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Using the Serial object pulls HardwareSerial0.o into your program linkage. This file defines, among other things, the ISRs (interrupt service routines) associated with the serial port. ISRs are defined using the ISR() macro from the acr-libc. This macro creates a function prototype and assigns some attributes to the function, including the used attribute which tells the linker that it should consider the function to be used, even if it cannot spot a single call to it within the compiled program. The ISR cannot then be removed, and it in turn keeps lots of code from the Serial implementation.