The eight Game Boy action/direction buttons are arranged as a 2x4 matrix. Select either action or direction buttons by writing to this register, then read out the bits 0-3.
Bit 7 - Not used Bit 6 - Not used Bit 5 - P15 Select Action buttons (0=Select) Bit 4 - P14 Select Direction buttons (0=Select) Bit 3 - P13 Input: Down or Start (0=Pressed) (Read Only) Bit 2 - P12 Input: Up or Select (0=Pressed) (Read Only) Bit 1 - P11 Input: Left or B (0=Pressed) (Read Only) Bit 0 - P10 Input: Right or A (0=Pressed) (Read Only)
Most programs read from this port several times in a row (the first reads are used as a short delay, allowing the inputs to stabilize, and only the value from the last read is actually used).
Beside for normal joypad input, SGB games mis-use the joypad register to output SGB command packets to the SNES, also, SGB programs may read out gamepad states from up to four different joypads which can be connected to the SNES. See SGB description for details.