Real processing of ISAM files

ISAM files can be processed directly on disk without first having to be read fully into the EDT memory area. When this is done, work file 0 must be the current work file.
It must either be empty or a file must have been opened for real processing.
In the former case, the character set of work file 0 must be *NONE or must correspond to the character set of the file that is to be opened.
In the latter case, the opened file is implicitly deleted. In this case, the work file is implicitly deleted and the character set *NONE is set.