Mode Notes:

Joystick modes:
  000 = Sinclair 2 (12345)
  001 = Kempston 1 (port 0x1F)
  010 = Cursor (56780)
  011 = Sinclair 1 (67890)
  100 = Kempston 2 (port 0x37)
  101 = MD 1 (3 or 6 button joystick port 0x1F)
  110 = MD 2 (3 or 6 button joystick port 0x37)
  111 = User Defined Keys Joystick
  on nextreg 0x28, nextreg 0x29 and nextreg 0x2B:
  1. Write 128 to nextreg 0x28
  2. Write 0 (left joystick) or 16 (right joystick) to nextreg 0x29
  3. Write twelve bytes to nextreg 0x2B in order. The bytes correspond to the twelve
  buttons on an md pad (MODE=11 X Z Y START A C B U D L R=0)
  4. Each byte written identifies a key in the 8x7 membrane; bits 5:3 select the row
  and bits 2:0 select the column with 111 meaning no action
  generate key input if so programmed