of the __pcb_spare longs. Except that fields were changed and one of the
spare values was used and the __pcb_spare field was reduced from two to one
long. Now VM86 bios calls can trash the first 4 bytes of the next page
following the kernel stack/pcb. This Is Bad(TM). This bug has been
present in 5.2-release and onwards, and is still in RELENG_5.
Instead of tempting fate and trying to use "spare" fields, explicitly
reserve them.