Vector table swap

The first $80 bytes of the 68k memory map are swappable between the onboard BIOS and the cartridge's first P ROM.

When the vector tables are swapped, $00~$7F point to $C00000~$C0007F, and $C00000~$C0007F point to the data in the P ROM (if any).

On startup or reset, the first $80 bytes are those found at $C00000~$C0007F, so that the system always starts by executing the initialisation routine.

Right before starting a game, the cart's ROM is swapped in as well as the S ROM and M1 ROM.

Registers $3A0003 (REG_SWPBIOS) and $3A0013 (REG_SWPROM) are used for this.