SYSTEM INT1

BIOSF_VBL ($C0044A): Multiple purpose VBL routine

Sets values in RAM according to buttons states, checks CDDA commands, probably other things... Always (?) called each frame by games.

Memory locations set (all 8bit):

BIOS_P1CURRENT     $10FD95 Joystick 1 state
BIOS_P1PREVIOUS    $10FD96 Previous joystick 1 state
BIOS_P1CHANGE      $10FD97 Joystick 1 state change (edges)
BIOS_P1REPEAT      $10FD98 Joystick 1 repeat (edge, and strobe if held more than ? calls)

BIOS_P2CURRENT     $10FD9B Joystick 2 state
BIOS_P2PREVIOUS    $10FD9C Previous joystick 2 state
BIOS_P2CHANGE      $10FD9D Joystick 2 state change (edges)
BIOS_P2REPEAT      $10FD9E Joystick 2 repeat (edge, and strobe if held more than ? calls)

BIOS_STATCURNT     Joystick 1/2 start/select state
BIOS_STATCHANGE    Previous Joystick 1/2 start/select state

Bit definitions: see Joypad