Serial I/O
----------

  The serial I/O port on the Gameboy is a very simple setup
 and is crude compared to standard RS-232 (IBM-PC) or RS-485
 (Macintosh) serial ports. There are no start or stop bits
 so the programmer must be more creative when using this port.

  During a transfer, a byte is shifted in at the same time
 that a byte is shifted out. The rate of the shift is deter-
 mined by whether the clock source is internal or external.
 If internal, the bits are shifted out at a rate of 8192Hz
 (122 microseconds) per bit. The most significant bit is
 shifted in and out first.

  When the internal clock is selected, it drives the clock
 pin on the game link port and it stays high when not used.
 During a transfer it will go low eight times to clock
 in/out each bit.

  A programmer initates a serial transfer by setting bit 7
 of $FF02. This bit may be read and is automatically set
 to 0 at the completion of transfer. After this bit is set,
 an interrupt will then occur eight bit clocks later if the
 serial interrupt is enabled.
  If internal clock is selected and serial interrupt is
 enabled, this interrupt occurs 122*8 microseconds later.
  If external clock is selected and serial interrupt is
 enabled, an interrupt will occur eight bit clocks later.

  Initiating a serial transfer with external clock will
 wait forever if no external clock is present. This allows
 a certain amount of synchronization with each serial port.

  The state of the last bit shifted out determines the
 state of the output line until another transfer takes
 place.

  If a serial transfer with internal clock is performed
 and no external GameBoy is present, a value of $FF will
 be received in the transfer.

  The following code causes $75 to be shifted out the
 serial port and a byte to be shifted into $FF01:

    ld   a,$75
    ld  ($FF01),a
    ld   a,$81
    ld  ($FF02),a