Jason R Thorpe <thorpej@netbsd.org> comment inspired me;

> Log Message:
> * Rewrite the 32-bit ARM pte.h based on the ARM architecture manual.
>  Significant cleanup, here, including better PTE bit names.

There is no siginificant rationale to have PTE_BASE below KERNEL_BASE,
which looks inherited from acent i386 arrangement.  The address range of
PTE_BASE and APTE_BASE is considered subject to process context
switch even if it was maintained in kernel space and disjoint to user address
space.   It'd be better to move adjacent to APTE_BASE.  sh3 folks, it's not
optimal to have and maintain the pmap as a copy of i386.  It could be
implemented in a saner (clean, handy and managable) way.

Toru Nishimura