Library /sys$common/syshlp/dbg$help.hlb DEBUG, SET, BREAK, Qualifiers, /MODIFY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Causes the debugger to break on every instruction that writes to
    and modifies the value of the location indicated by the address
    expression. The address expression is typically a variable name.

    The SET BREAK/MODIFY command acts exactly like a SET WATCH
    command and operates under the same restrictions.

    If you specify an absolute address for the address expression,
    the debugger might not be able to associate the address with
    a particular data object. In this case, the debugger uses a
    default length of 4 bytes. You can change this length, however,
    by setting the type to either WORD (SET TYPE WORD, which changes
    the default length to 2 bytes) or BYTE (SET TYPE BYTE, which
    changes the default length to 1 byte). SET TYPE LONGWORD restores
    the default length of 4 bytes.