Env Wiring: Frontdoor + Prediction Paths

Two parallel RAL paths

A complete register environment wires two independent paths : the stimulus path (RAL → bus) and the observe path (bus → mirror). Both are required. Wiring only set_sequencer gives you frontdoor writes but a stale mirror. Wiring only the predictor gives you mirror updates from external masters but no RAL-driven stimulus.

Legend: [RAL] [UVM]

  FRONTDOOR PATH (stimulus)                    OBSERVE PATH (prediction)
  ─────────────────────────                    ──────────────────────────

  reg_seq.write()                              monitor.sample()
       │                                            │
       ▼                                            ▼
  reg_model.default_map                     apb_item on ap
  .set_sequencer(sqr, adapter)                  │
       │                                            │
       ▼                                            ▼
  adapter.reg2bus()                         predictor.bus_in
       │                                            │
       ▼                                            ▼
  apb sequencer → driver → DUT              adapter.bus2reg()
       │                                            │
       ▼                                            ▼
  DUT register updated                      reg_map.predict(rw)
                                                    │
                                                    ▼
                                              mirror updated ✓

Complete reg_env class

class reg_env extends uvm_env;
  `uvm_component_utils(reg_env)

  apb_agent          apb;
  reg_block_model    reg_model;
  apb_reg_adapter    adapter;
  uvm_reg_predictor  #(apb_item) predictor;

  function void build_phase(uvm_phase phase);
    super.build_phase(phase);
    apb       = apb_agent::type_id::create("apb", this);
    reg_model = reg_block_model::type_id::create("reg_model");
    reg_model.build();
    reg_model.lock_model();
    reg_model.set_auto_predict(0);   // explicit prediction only
    adapter   = apb_reg_adapter::type_id::create("adapter");
    adapter.base_addr = 32'h4000_0000;
    predictor = uvm_reg_predictor#(apb_item)::type_id::create("predictor", this);
    uvm_config_db#(reg_block_model)::set(this, "*", "reg_model", reg_model);
  endfunction

  function void connect_phase(uvm_phase phase);
    super.connect_phase(phase);

    // STIMULUS path [RAL]: RAL frontdoor → sequencer → driver
    reg_model.default_map.set_sequencer(apb.sqr, adapter);

    // OBSERVE path [RAL]: monitor → predictor → mirror
    predictor.map     = reg_model.default_map;
    predictor.adapter = adapter;
    apb.mon.ap.connect(predictor.bus_in);
  endfunction
endclass

connect_phase walkthrough

Line-by-line explanation

1. set_sequencer(apb.sqr, adapter) — binds RAL default_map to the APB sequencer; all reg.write()/read()/mirror() frontdoor calls route through adapter.reg2bus() to apb.sqr.

2. predictor.map = reg_model.default_map — tells predictor which address map to use for offset lookup and mirror update.

3. predictor.adapter = adapter — same adapter instance as set_sequencer; ensures addr translation is consistent on both paths.

4. apb.mon.ap.connect(predictor.bus_in) — feeds every observed APB transaction into the predictor; this is the observe-path entry point.

5. set_auto_predict(0) in build_phase — disables RAL self-prediction; only the predictor updates the mirror.

WIRING VERIFICATION CHECKLIST [RAL]

  □ reg_model.default_map.get_sequencer() returns apb.sqr
  □ predictor.map == reg_model.default_map
  □ predictor.adapter == adapter used in set_sequencer
  □ apb.mon.ap is connected (not apb.drv.seq_item_port)
  □ set_auto_predict(0) confirmed in build_phase
  □ adapter.base_addr matches system memory map

Key takeaways

• set_sequencer + predictor.bus_in are both required for full RAL flow.

• Use the same adapter instance on both paths — addr translation must match.

• Connect monitor.ap, never driver — driver items are intent, not observed reality.

Common pitfalls

• Predictor connected to driver seq_item_port — mirror never sees real bus.

• set_sequencer without predictor — frontdoor works, mirror checks fail.

• Predictor without set_sequencer — mirror updates but RAL cannot drive stimulus.