Joystick I/O Mode Register

Number	TBRegisterNumber::$0B
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::Joystick port I/O control

 bit 7 = 1 to enable i/o mode (soft reset = 0)
 bit 6 = Reserved, must be 0
 bits 5:4 = I/O Mode (soft reset = 0)
   00 = bit bang
   01 = clock
   10 = uart on left joystick port
   11 = uart on right joystick port
 bits 3:1 = Reserved, must be 0
 bit 0 = Parameter (soft reset = 1)
   bit bang : copied to pin 7
   clock    : 0 = hold high when clock becomes high, 1 = run *
   uart     : 0 = redirect esp uart0 to joystick, 1 = redirect pi uart1 to joystick
              (Tx out on pin 7, Rx in from pin 9, CTS_n in from pin 6 **)

The state of output pin 7 is stored internally in a register and is retained across changing modes and while i/o mode is disabled. While in i/o mode, keyboard joystick types (Sinclair, Cursor, etc) produce no readings but the current state of pins can still be read via the Kempston ports. When leaving i/o mode, joystick operation resumes after ~64 scan lines have passed.

 * CTC channel 3 is currently used to drive pin 7 in clock mode.  Freq = Fctc3 / 2.
 ** CTS_n is only active if the seleced uart is in hw flow control mode.