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SystemVerilog Blocking vs. Nonblocking Assignments

EasySystemVerilogMultiple choice

What values are printed by display and strobe, in that order?

Question code
logic a;
initial begin
  a = 0;
  a <= 1;
  $display("D=%0b", a);
  $strobe("S=%0b", a);
end

Answer choices

  1. A. D=0, S=0
  2. B. D=x, S=1
  3. C. D=1, S=1
  4. D. D=0, S=1

Concise answer

The answer and the key reason

Choose D: $display prints D=0 and $strobe prints S=1. The blocking assignment changes a immediately, while the nonblocking assignment schedules its update for the NBA region. $display runs before that scheduled update; $strobe reports later, after the new value has taken effect.

Deeper explanation

Work through the implementation and tradeoffs

At the start of the initial block, the blocking assignment places 0 into a in the current active-region execution. The following <= statement evaluates its right-hand side but defers writing 1. Therefore the immediate $display call still observes the old current value and emits D=0.

Nonblocking updates are committed later in the same simulation time slot, after active-region work. $strobe waits until the postponed region, when those updates are visible and the time slot has settled, so it emits S=1. No simulation time advances; the difference comes entirely from event-region ordering, which is central to race-free modeling in synchronous RTL.

What to remember

  • $display sees active-region state
  • NBA updates occur later
  • $strobe sees settled state

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