Wasn't sure which topic to put this under.   Anyway, I want to program a game and put it onto either a NES cartridge or Sega Genesis cartridge.  Is this possible and if so how does one go about doing this?   I know you can buy blank cartridges, but how do you go about putting the game onto the cartridge?  Lastly, is there any primary language to use to do this?  I was thinking Python/Pygame but did not know if the language mattered much.   Thanks for all your help in advance.

the other day i was reading an article or something i forgot what but i remember there is some authentication required from the cartridge its self, you might wanna check that out before u start

You'll have to use Assembly I believe. And you'll need the documentation for the all the chips.

Is Assembly a language or script or something like that?  And do you know how to go about getting the documentation for the chips?   And I am not sure if this is possible, but if is it possible to make your own cartridge if you got all the parts?

The cartridge is not a problem. You can buy a special cartridge that you can load files into from a docking station connected to your PC. I don't know where but I know they exist.

The Sega2.doc has info on the megadrive, but it's incomplete. I've found a few guides about programming the megadrive by googling "Programming the MegaDrive" I'm sure the NES homebrew community has info about the NES components, but I've never looked it up.

Assembly is basically as close as you can get to machine code without it being unreadable to all but very few programmers. It's basically direct commands for telling the processor to move, compare, add or subract, etc etc. It's very tedious to program, but other people have done it so you can too!

Machine code isn't all that hard to read :D.  Both consoles don't have a C compiler, everything is done through assembly.

NES tutorials: http://www.patater.com/nes-asm-tutorials

Genesis Start (Not necessarily a tutorial, but will give you a start for searching): http://www.gamedev.net/topic/575801-making-a-game-for-the-sega-genesis/

Thanks a bunch you guys.  I really want to get into doing my own gaming hardware stuff.  Are there any other recommendations for uploading a game to a popular system like NES or Genesis?   I would like to build a completely homegrown 8 or 16 bit console to play my own games and stuff on.  Do either of you know if it is possible to get a NES or Genesis or SNES or some other popular old school console schematic?   I would like to build one on my own if possible.   

That's a bit intense, I say just stick to emulation.  Though, if you politely e-mail EA they might be able to give you some info on their Genesis dev kit, which they engineered themselves.  Literally, they borrowed a dev kit from another developer, got a bunch of engineers, and built their own.  They supposedly threatened Sega with it since they could make games themselves without having to go to Sega.  That said, now that I think about it, you can just upload your roms to a NES cartridge and run it on a top loader.  Not sure if the Genesis had any copy protection, but you should be able to do the same.

Yea, it is a little intense.  Hardware is just more exciting to me than software for some reason.  Thanks for your help again. 

Not an easy task you have chosen. Do you want to emulate an old console and add own games, or do you want to build a new console with modern tech? The second task may be much easier to pull off. Today there are many chips, that can be easily used. Most common used are ARM-processors (that also is used in the DS for example).

http://en.wikipedia.org/wiki/ARM_architecture

Another option to build an own project on is Arduino. That is a complete board.

http://en.wikipedia.org/wiki/Arduino

That modern tech is not compatible with the old consoles though. The NES used a Ricoh 2A03 as processor, the Genesis a Motorola 68000. I don't believe the Ricoh is produced anymore. The Motorola-chip was very popular, it was also used in the some Apple machines. So someone may produce clones of this chip. But the complete setup of the old consoles may contain even more old chips, that are hard to obtain these days.