SystemVerilog Interview Questions and Answers

logic is a 4-state type that can be driven procedurally or by a single continuous assignment, unifying the roles of reg and wire for single-driver nets. It cannot be used for a multi-driver net — that still needs wire. Using logic everywhere you have one driver removes the old reg/wire confusion.

bit is 2-state (0/1); logic is 4-state (0/1/x/z). 2-state types simulate faster and use less memory but cannot represent unknown/high-impedance, so uninitialised 2-state variables read as 0 and can hide bugs that a 4-state x would expose. Use 4-state for DUT-facing signals, 2-state for scoreboard/model counters.

They declare design intent and let the tool check it. always_comb is combinational with an automatic complete sensitivity list and errors on inferred state; always_ff is a clocked register; always_latch is an intentional latch. They catch the classic accidental-latch and missing-signal bugs at compile time.

A struct groups fields that all exist; a union overlays fields in the same storage (one at a time). packed lays the members out as a contiguous bit vector you can treat as one number and put on a wire; unpacked is a collection of separate elements. Packed structs are synthesizable as buses; unpacked are usually for testbench data.

A dynamic array is a contiguous array sized at runtime (new[n]). A queue is an ordered, growable list with push/pop at both ends ([$]) — ideal for scoreboards. An associative array is a sparse map keyed by any type ([string], [int]) — ideal for large, sparse, or name-indexed data like a register model.

Because frameworks hold derived objects through base-class handles. A virtual method dispatches to the actual object’s override at run time; a non-virtual method resolves by the handle type and would call the base version. UVM marks its hooks (do_copy, do_compare, phase methods) virtual for exactly this reason.

rand picks a value each randomize() independently (values can repeat). randc is random-cyclic: it visits every value in the range once before any repeats, useful for sweeping all opcodes or addresses without duplication across a cycle.

A mailbox is a thread-safe FIFO for passing transactions between processes (e.g. generator to driver) with blocking put/get. A semaphore is a counting lock for arbitrating access to a shared resource with get/put of keys. One moves data; the other grants permission.

$display prints immediately when executed (Active region — may show pre-NBA values). $strobe prints at the end of the time step (Postponed region) after all non-blocking updates settle. $monitor prints automatically whenever any of its arguments change. Use $strobe/clocking to sample stable end-of-cycle values.

An immediate assertion (assert(expr)) checks a condition at the instant it executes, procedurally. A concurrent assertion (assert property(...)) is clocked and checks temporal behaviour over cycles using sequences, the |-> implication and ##n delays — the basis of protocol checking and formal.