Serial Data Transfer (Link Cable)
Communication between two Gameboys happens one byte at a time. One Gameboy acts as the master, uses its internal clock, and thus controls when the exchange happens. The other one uses an external clock (i.e., the one inside the other Gameboy) and has no control over when the transfer happens. If it hasn't gotten around to loading up the next data byte at the time the transfer begins, the last one will go out again. Alternately, if it's ready to send the next byte but the last one hasn't gone out yet, it has no choice but to wait.
FF01 - SB - Serial transfer data (R/W)
Before a transfer, it holds the next byte that will go out.
During a transfer, it has a blend of the outgoing and incoming bytes. Each cycle, the leftmost bit is shifted out (and over the wire) and the incoming bit is shifted in from the other side:
o7 o6 o5 o4 o3 o2 o1 o0
o6 o5 o4 o3 o2 o1 o0 i7
o5 o4 o3 o2 o1 o0 i7 i6
o4 o3 o2 o1 o0 i7 i6 i5
o3 o2 o1 o0 i7 i6 i5 i4
o2 o1 o0 i7 i6 i5 i4 i3
o1 o0 i7 i6 i5 i4 i3 i2
o0 i7 i6 i5 i4 i3 i2 i1
i7 i6 i5 i4 i3 i2 i1 i0
FF02 - SC - Serial Transfer Control (R/W)
Bit 7 - Transfer Start Flag (0=No transfer is in progress or requested, 1=Transfer in progress, or requested) Bit 1 - Clock Speed (0=Normal, 1=Fast) ** CGB Mode Only ** Bit 0 - Shift Clock (0=External Clock, 1=Internal Clock)
The gameboy acting as master will load up a data byte in SB and then set SC to 0x81 (Transfer requested, use internal clock). It will be notified that the transfer is complete in two ways: SC's Bit 7 will be cleared (i.e., SC will be set up 0x01), and also the Serial Interrupt handler will be called (i.e., the CPU will jump to 0x0058).
The other gameboy will load up a data byte and can optionally set SC's Bit 7 (i.e., SC=0x80). Regardless of whether or not it has done this, if and when the master gameboy wants to conduct a transfer, it will happen (pulling whatever happens to be in SB at that time). The passive gameboy will have its serial interrupt handler called at the end of the transfer, and if it bothered to set SC's Bit 7, it will be cleared.
In Non-CGB Mode the gameboy supplies an internal clock of 8192Hz only (allowing to transfer about 1 KByte per second). In CGB Mode four internal clock rates are available, depending on Bit 1 of the SC register, and on whether the CGB Double Speed Mode is used:
8192Hz - 1KB/s - Bit 1 cleared, Normal 16384Hz - 2KB/s - Bit 1 cleared, Double Speed Mode 262144Hz - 32KB/s - Bit 1 set, Normal 524288Hz - 64KB/s - Bit 1 set, Double Speed Mode
The external clock is typically supplied by another gameboy, but might be supplied by another computer (for example if connected to a PCs parallel port), in that case the external clock may have any speed. Even the old/monochrome gameboy is reported to recognizes external clocks of up to 500KHz. And there is no limitation into the other direction - even when suppling an external clock speed of "1 bit per month", then the gameboy will still eagerly wait for the next bit(s) to be transferred. It isn't required that the clock pulses are sent at an regular interval either.
When using external clock then the transfer will not complete until the last bit is received. In case that the second gameboy isn't supplying a clock signal, if it gets turned off, or if there is no second gameboy connected at all) then transfer will never complete. For this reason the transfer procedure should use a timeout counter, and abort the communication if no response has been received during the timeout interval.
Delays and Synchronization
The gameboy that is using internal clock should always execute a small delay between each transfer, in order to ensure that the opponent gameboy has enough time to prepare itself for the next transfer, ie. the gameboy with external clock must have set its transfer start bit before the gameboy with internal clock starts the transfer. Alternately, the two gameboys could switch between internal and external clock for each transferred byte to ensure synchronization.
Transfer is initiated by setting the master Gameboy setting its Transfer Start Flag, regardless of the value of this flag on the other device. This bit is automatically set to 0 (on both) at the end of Transfer. Reading this bit can be used to determine if the transfer is still active.
INT 58 - Serial Interrupt
When the transfer has completed (ie. after sending/receiving 8 bits, if any) then an interrupt is requested by setting Bit 3 of the IF Register (FF0F). When that interrupt is enabled, then the Serial Interrupt vector at 0058 is called.
Transmitting and receiving serial data is done simultaneously. The received data is automatically stored in SB.
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.
During a transfer, a byte is shifted in at the same time that a byte is shifted out. The rate of the shift is determined by whether the clock source is internal or external. 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.
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 initiates the process of shifting $75 out the serial port and a byte to be shifted into $FF01:
ld a,$75 ld ($FF01),a ld a,$81 ld ($FF02),a