Mark Real Mode Region as Pageable

Advises the DPMI host that the specified memory below the 1
MB boundary may be paged to disk.

Call With:

   AX     = 0602H
   BX:CX  = starting linear address of memory to mark as
           pageable
   SI:DI  = size of region to be marked (bytes)

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8002H   invalid state (region already marked as
                   pageable)
           8025H   invalid linear address (region is above 1
                   MB boundary)

Notes:

o  If the function returns an error, none of the memory has
   been marked as pageable.
o  If the specified region overlaps part of a page at the
   beginning or end of the region, the page(s) will not be
   marked as pageable.
o  Pageability information for a real mode region is
   maintained as a binary state, not a count.  Therefore,
   multiple calls to this function for the same region have
   no effect.
o  For compatibility with DPMI version 0.9 hosts, a client
   must call the Relock Real Mode Region function (Int 31H
   Function 0603H) to relock the memory region before
   terminating.  Memory that remains unlocked after the
   client has terminated could result in fatal page faults
   when another program is executed in the same address
   space.  DPMI 1.0 hosts automatically relock real mode
   memory at client termination.
o  Under some DPMI hosts, all conventional memory may be
   locked by default.  If a protected mode program is using
   memory in the first megabyte of address space, it is
   recommended that this function be used to turn off
   automatic page locking for regions of memory that will
   not be touched at interrupt time.
o  The client must not mark memory as pageable in regions
   that it does not own; i.e. it may only mark as pageable
   memory that it has previously allocated with Int 31H
   Function 0100H or by a direct call to DOS via the
   translation functions.  For example, marking all free DOS
   memory as pageable under some DPMI hosts could cause a
   page fault to occur while inside of DOS, resulting in a
   crash.  Also, a client should not mark the DPMI host data
   area as pageable.
o  Note that address space marked as pageable by this
   function can still be locked using the Lock Linear Region
    function (Int 31H Fn 0600H).  This function is
   just an advisory service to allow memory that does not
   need to be locked to be paged out; it disables any
   automatic locking of real mode memory performed by the
   DPMI host.
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.