Modules/input/Usage
From MSX Game Library
Check joystick bouton example:
u8 joyStat = Joystick_Read(JOY_PORT_1); if ((joyStat & JOY_INPUT_TRIGGER_A) == 0) { // Joystick A boutton pressed } else if ((joyStat & JOY_INPUT_TRIGGER_B) == 0) { // Joystick B boutton pressed }