For the details, see http://www.autosec.org/pubs/cars-usenixsec2011.pdf. (Pretty scary reading. In this case they are also able to disable the brakes and they are also able to engage the brakes on only one of the front wheels for all sorts of "fun"...)
"When set to 1, the ALTINST bit in the FCR enables ex ecution of an alternate (not x86) instruction set. While setting this FCR bit is a privileged operation, ex ecuting the alternate instructions can be done from any protection level.
This alternate instruction set includes an extended set of integer, MMX, floating-point, and 3DNow! in- structions along with additional registers and so me more powerful instruction forms over the x86 instruction architecture. For example, in the alternat e instruction set, privileged functions can be used from any protection level, memory descriptor checki ng can be bypassed, and many x86 exceptions such as alignment check can be bypassed.
This alternate instruction set is intended for testing, debug, and special application usage. Accordingly, it is not documented for general usage. If you have a ju stified need for access to these instructions, contact your VIA representative. "
I have tried to find some details about this alternate instruction set but haven't been able to find anything unfortunately. (And I'm not so interested in this any longer as my remaining Via C3 machine is now only used for backups and does not require very high performance...) Anyway, I'm guessing that it didn't become very popular due to the fact that they kept the details secret.
I have been wondering about this myself. This situation is not really that well explored and I'd really like to see a license for HDL-like code with some high quality lawyering behind it.
One problem is that hardware such as ASICs are typically not protected by copyright. Instead it is protected by maskworks laws which are similar to, but not as strong as copyright. The intention is that ASIC-like hardware should be protected by patents. (Although I guess an exception would be made for ROMs where the layout itself would be protected by maskworks whereas the actual metal configuration of the ROM array would be protected by copyright.)
For FPGAs I have heard people claim that their lawyers consider the bitstream to fall under copyright law -> e.g., the GPL would be fully enforcable. Another interesting case in point is that SUN choose the GPLv3 as the license when releasing the source code for the Niagara processor. Personally I've used an MIT-style license when releasing open source hardware because I was more interested in getting the (fairly small) designs out there than to try to enforce the GPL license for something it wasn't really designed to protect.
Also note that the GNU FAQ has the following to say about hardware:
Any material that can be copyrighted can be licensed under the GPL. GPLv3 can also be used to license materials covered by other copyright-like laws, such as semiconductor masks. So, as an example, you can release a drawing of a physical object or circuit under the GPL.
In many situations, copyright does not cover making physical hardware from a drawing. In these situations, your license for the drawing simply can't exert any control over making or selling physical hardware, regardless of the license you use. When copyright does cover making hardware, for instance with IC masks, the GPL handles that case in a useful way.
A list is only as strong as its weakest link. -- Don Knuth