Joystick Ports
The interface two came with two joystick ports that (unlike the Kempston which used the IN31 command) were mapped to actual key presses. Player 1 was mapped to 1-5 and player 2 was mapped to 6-0. This initially seemed at odds with Sinclair's own keyboard layout, given that the keyboard itself had the cursor keys mapped to 5-8 with 0 typically being used by games as a fire button. Joystick interfaces that mapped to the cursor keys were available, but like the popular Kempston interface they were limited to supporting a single joystick only. It was the twin joystick feature of the ZX Interface 2 that turned out to be its major selling point.
Read more about this topic: ZX Interface 2
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