when hardware maps register into port or address space, it has to detect what i read or write to it.
how is it done? does hardware has something like OnRead/OnWrite functions?

when hardware has mapped 2 byte register into port 0, and i write only 1 byte to 0 or 1, will cpu detect it, or hardware just get partial (or padded) data?

b1528932,

Your question is too vague to give exact answer — particular buses can behave quite differently (according to specification and implementation details).