FAKE_VM X01-005
                                 23-May-1989

                                Mark Bramhall
                        Digital Equipment Corporation
                             110 Spit Brook Road
                              Nasuha, NH  03062

    The only "documentation" for FAKE_VM is the initial block comment that
    lives at the beginning of the module's source code.  Please read those
    comments carefully.  There is a lot of helpful information, even if it
    is hard to dig out.

    Quite useful and often overlooked are the settings of 3 and 5 in the
    location FAKE_VM_REAL_FREE_OFF.  If you suspect "stale" pointers
    (pointers still pointing at deallocated memory) and/or other memory
    corruption problems then the setting of 3 is very useful.  This forces
    each memory allocation to be on a unique page (or set of pages) and
    will actually delete the page(s) from your address space upon
    deallocation.  This forces ACCVIOs when stale pointers are used
    (instead of memory corruption).  If you have soo many bugs that you get
    immediate ACCVIOs when using the setting of 3 then you can try the
    setting of 5.  This purposely keeps (avoids reusing) deallocated memory
    so that stale pointers, etc. will not be clobbering active memory.  The
    5 setting is there to let you debug a specific problem in an otherwise
    totally buggy program!

    One final note, FAKE_VM announces errors as soon as it detects them.
    This means that you might get an error announcement seemingly
    unconnected with what your program was doing.  For example, if your
    program is deallocation some piece of memory, FAKE_VM runs through its
    still-allocated memory list to find said piece.  But, if while looking
    for the piece being returned, it finds another piece that has been
    corrupted FAKE_VM will announce the corrupted bit.  Just carefully look
    at the error message.  Messages about corruption can occur at almost
    any time.  In fact, to force this type of error announcement to happen
    as soon as possible, you can set FAKE_VM_CHECK_ACTIVE_LIST to 1.