uvm_set_before_get_dap

Provides a ‘Set Before Get’ Data Access Policy.

The ‘Set Before Get’ Data Access Policy enforces that the value must be written at least once before it is read.  This DAP can be used to pass shared information to multiple components during standard configuration, even if that information hasn’t yet been determined.

Such DAP objects can be useful for passing a ‘placeholder’ reference, before the information is actually available.  A good example of this would be the virtual sequencer:

typedef uvm_set_before_get_dap#(uvm_sequencer_base) seqr_dap_t;
virtual_seqeuncer_type virtual_sequencer;
agent_type my_agent;
seqr_dap_t seqr_dap;

function void my_env::build_phase(uvm_phase phase);
  seqr_dap = seqr_dap_t::type_id::create("seqr_dap");
  // Pass the DAP, because we don't have a reference to the
  // real sequencer yet...
  uvm_config_db#(seqr_dap_t)::set(this, "virtual_sequencer", "seqr_dap", seqr_dap);

  // Create the virtual sequencer
  virtual_sequencer = virtual_sequencer_type::type_id::create("virtual_sequencer", this);

  // Create the agent
  agent = agent_type::type_id::create("agent", this);
endfunction

function void my_env::connect_phase(uvm_phase phase);
  // Now that we know the value is good, we can set it
  seqr_dap.set(agent.sequencer);
endfunction

In the example above, the environment didn’t have a reference to the agent’s sequencer yet, because the agent hadn’t executed its build_phase.  The environment needed to give the virtual sequencer a “Set before get” DAP so that the virtual sequencer (and any sequences one it), could eventually see the agent’s sequencer, when the reference was finally available.  If the virtual sequencer (or any sequences on it) attempted to ‘get’ the reference to the agent’s sequencer prior to the environment assigning it, an error would have been reported.