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        Virtual Memory and Page Locking


            Many implementations  of DPMI  support virtual  memory.   In
            these environments,  it will be necessary to lock any memory
            that can  be touched while executing inside of DOS.  This is
            necessary because  it may  not be possible for the operating
            system to demand load a page if DOS is busy.

            Some DPMI implementations will not call DOS to read or write
            virtual memory  to disk  and under these implementations the
            page locking services may be ignored.  Since the entire DPMI
            session is  virtualized, a  page fault can be handled at any
            point while  executing the  program.    However,  under  all
            implementations, DPMI  applications  should  lock  interrupt
            code and  data.   The lock  calls will always return success
            under implementations that ignore these calls.