There is no hardware that is driving these registers so the data is what is last on the z80 data bus.
===Digital joysticks===
There is no dreaded "keyboard clash" between the digital joysticks, the fire buttons of the analogue joystick or the keyboard. This means that pressing a combination of buttons on the joystick and pressing keys on the keyboard don't result in a phantom key as they would on the CPC.
The keyboard itself suffers from keyboard clash however.
===Power requirements===