Technical data

Access of Misaligned Data

The Fortran compiler allows misalignment of data if speciﬁed by the use of

special options.

As discussed in the previous section, the architecture of the IRIS-4D series

assumes a particular alignment of data. ANSI standard Fortran 77 cannot

violate the rules governing this alignment. Common extensions to the

dialect, particularly small integer types, allowing intermixing of character

and non-character data in COMMON and EQUIVALENCE statements and

mismatching the types of formal and actual parameters across a subroutine

interface, provide many opportunities for misalignment to occur.

Code using the extensions that compiled and executed correctly on other

systems with less stringent alignment requirements may fail during

compilation or execution on the IRIS-4D. This section describes a set of

options to the Fortran compilation system that allow the compilation and

execution of programs whose data may be misaligned. Be forewarned that

the execution of programs that use these options will be signiﬁcantly slower

than the execution of a program with aligned data.

This section describes the two methods that can be used to create an

executable object ﬁle that accesses misaligned data.

Accessing Small Amounts of Misaligned Data

Use the ﬁrst method if the number of instances of misaligned data access is

small or to provide information on the occurrence of such accesses so that

misalignment problems can be corrected at the source level.

This method catches and corrects bus errors due to misaligned accesses. This

ties the extent of program degradation to the frequency of these accesses.

This method also includes capabilities for producing a report of these

accesses to enable their correction.