Ok I have two (somewhat) unrelated questions:
The first is in regards to timer interrupts I am planning on using Timer3 for a very fast (at least twice a millisecond) interrupt which will create a pulse on one of the various Digital I/O pins on the Arduino Mega 2560. So I went here http://arduino.cc/playground/Code/Timer1 and found the Timer3 library which I am using. It says I can set the interrupt to be any amount of microseconds I want, but really won't it be limited by the combinations of the prescalar? Is it just selecting the closest one to the microseconds you provide?
The second question has to do with Port Manipulation as shown here (http://www.arduino.cc/en/Reference/PortManipulation) Unfortunately it doesn't specify the registers to use for the Arduino Mega 2560, so does anyone have that information? I would be grateful.