Obsolete Fortran-77 access routines

Caution: These routines are obsolete. Documentation only provided for historical reasons. It may disappear at some point or be moved to another document.
In Fortran-77, neither virtual memory handling, nor data structures, are part of the language. We had written a number of routines to access images. These routines are more complex than the new Fortran-90 method of access, and are provided here just for completeness.

To read a file requires six basic steps:

• allocation of an Image Slot using GDF_GEIS.
• connection of an image to the slot using GDF_REIS or GDF_WRIS.
• reading the header using GDF_RHSEC
• connection of a Memory Slot to the image (or a subset of it) using GDF_GEMS
• once the Memory Slot is no longer needed, disconnection using GDF_FRMS
• once the image slot is no longer required, disconnection using GDF_FRIS

Creation of a new file is slightly different:

• allocation of an Image Slot using GDF_GEIS
• preparatin of the header using e.g. GDF_WHSEC
• creation of the image to the slot using GDF_CRIS. The header is written at this stage.
• connection to the image (or a subset of it) using GDF_GEMS
• once the Memory Slot is no longer needed, disconnection using GDF_FRMS. This is the step which actually does the writing.
• once the image slot is no longer required, disconnection using GDF_FRIS