For which the instruction set is not documented.
For which the x86 access instruction ("bound eax") is not documented.
For which the capabilities are not documented.
From which you can circumvent all of the processor's security checks.
> For which the instruction set is not documented.
Why does it matter?
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.
> For which the x86 access instruction ("bound eax") is not documented.
The instruction is documented to be LEA (which I presume is correct for this particular processor) and:
While all VIA C3 processor processors contain this alternate instruction feature, the invocation details (e.g., the 0x8D8400 “prefix”) may be different between processors. Check the appropriate processor data sheet for details.
> For which the capabilities are not documented.
It's documented that you can do pretty much anything:
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.
For which the instruction set is not documented. For which the x86 access instruction ("bound eax") is not documented. For which the capabilities are not documented. From which you can circumvent all of the processor's security checks.
This is a _textbook_ definition of a backdoor.