Not sure about assembly as I don't use it much for the boards I used (mainly arm) but there has been quite a bit of discussion on accessing I/O registers:
Compatibility: digitalPinToPort and digitalPinToBitMask - UNO R4 / UNO R4 WiFi - Arduino Forum
AVR Compatibility: PORT and PIN registers? - UNO R4 / UNO R4 WiFi - Arduino Forum
This is a good one:
digitalWriteFast with UNO R4 - UNO R4 / UNO R4 Minima - Arduino Forum
There was another one to a library someone had put together but I can not find the reference right now. If I come across it I will post it here.