Rule #8.7 for "special function registers" and others?

[mishak]

So far I haven't heard any convincing arguments why it couldn't be used either.

Many micros have registers in a memory space that can't be accessed with a macro unless an extension (typically of the form of a qualifier) is used.

Some also require the use of extensions for efficiency reasons. For example, the 8051 (an many, many derivatives) have bit-addressable locations and registers which cannot be (efficiently) accessed using standards-compliant 'C' code.