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

    Causes the debugger to trace when an instruction writes to and
    changes the value of a location indicated by a specified address
    expression. The address expression is typically a variable name.

    The SET TRACE/MODIFY X command is equivalent to SET WATCH X
    DO(GO). The SET TRACE/MODIFY command operates under the same
    restrictions as SET WATCH.

    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). The SET TYPE LONGWORD
    command restores the default length of 4 bytes.