o Use sysctl machdep.acpi_root to get the physical address of the

o  Use sysctl machdep.acpi_root to get the physical address of the
   RSDP. Scan the first 1MB on i386 if the sysctl fails,
o  Extend struct ACPIrsdp with the ACPI 2.0 fields which involves
   changing a prior reserved field into the ACPI revision,
o  Only calculate the RSDP checksum on the first 20 bytes to remain
   compatible with ACPI 1.0 tables; we don't check the extended
   checksum covering the whole table,
o  Use the length field in the RSDP to map the RSDP into the address
   space so that we don't have to know about future extensions here.