/sys$common/syshlp/helplib.hlb FORTRAN, /CHECK, OVERFLOW  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Enables or disables integer overflow traps.  If /CHECK=OVERFLOW is
  specified, fixed-point calculations involving BYTE, INTEGER*2, and
  INTEGER*4 data types are checked for arithmetic overflow.  Real and
  complex calculations are always checked for overflow and are not
  affected by /NOCHECK.  Integer exponentiation is performed by a
  routine in the mathematical library.  The routine in the
  mathematical library always checks for overflow, even if
  /CHECK=NOOVERFLOW is specified.