Joypad

Joypads have 4 directions and 4 buttons contacts.

Registers used to read the button's states directly (all bits are active low):
 * REG_P1CNT ($300000), Player 1:
 * REG_P2CNT ($340000), Player 2:


 * REG_STATUS_B ($380000), SELECTs and STARTs:

The SYSTEM_IO ($C0044A) BIOS call can be used to read joypads more easily: it sets values in the BIOS's RAM space which gives currently pressed, state change, and "repeat" values for both joypads. Need details...