Like other systems from its generation, the CPU is primarily responsible for handling I/O. In this case, the Z80 processor is unique for its specialised I/O addressing, but CPU cycles are still spent moving bits between components.
On the other side, the SMS uses a dedicated I/O controller chip to interface with the joypads and to enable or disable parts of the system, which alters the address map. Furthermore, this controller is essential for supporting the FM expansion, as the FM module exposes ports that conflict with the rest of the system.
Available interfaces
In addition to the two controller ports, the system contains one proprietary cartridge slot, one 'Sega Card' slot, and one expansion slot reserved for 'future accessories'. The latter was never utilised, except for the FM expansion in the Mark III. Even so, the SMS and Mark III featured different expansion port designs .
Top interruptors
Another speciality about this console is the inclusion of two buttons on the top of its case: PAUSE and RESET. You can already guess what they do!
Top of the case .
Pressing the PAUSE button sends a non-maskable interrupt to the CPU . The interrupt handler is stored within the game itself, meaning it is up to the game to honour the press.
In contrast, and for some strange reason, the RESET button is handled as a keypress on the controller.