SYSTEM IO

BIOSF_SYSTEM_IO ($C0044A)
Should be called at the end of the VBlank interrupt routine.

Reads coin and game select inputs, jumps to COIN_SOUND, PLAYER_START or DEMO_END accordingly.

Reads the player inputs and sets values in RAM accordingly:


 * BIOS_P1STATUS ($10FD94):
 * 0:No joypad connected
 * 1:Normal joypad
 * 2:Expanded joypad (?)
 * 3:Mahjong controler
 * 4:Keyboard


 * BIOS_P1CURRENT ($10FD95) inverted with P1_PREVIOUS ?
 * BIOS_P1PREVIOUS ($10FD96)
 * BIOS_P1CHANGE ($10FD97) rising edge detection (OFF to ON)
 * BIOS_P1REPEAT ($10FD98) rising edge detection + repeat each 8th frame after 16 frames
 * BIOS_P1TIMER ($10FD99) repeat timer (used ?)


 * BIOS_P2STATUS ($10FD9A)
 * BIOS_P2CURRENT ($10FD9B)
 * BIOS_P2PREVIOUS ($10FD9C)
 * BIOS_P2CHANGE ($10FD9D)
 * BIOS_P2REPEAT ($10FD9E)
 * BIOS_P2TIMER ($10FD99)

All bits are active high.