Return from subroutine, then read $DD00 , and do something with it.

Again, $DD00 is a CIA IO register, and so there are a number 
of routines that are required that simply check or modify the
contents of this register in order to implement the IEC serial
protocol.  Thus it is totally unsurprising to find routines that
start by reading $DD00 and then processing it in some way.