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

  /[NO]SYNCHRONOUS_EXCEPTIONS    D=/NOSYNCHRONOUS_EXCEPTIONS

  Controls whether vector arithmetic exceptions (such as floating
  overflow on a vector add instruction) are reported immediately
  after the instruction is executed that caused them and before
  operations continue.

  If the vector emulator is being used, exceptions are reported after
  the instruction that caused them, regardless of which qualifier was
  specified (/NOSYNCHRONOUS_EXCEPTIONS or /SYNCHRONOUS_EXCEPTIONS).

  If vector hardware is being used, the exception is reported
  immediately only if the /SYNCHRONOUS_EXCEPTIONS qualifier was
  specified.  If /NOSYNCHRONOUS_EXCEPTIONS was specified, exceptions
  can be reported several instructions after the instruction that
  caused the error.

  If the default (/NOSYNCHRONOUS_EXCEPTIONS) is in effect, exceptions
  are reported at any point after completion of the operation and
  before execution of the next vector instruction, always in the
  procedure containing the error condition.  This setting allows
  uninterrupted completion of the vector operation, but it also makes
  it more difficult to isolate the location of any exceptions.

  The default behavior does not apply to special entry points used
  for the Run-Time Library (RTL) routines that are implementing the
  Fortran intrinsic functions.