Phase Overview Hub: Scheduler Traversal & the Common List

Overview

The UVM phase scheduler walks the component tree and invokes the same named callback on every component for each phase. The order of phases is fixed; the traversal direction changes per phase.

Only run_phase (and the 12 runtime sub-phases) consume simulation time. All other common phases are zero-time function phases.

Sub-lessons in this topic

1. phase-scheduler-overview — how uvm_phase and the scheduler cooperate.

2. common-phase-list — the nine common phases and what each does.

3. traversal-order-rules — top-down vs bottom-up and why.

4. phase-timeline-map — build, run, cleanup on one axis.

5. scheduler-vs-user-code — what the library owns vs what you implement.

6. phase-overview-debug — trace switches and traversal verification.

[UVM][PHASE] scheduler at a glance

for each phase in fixed order:
  scheduler selects traversal direction (TD or BU)
  visits every uvm_component in tree
  calls <phase>_phase(phase) on each node
  waits for task phases / sub-phases to complete
  advances to next phase

user code:
  implement callbacks
  never call phase methods on neighbors directly

// The scheduler calls YOU — not the other way around
function void connect_phase(uvm_phase phase);
  super.connect_phase(phase);
  // wiring only — scheduler already ran all build_phase callbacks
endtask

Key takeaways

• Nine common phases run in a fixed global order for the entire testbench.

• Traversal direction alternates to satisfy structural dependencies.

• run_phase is the only common phase that advances simulation time.

Common pitfalls

• Manually calling a neighbor's phase method — breaks scheduler contract.

• Assuming connect_phase is top-down — it is bottom-up.

• Expecting portable sibling order within a single phase.