The cartridge bus allows both memory access (to linear addresses 0x10000-0xFFFFF) and I/O access (to ports 0xC0-0xFF).
To facilitate this, the Memory/IO pin is used:
- When Memory/IO is asserted high, all bus address pins specify the 20-bit linear memory address.
- When Memory/IO is asserted low, bus address pins 0-7 specify the port address bits 0-7, bus address pins 8-15 are low, bus address pins 16-19 specify the port address bits 4-7.
The cartridge bus provides an /IRQ pin which can be used by the cartridge to assert an interrupt.