Don't put any members after these buffers, since only the first 32 bytes of this struct can be accessed quickly using the ldd instruction

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@mustaq_ahm Are you referring to the AVR library: https://github.com/arduino/ArduinoCore-avr/blob/master/cores/arduino/HardwareSerial.h#L110
There is a 'ldd' instruction and maybe some things are faster. However in this situation I think that text is totally nonsense.

@gcjr In HardwareSerial.h of the AVR branch, the buffer is set to 64 bytes. For microcontrollers with little RAM, it will be 16 bytes. Two blocks of 16 bytes make a block of 32 bytes and maybe that can be optimized for certain instructions. If you have a Arduino board with a ATmega processor with less then 1kbyte, then it is still trivial if that optimization can be noticed. Even the ATmega8 has 1024 bytes of RAM.