AVR-Chat

All,

I just installed Studio 4.14 and WinAVR 20080610 on my home machine for
a personal project I have in mind using an ATmega324P. I've never used
WinAVR before so I've been doing a lot of reading to get up on the curve
and, while reading through some of the header files, I found something
puzzling. 

At the bottom of the iomxx4.h header file, where the interrupt table is
defined, the last three vectors, for the second usart, are omitted for
'644P CUPs. Looking over the 164/324/644 datasheet, I can't find any
difference in I/O structure among the parts. In particular, all three
seem to have two USARTS so why is the vector table different? 

Is this a bug or should I keep reading?

Bruce