UVM RAL Callbacks — Extending Register Behaviour

Callbacks exist because real registers do things a generic model cannot express: a read that clears, a write that triggers a side effect elsewhere, a field whose legal values depend on another register. A callback lets that behaviour be attached to a generated model without editing generated code, which is what keeps regeneration safe. The failure mode is registration — an unregistered callback simply never fires, and the model quietly behaves as though the special case did not exist.