o) Use <machine/asm.h> macros for register-width, etc., rather than doing it
by hand in a few more assembly files.
o) Reduce diffs between various bits of TLB refill code in exception.S and
between interrupt processing code.
o) Use PTR_* to operate on registers that are pointers (e.g. sp).
o) Add and use a macro, CLEAR_PTE_SWBITS rather than using the
mysteriously-named WIRED_SHIFT to select bits to truncate when loading PTEs.
o) Don't doubly disable interrupts by moving zero to the status register,
especially since that has the nasty side-effect of taking us out of 64-bit
mode.
o) Use CLEAR_STATUS to disable interrupts the first time.
o) Keep SR_PX set as well as SR_[KSU]X when doing exception processing. This
is the bit that determines whether 64-bit operations are allowed.
o) Don't enable interrupts until configure_final(), like most other ports.
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