45Implementation

Make an action method always enabled

Situation

FreeRunningCounter.tick has no guard and calls no state-dependent method. The design drives it on every cycle, but its interface currently describes an ordinary action method with an Enable input. The implementation works in BSV; the missing piece is the stronger generated-RTL protocol.

The editable area is the starter interface declaration. Keep the method bodies, state, and supplied mkTop structure intact.

Read and predict before editing

Read in this order:

1. Compare the tick and count declarations in FreeRunningCounter.
2. Follow tick to the only register it updates.
3. Check whether drive has a predicate and whether any other rule can stop it.
4. Trace cycle from reset until show becomes enabled.

Predict the starter behavior before Run. Compilation and simulation can already succeed because an ordinary Enable protocol is legal. The counter should advance once for each cycle before show observes it. The problem is therefore not a wrong arithmetic result; it is that generated RTL still permits an external caller to omit tick.

Make the minimal change

Declare that tick is part of every cycle's module behavior. Change only the method declaration in the interface. Do not add a guard, duplicate the call, or alter the update expression to imitate the protocol in datapath logic.

The reasoning is short: tick is permanently ready, and drive requests it on every cycle, so removing the request port accurately describes both callee and caller behavior. Remember that this protocol also includes permanent readiness.

Observable completion

Run succeeds, and the displayed count reflects one increment for each completed pre-display cycle. Independently synthesizing mkFreeRunningCounter no longer requires an Enable input for tick; count remains a read-only value method. The contract must continue to hold for every 8-bit delta, including zero and wraparound values, rather than only for the supplied call with 1.

Progressive hints

Transfer experiment

After completing the required change, temporarily guard drive so it runs only while cycle < 2. Predict why the stronger interface contract is then false even though the method body remains unguarded. Revert the experiment before submitting; it is not part of grading.