In reply to robert.liu:
It seems src = 3 is the right answer. This port connection is an implicit continuous assignment, due to below LRM subsection:
3.9.6 Port connections
Ports connect processes through implicit continuous assignment statements or implicit bidirectional
connections. Bidirectional connections are analogous to an always-enabled tran connection between the
two nets, but without any strength reduction.
Ports can always be represented as declared objects connected, as follows:
— If an input port, then a continuous assignment from an outside expression to a local (input) net or
variable
— If an output port, then a continuous assignment from a local output expression to an outside net or
variable
— If an inout port, then a non-strength-reducing transistor connecting the local net to an outside net