Int 67 Fn 5DE4  - Rm386 - Set Write-protection For Page In First Megabyte  [m]

   AX = 5DE4h
   BL = page number
   BH = access (00h read-only, 01h read-write)

Note:  RM386 traps this function on the initial transition to protected
     mode caused by the INT instruction, which means it can not be
     overridden simply by hooking the interrupt