on success and failure, preventing partially coalesced state from escaping.

Requirements (4)

Use one lock for every operation touching allocator state.
Release the lock on every control path.
Do not expose partially updated range or allocation metadata.
Document that the wrapper blocks while another process owns the lock.