Used by the routines at UPDATE_WIZARD, MOVE_PLAYER, UPDATE_SERF and UPDATE_KNIGHT.

Returns one byte for all three control methods, in Kempston's bit order: bit 0 right, bit 1 left, bit 2 down, bit 3 up, bit 4 fire. A bit is 0 when that direction is being asked for.

Kempston reads the other way round, so its byte is inverted; the keyboard needs no inversion but does need its bits shuffled, because Q and W sit in the opposite order to left and right. That swap is the whole reason this routine looks fiddly -- it is what lets the movement code downstream be written once instead of three times.

This is what pins down the key map: the half-row selected is $FBFE, which is Q W E R T, and after the swap Q lands on the "left" bit and W on the "right" bit. So the keyboard controls are Q left, W right, E down, R up, T fire. Confirmed live -- holding W drives the player's x up to the room's right-hand limit and E drives y down to the bottom one.