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ALTERNATE INSTRUCTION EXECUTION
When set to 1, the ALTINST bit in the FCR enables execution of an alternate (not x86) instruction set.
While setting this FCR bit is a privileged operation, executing 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 some more powerful instruction forms over the x86
instruction architecture. For example, in the alternate instruction set, privileged functions can be used
from any protection level, memory descriptor checking 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 justified need for access to these instructions, contact
your VIA representative.

Только я не понял, можно ли переключиться в непривелигированном режиме? Может ли ОС это контролировать?

ALTERNATE INSTRUCTION EXECUTION
When set to 1, the ALTINST bit in the FCR enables execution of an alternate (not x86) instruction set.
While setting this FCR bit is a privileged operation, executing 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 some more powerful instruction forms over the x86
instruction architecture. For example, in the alternate instruction set, privileged functions can be used
from any protection level, memory descriptor checking 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 justified need for access to these instructions, contact
your VIA representative.