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 having the SMS main hardware integrated into the console's VDP, and most Master System games only require a simple cartridge adapter to play on a Genesis.
Sega made this backward compatibility a selling point 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.
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 Z80 CPU will take control of the system, leaving the 68000 to idle. Like the Mark III, the Genesis has no BIOS and hence does not perform the header check which non-Japanese systems do.
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.