Kempston Joystick Interface
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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.
Port $1f
Bits are active high so 00=nothing pressed.
| Bit | Mask | Usage |
|---|---|---|
| 0 | 00000001 | Right |
| 1 | 00000010 | Left |
| 2 | 00000100 | Down |
| 3 | 00001000 | Up |
| 4 | 00010000 | Fire! |
| 5 | 00100000 | Very occasionally a second fire button on some clones, but usually 0 |
| 6 | 01000000 | Always 0 |
| 7 | 10000000 | Always 0 |