The File Handler uses four data structures during file I/O operations:
| Data Structure | Description |
|---|---|
| File Control Description (FCD) | A 100-byte data area containing information about the file in use. |
| Record area | A data area into which records are read and from which records are written. |
| Filename area | A data area which contains the name of the file in use, as recognized by the operating system. |
| Key definition block | Used by the File Handler to hold index key information during operations on indexed files. |