Machine-dependence

  Machine-dependence has an impact on several areas, in particular in:

• precision that is used in the computations
• open statements
• machine-dependent constants
• plot commands

We shall consider each of these items separately;

• precision that is used in the computations Most present day computers use 32-bits arithmetic (except CRAY and ALPHA processor-based computers). This means that for large-scale numerical applications it is necessary to use double precision arithmetic. In order to translate the double precision version easily into a single precision version by some tool, it is necessary to apply the following rules:
  • declare explicit with double precision or complex 16. Use small letters for these declarations.
  • use always generic names for intrinsics.
  • use always small letters when BLAS subroutines are called en do not use spaces in these names. Unfortunately no generic names for BLAS subroutines can be used. For each precision a separate subroutine must be used.

• open statements Open statements may be machine-dependent. Especially the name of a file is in general machine-dependent, but in general also some other options may have machine-dependent aspects. For that reason it is advised to put all open statements in a general machine-dependent subroutine.
  In SEPRAN subroutine INIFIL is used for all open statements, in ISNaS it is the subroutine ISINIFIL.
  The names of the files are stored in a common, containing machine-dependent names.
• machine-dependent constants Some machine-dependent constants, like machine accuracy, largest real number and so on, are used in many programs. It is a good practice to initialize these constants in general machine-dependent subroutine and to put them in a common block.
• plot commands Plot commands are not only machine-dependent, they may also be device or even package-dependent. It is for that reason sensible to avoid actual calls to low level plotting subroutines in the programs. This is possible if some intermediate plotting subroutine is defined, that simulates the actual plot calls. This own subroutine may contain machine-dependent calls or even make a "local" neutral plot file. It is good practice to restrict the number of local machine-dependent plot subroutines to a small number (for example 3 or 4).

In SEPRAN a number of machine-dependent constants and file names may be defined by an environment file (sepran.env), in ISNaS this environment file is called isnas.env.