In arduino-1.0\hardware\arduino\cores\arduino\HardwareSerial.cpp is the following code:
It looks tied in to the bootloader for some reason!?
I'm going to look through it and see what I can make of it.
void HardwareSerial::begin(unsigned long baud)
{
uint16_t baud_setting;
bool use_u2x = true;
#if F_CPU == 16000000UL
// hardcoded exception for compatibility with the bootloader shipped
// with the Duemilanove and previous boards and the firmware on the 8U2
// on the Uno and Mega 2560.
if (baud == 57600) {
use_u2x = false;
}
#endif
try_again:
if (use_u2x) {
*_ucsra = 1 << _u2x;
baud_setting = (F_CPU / 4 / baud - 1) / 2;
} else {
*_ucsra = 0;
baud_setting = (F_CPU / 8 / baud - 1) / 2;
}
if ((baud_setting > 4095) && use_u2x)
{
use_u2x = false;
goto try_again;
}
// assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
*_ubrrh = baud_setting >> 8;
*_ubrrl = baud_setting;
sbi(_ucsrb, _rxen);
sbi(_ucsrb, _txen);
sbi(_ucsrb, _rxcie);
cbi(_ucsrb, _udrie);
}