Kempston Joystick Interface
Until the release of the +2 and +3, the Kempston joystick interface and its compatibles were the most common Joystick interfaces available to Speccy owners.
The original version also worked with the ZX-81, but joystick support in ZX81 games was much less common.
The interface uses standard Atari-style joysticks.
Bits are active high so 00=nothing pressed.
|5||00100000||Very occasionally a second fire button on some clones, but usually 0|