10.3 Fortran 90  (Page 6/5)

The previous American National Standards Institute (ANSI) FORTRAN standard release, FORTRAN 77 (X3.9-1978), was written to promote portability of FORTRAN programs between different platforms. It didn't invent new language components, but instead incorporated good features that were already available in production compilers. Unlike FORTRAN 77, FORTRAN 90 (ANSI X3.198-1992) brings new extensions and features to the language. Some of these just bring FORTRAN up to date with newer languages like C (dynamic memory allocation, scoping rules) and C++ (generic function interfaces). But some of the new features are unique to FORTRAN (array operations). Interestingly, while the FORTRAN 90 specification was being developed, the dominant high performance computer architectures were scalable SIMD systems such as the Connection Machine and shared-memory vector-parallel processor systems from companies like Cray Research.

FORTRAN 90 does a surprisingly good job of meeting the needs of these very different architectures. Its features also map reasonably well onto the new shared uniform memory multiprocessors. However, as we will see later, FORTRAN 90 alone is not yet sufficient to meet the needs of the scalable distributed and nonuniform access memory systems that are becoming dominant at the high end of computing.

The FORTRAN 90 extensions to FORTRAN 77 include:

• Array constructs
• Dynamic memory allocation and automatic variables
• Pointers
• New data types, structures
• New intrinsic functions, including many that operate on vectors or matrices
• New control structures, such as a WHERE statement
• Enhanced procedure interfaces

Fortran 90 array constructs

With FORTRAN 90 array constructs, you can specify whole arrays or array sections as the participants in unary and binary operations. These constructs are a key feature for "unserializing" applications so that they are better suited to vector computers and parallel processors. For example, say you wish to add two vectors, A and B. In FORTRAN 90, you can express this as a simple addition operation, rather than a traditional loop. That is, you can write:

instead of the traditional FORTRAN 77 loop:

The code generated by the compiler on your workstation may not look any different, but for some of the parallel machines available now and workstations just around the corner, the difference are significant. The FORTRAN 90 version states explicitly that the computations can be performed in any order, including all in parallel at the same time.

One important effect of this is that if the FORTRAN 90 version experienced a floating-point fault adding element 17, and you were to look at the memory in a debugger, it would be perfectly legal for element 27 to be already computed.

You are not limited to one-dimensional arrays. For instance, the element-wise addition of two two-dimensional arrays could be stated like this: Just in case you are wondering, A*B gives you an element-wise multiplication of array members­­— not matrix multiplication. That is covered by a FORTRAN 90 intrinsic function.