The old FCB File I/O functions funnel all data through a program-global
address called the DTA or Data Transfer Address.

DOS fn 1aH (set DTA) sets or changes the DTA.
DOS fn 2fH (query DTA) obtains the current address of the DTA.

When a program starts, its DTA is initially set to PSP:0080.  See

About the only commonly-used DOS services that uses the DTA are:
4eH (find first matching file)
4fH (find next matching file)

If you use these functions in a recursive search through directories,
remember to set the DTA to an instance-local area at each level!

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DTA: Disk Transfer Address