From Sega Retro
Kega Fusion (also known simply as Fusion) is a Sega Mega Drive/Genesis emulator by Steve Snake and the third and final emulator of the Kega-series. It also emulates the Mega CD, Sega 32X, Sega Game Gear, Sega Master System, SG-1000 and SC-3000. It runs only on Microsoft Windows Systems, Mac OS X and Linux. Kega Fusion is based on the earlier (and now discontinued) Kega and its successor Kega Lazarus. Previous emulators by Steve were KGen98, KGen (both DOS) and a Windows conversion used in SEGA's Sega Smash Pack.
Previously, Kega Fusion used version number 0.1 with a letter and beta suffix, with the last 0.1 revision 0.1e. The version change from Kega Fusion 0.1e beta to Kega Fusion 3.0 Beta is not the unimaginable leap this would indicate. The developer, Steve Snake said that the low version number made some people shy away; sub 1.0 version software is known (though often incorrectly) to be incomplete and unstable (most freeware and open source programs are indefinitely in a beta like state since there is no need to go gold). While this was not the case, Steve dropped the 0.1 version numbering in favor of starting at version number 3.0. The logic in this version number is that Kega Fusion is actually the third major revision of the Kega heritage, in the order of Kega, Kega Lazarus and Kega Fusion. The beta moniker was dropped after version 3.2, because yet again, someone complained about not wanting to use "incomplete" software.
Unlike most Sega Genesis Emulators, Kega Fusion puts an extremely high importance on accuracy as opposed to speed or other features (such as netplay, movie recording, or many other features found in emulators like Gens). However, Fusion is highly optimized and runs very efficiently. On a Pentium III class or a similar microprocessor, Kega Fusion runs at 60 frames per second with Sega Genesis ROMs, though with the popular 2xSaI graphics filter plugin the processing power needed to achieve 60 frames per second is much higher (it can also be noted that emulating Sega 32X ROMs at full speed, an 850 MHz or faster microprocessor is strongly recommended). The emphasis on accuracy shows in the Yamaha YM2612 coding, which is reported by Steve Snake to be almost perfect (indeed, there are many games where sound is buggy or wrong in any Mega Drive emulator except for Kega Fusion such as the Title Screen Theme for Sonic 3D Blast and Sonic & Knuckles). Compatibility is very high with Kega Fusion, even on the infamous Sega 32X, which has many games that most if not all emulators cannot handle. Since the Sega 32X BIOSs have been more thoroughly reverse engineered than the BIOSs for the Mega CD, as of version 3.3 Kega Fusion does not require a Sega 32X BIOS (though you can still use an external BIOS). Kega Fusion can emulate most (if not all) Sega home video game consoles released before the Sega Saturn (though there is some cryptic indication by Steve Snake that Saturn emulation could be possible in the future).
Along with being able to emulate most Sega home console systems Kega Fusion also supports logging to a WAV file or the successor to GYM format, VGM. VGM is much like GYM in terms of how it's created but widely superior in its accuracy to YM2612 synthesis, compression (in the VGZ format) and looping. It also has support for the YM-2413, a synthesizer found on the Sega Mark III (the Japanese equivalent of the Master System), which the GYM format does not. As of version 3.5, Kega Fusion supports multiplayer over the internet or LAN. While multiplayer support is still in its preliminary stages, Kega Fusion 3.51 has made multiplayer more stable. Kega Fusion 3.6 currently supports AVI movie logging in the form of a "Video 1 lossless codec."
Even though Kega Fusion is being developed separately from Gens, the development teams are more cooperative than antagonistic towards each other. The differences between Kega Fusion and Gens appeal to different audiences, with Kega Fusion having a more hardcore audience mainly because Kega Fusion is not as well known among new and/or novice emulation fans as Gens even though it is much more accurate.