Hi, in a serial communication program, do I have to "SETB P3.0" in order to receive from the serial port ??
"How does that work when you "set" the pretend DDR for output?"
nothing. absolutely nothing.
So your code doesn't just "set a DDR register" - you need a macro (or macros) that expands to one thing to select "input" mode, and to nothing at all to select "output" mdoe?
depending on the target, the code, macros or calls, may do something, or different things, or nothing at all.
and the 8051 happens to be the platform where the code does absolutely nothing.