UVM Component Hierarchy

Before diving into phases and components, it's crucial to understand how UVM organizes your testbench. Think of UVM like a family tree—every component has a parent, and together they form a hierarchy.

The Root of Everything: uvm_root

At the very top of every UVM testbench sits uvm_root, also known as uvm_top. This is a singleton object (only one exists) that manages the entire simulation. You never create it yourself—UVM does this automatically.

The Component Tree Structure

Every UVM testbench follows a standard hierarchy. Here's what a typical tree looks like:

uvm_root (uvm_top)
  +-- my_test
      +-- env
          ├-- agent
          |   ├-- sequencer
          |   ├-- driver
          |   +-- monitor
          +-- scoreboard
                            

Parent-Child Relationships

When you create a component using the factory (type_id::create()), you pass this as the second argument. This tells UVM: "I am the parent of this new component."

// Inside the Test's build_phase
function void build_phase(uvm_phase phase);
    // Create the environment. "this" (the test) is the parent.
    env = my_env::type_id::create("env", this);
endfunction
// Inside the Environment's build_phase
function void build_phase(uvm_phase phase);
    // Create the agent. "this" (the env) is the parent.
    agent = my_agent::type_id::create("agent", this);
    scoreboard = my_scoreboard::type_id::create("scoreboard", this);
endfunction
// Inside the Agent's build_phase
function void build_phase(uvm_phase phase);
    // Create driver, monitor, sequencer. "this" (the agent) is the parent.
    sequencer = my_sequencer::type_id::create("sequencer", this);
    driver = my_driver::type_id::create("driver", this);
    monitor = my_monitor::type_id::create("monitor", this);
endfunction
                            

Why Does Hierarchy Matter?

The hierarchy isn't just for organization—it's the foundation for three critical UVM features:

• Phasing: UVM uses the tree to execute phases in the correct order (top-down for build, bottom-up for connect).
• Configuration: The uvm_config_db uses hierarchical paths to deliver settings to the right components.
• Factory Overrides: Instance overrides target specific components using their hierarchical path (e.g., "env.agent.driver").