我翻了一下手册, 原来答案在此:
Tip
SMB2 and SMB3 are shared between Port 0 and Port 1. When the
reception of a character on Port 0 results in the execution of the
interrupt routine attached to that event (interrupt event 8), SMB2
contains the character received on Port 0, and SMB3 contains the
parity status of that character. When the reception of a character on
Port 1 results in the execution of the interrupt routine attached to
that event (interrupt event 25), SMB2 contains the character received
on Port 1 and SMB3 contains the parity status of that character.