Genesis:Master System Games Info
One of the more interesting aspects of the Genesis/Mega Drive hardware is that it is almost totally backwards compatible with Master System/Mark III games, much like how the Master System itself was backwards compatible with SG-1000 games. This is made possible by the Genesis having a Z80 CPU and the Master System VDP being integrated into the Genesis VDP as an additional graphics mode. As a result, most Master System games only require a simple cartridge adapter to play on a Genesis.
Sega made this backward compatibility a selling point[1] of the console early in its life, especially in PAL regions and Brazil where the Master System enjoyed significant popularity. Even in Japan, where the Mark III failed to gain ground against the Nintendo Famicom, Sega prioritized the release of a Mark III adapter after the Mega Drive's launch and considered it a "complimentary gift" to Mega Drive owners as it was sold at cost.[2]
Power Base Converter
The Power Base Converter, also known as the Master System Converter in PAL regions and Brazil, and as the Mega Adaptor in Japan, was the official add-on for playing SMS games on a Genesis/Mega Drive. It is designed to sit atop a model 1 console and even shipped with a thumbscrew so that it could be secured onto the back of the host console. The cartridge port is hidden behind a locking flap with the card slot and pause button out on the front. Much of the Power Base Converter's circuitry is passive, but it does contain a small programmable logic array chip which sets up the stack register for certain games that need it set up before booting (e.g. After Burner, Shinobi). The Japanese Mega Adaptor does not include the YM2413 FM chip for games that support FM audio, so games played through the Mega Adaptor will only have PSG audio and function as they do on a stock Mark III.
Though the Power Base Converter is technically compatible with the model 2 Genesis, it will not fit since it was only designed to sit on top of a model 1. It will work on a CDX and will fit on a Genesis 3, but will not work on that console since it has no SMS support. This can be fixed on certain Genesis 3 consoles though (see below for more information).
Master System Converter II
In Europe, Sega released the Master System Converter II in 1993, which was designed to accompany the Mega Drive II as the original converter did not fit on the new console and the Master System was still a fairly popular system there. It only supports cartridge games so Sega Cards and the 3-D glasses adapter are not compatible.
Technical Notes
Master System compatibility is triggered by pin B30 of the cartridge port, which goes to a corresponding pin labeled "M3" on the Genesis VDP. When this pin is held low (connected to ground), the VDP will go into video mode 4 (Master System mode) and the bus arbiter will place the Z80 CPU in control of the system bus, with the 68000 left in an idle state. Normally on the Master System, its BIOS checks to see if a header is present in the game code, and if the header isn't there, the game will not boot. Because the Genesis has no BIOS, this header check is not performed and allows for the play of certain Japanese Master System/Mark III games on non-Japanese hardware. This is similar behavior to the Mark III, which also has no BIOS.
On the Genesis 3 and Nomad, pin B30 is disconnected so SMS games are not compatible on these consoles. On both the Nomad and VA1 Genesis 3, the M3 pin is still present on the main ASIC and can be reconnected to pin B30, fully restoring SMS compatibility. This is not possible on the VA2 Genesis 3 as its revised ASIC lacks M3 entirely.
While the JVC X'Eye is compatible with SMS games, it is not compatible with SMS games played through a Genesis flash cart due to how the X'Eye's reset circuit works. SMS games played through an SMS flash cart via an adapter still work however.
While most SMS games will work fine with a Genesis/Mega Drive controller, some games will require an SMS controller because of how the console handles pin 7 of the controller port, an issue which is also present when using Genesis controllers on Western SMS consoles. This issue does not affect consoles which are set to Japanese mode, including stock Japanese Mega Drives.
SG-1000 Games
One particular note about Master System compatibility is that it unfortunately does not extend to SG-1000 games. This is because the Genesis VDP does not have support for the legacy TMS9918 video modes (modes 0 through 3) that were included with the Master System hardware for SG-1000 compatibility. It is due to this same reason that the Master System game F-16 Fighting Falcon is incompatible, as the main gameplay makes use of one of these legacy video modes. Although SG-1000 games will boot and run on a Genesis, one would be playing them blind as it only displays a black screen.