Selected by the menu at GameMenu. Bits 1 and 2 hold the control method read by ReadControls:

Value	Control Method
0	Keyboard
1	Kempston Joystick
2	Cursor Joystick
3	Interface II

Bit 3 is never set by the menu or by anything else, so the game never uses what it would do: it would make the menu flash the line that starts the game, make TestPickupDropKey read the action bit from one place lower, and make HandleTurning move the player by itself.