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Q: How exactly can I share my 360 game to other 360 users? Will my game only be available to people with the XNA Creators Club subscription? Will it be available to all 360 users that have an Xbox Live account?
A: There is currently no supported way to share binaries on the Xbox 360. Currently, there are four requirements that must be met in order to share a game targeting Xbox 360 which is developed with XNA Game Studio Express.
1. The individual you are planning to share the game with must be logged in to Xbox Live and have an active subscription to the XNA Creators Club
2. The receiving user must have downloaded the XNA Framework runtime environment for the Xbox 360
3. The receiving user must have XNA Game Studio Express installed on their own development PC
4. The game project, including all source and content assets, must be shared with the receiving user. The receiving user then compiles and deploys the game to their Xbox 360.
So only homebrew for 99$ a year or 49$ for 4 months.
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Q: Can I create non-gaming applications (such as a Media Center/Player) with XNA Game Studio Express?
A: On Windows this is possible, but the initial release on Xbox 360 is tuned to writing games. This is an area we are actively looking to the community for feedback on the types of applications they want to write for their Xbox 360.
No XBMC360...
bottom line, have fun with mariowar 360 (a game which i thought was kinda cool though!)
Also:
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Q: Isnt managed code in the XNA Framework interpreted and therefore slow?
A: No, it is not interpreted. The IL is just-in-time (JIT) compiled into native code when it is initially loaded by a process, prior to execution. This allows hardware-specific optimizations unique to the PC and Xbox 360 architectures.
Dont get your hopes up for anything "illegal" or something what MS not wants, since they will be checking every line of code before executing it.
Source:
http://msdn.MS.com/directx/xna/faq/