Games are primarily written in 68000 assembly, while the sound driver is implemented in Z80 assembly. Both reside in the cartridge ROM and can be up to 4 MB in size without the need for a mapper.
Extra functions
In terms of expandability, this design wasn't as modular as the NES or the SNES. Hence, later add-ons like the 32x (which included a new chipset that took over the 68k) had to bypass the VDP, necessitating the use of a 'Connector Cable'.
Only one custom chip was produced for cartridges: the Sega Virtua Processor (SVP) , a rebranded Samsung SSP1601 (itself a 16-bit Digital Signal Processor). The new chip produced polygons that were subsequently encoded as tiles for the VDP to read. In any case, only one game shipped with it, as the SVP proved too expensive to manufacture.
Early network attempts
Before online services became widely adopted and standardised, Sega tried its luck with Sega Meganet, a dial-up service for games to use. Meganet required users to purchase a separate accessory called the Sega Mega Modem, plug it to the back of the console (where the DE-9 connector is), and finally connect it to the phone line. Games treated the modem unit as another controller, with the addition of communicating with it serially (as opposed to the parallel encoding used for controllers).
Be that as it may, this feature only lasted a couple of years before Sega removed the DE-9 connector in later revisions and discontinued the service entirely.