MBC6 (Game Boy mapper)
MBC6 is a Game Boy mapper chip by Nintendo, however it is believed it is only used in the third-party game Net de Get: Minigame @ 100 (this game however was developed by Mobile21 and published by Konami, not Nintendo; the licenses are unclear since Mobile21 was partly owned by Nintendo and Konami, though strictly it is not Nintendo) leaving it as a more experimental chip like the unreleased MBC4. This chip is succeeded in its naming convention by the MBC7.
The main addition is support for a Flash memory chip which lasts longer than cell batteries; though MBC6 is associated with downloading long-lasting data over the Internet. Curiously, some Game Boy Advance games use Flash memory for their save files, but the relevance of this in relation to MBC6 is unknown.
Furthermore, in relation to licensing of the chip itself, Macronix produced the Flash chip for the MBC6 Game Boy games called MX29F008TC-14 and it is speculated there must have been some licensing agreements between them and Nintendo/probably that only the Game Boy can use it.
MBC6 is not yet well understood even though it has been possible to boot the game with less common emulators, but later emulators have ventured into understanding how it properly works.
Specifically, the MBC6 was used in conjunction with five chips; Mapper+ROM+Flash+RAM+RAM protector. In the past external save storage was typically restricted to a battery.
Following the invention of the now long-discontinued Mobile System GB and the Mobile GB Adapter (released before MBC6), Mobile21 used the MBC6 to enable downloading minigames within Net de Get on to the Flash chip as downloadable content over the Internet (hence the name, and it is believed these were paid 100 yen each). It was an intermediary transfer between the game on a Game Boy Color and possibly Game Boy Advance (regular Game Boy or Game Boy Light not supported), the server (the Game Boy Data Center, the SRAM, portions in WRAM, and finally the Flash chip).
The Flash chip secures data is preserved longer than a cell save battery, and most commonly games that make intensive use of cell batteries (like the Real-Time Clock Pokémon games on original Game Pak) now have erased/corrupted data due to the cell save battery issue.
When the game writes to the Flash chip, it will write in 128 byte segments at a time. It is also possible to erase sectors of data on the Flash chip. MBC6 uses its own indexed commands to perform operations related to the Flash chip.