Unlock Linear Region

Unlocks a linear address range that was previously locked
using the Lock Linear Region function (Int 31H Function
0600H).

Call With:

   AX     = 0601H
   BX:CX  = starting linear address of memory to unlock
   SI:DI  = size of region to unlock (bytes)

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8002H   invalid state (page not locked)
           8025H   invalid linear address (unallocated
                   pages)

Notes:

o  If the function returns an error, none of the memory has
   been unlocked.
o  If the specified region overlaps part of a page at the
   beginning or end of the region, the page(s) will be
   unlocked.
o  A lock count is maintained for each locked page; the page
   is not unlocked until the lock count is decremented to
   zero (i.e. the number of Lock Region Int 31H Function
   0600H calls has been balanced by the same number of
   Unlock Region Int 31H Fn 0601H calls).
o  This function is ignored by DPMI implementations that do
   not support virtual memory; the function will return the
   Carry flag clear to indicate success, but has no other
   effect.  DPMI hosts which support virtual memory may also
   choose to ignore this function, but such hosts must be
   able to handle page faults transparently at arbitrary
   points during a client's execution, including within
   interrupt and exception handlers.