Predictors: uvm_reg_predictor & Bus-to-RAL

Overview

The uvm_reg_predictor keeps the RAL register mirror synchronized with observed bus traffic. It is the bridge between your protocol monitor and the register abstraction layer — essential for frontdoor mirror checks, multi-master register access, and any test that calls reg.mirror(UVM_CHECK) after bus activity.

Predictors are often confused with reference models. A ref model predicts transaction responses for scoreboard checking. A reg predictor updates the RAL mirror from observed register bus cycles. They serve different purposes and wire to different consumers.

Sub-topics

1. Predictor Role — RAL mirror vs general ref model; mirror failure walkthrough.

2. Reg Adapter — bus2reg and reg2bus implementation with field-by-field explanation.

3. Env Wiring — frontdoor and prediction paths running in parallel.

4. Explicit vs Implicit Prediction — auto_predict tradeoffs and double-predict trap.

5. Multi-Master & Debug — several monitors, backdoor predict, debug checklist.

Legend: [RAL] [UVM]

  [RAL] PREDICTOR DATA PATH

  monitor.ap ──► predictor.bus_in
                      │
                      ▼
                 adapter.bus2reg(apb_item)
                      │
                      ▼
                 uvm_reg_bus_op { kind, addr, data, byte_en, status }
                      │
                      ▼
                 reg_map.predict(rw)
                      │
                      ▼
                 mirror[field] = observed value

  [UVM] STIMULUS PATH (parallel, not through predictor)

  reg_seq.write() ──► adapter.reg2bus() ──► sequencer ──► driver ──► DUT

Predictor vs scoreboard vs ref model

Component          Input              Output              Purpose
─────────────────────────────────────────────────────────────────────────
ref model [CHECK]  request txn        expected response   scoreboard exp stream
scoreboard [CHECK] exp + act txns     UVM_ERROR           value compare
predictor [RAL]    observed bus txn   mirror update       RAL state sync
coverage [COVER]   observed bus txn   bin hits            measurement

Key takeaways

• Predictor = bus-observed mirror update, not general scoreboard checking.

• Frontdoor tests need both set_sequencer (stimulus) and predictor (observe).

• Default to explicit prediction in integrated chip environments.