$UNLOCK_PAGE returns from the locked routine created by the
    $LOCK_PAGE and $UNLOCK_PAGE macro pair and then unlocks the pages
    and linkage section from the working set. This macro is placed
    inline in executable code after a $LOCK_PAGE macro.

    All registers are preserved by this macro unless the error
    address parameter is present and one of the calls fail. If that
    happens, R0 reflects the status of the failed call. R1 then
    contains 0 if the call to unlock the code failed or 1 if that
    call succeeded but the call to unlock the linkage section failed.

    If the error parameter is used, the error label must be placed
    outside the scope of the $LOCK_PAGE and $UNLOCK_PAGE pair. This
    is because the error routine is branched to after returning
    from the subroutine created by the $LOCK_PAGE and $UNLOCK_PAGE
    routines.