AMD64_PDRMASK
a = (pde & AMD64_PG_PS_FRAME) + (va & AMD64_PDRMASK);
return (AMD64_NBPDR - (va & AMD64_PDRMASK));
_Static_assert(PDRMASK == AMD64_PDRMASK, "PDRMASK mismatch");
a += (va & AMD64_PDRMASK) ^ offset;
((va & AMD64_PDRMASK) ^ VA_OFF(vm, va));