SGB Command $11 — MLT_REQ
Used to request multiplayer mode (that is, input from more than one joypad). Because this function provides feedback from the SGB/SNES to the Game Boy program, it can also be used to detect SGB hardware.
Byte Content 0 Command*8+Length (fixed length=1) 1 Multiplayer Control (0-3) (Bit0=Enable, Bit1=Two/Four Players) 0 = One player 1 = Two players 3 = Four players 2-F Not used (zero)
In one-player mode, the second joypad (if any) can only be used for the SGB BIOS. In two-player mode, both joypads are used for the game. Because SNES only has two joypad sockets, four-player mode requires an external “Multiplayer 5” adapter.
Changing the number of active players ANDs the currently selected player
minus one with the number of players in that mode minus one. For example,
if you go from four players to two players while player 4 was active,
player 2 will then be active because
3 & 1 = 1. However, sending the
MLT_REQ command will increment the counter several times so results may
not be exactly as expected. The most frequent case is going from one
player to two-or-four player which will always start with player 1
Reading Multiple Controllers (Joypads)
When having enabled multiple controllers by
MLT_REQ, data for each
joypad can be read out through the
P1 register as follows: First
set P14 and P15 both HIGH (deselect both Buttons and Cursor keys), you
can now read the lower 4 bits of
P1, which indicate the joypad ID for
the following joypad input:
Next, read joypad state normally. The next joypad is automatically selected when P15 goes from LOW (0) to HIGH (1) (source), so you can simply repeat reading the joypad state normally until all two (or four) joypads have been read out.