File architecture overview
Model 204 File Architecture Overview
A Model 204 file consists of one or more data sets. Each data set consists of fixed-length physical records called pages.
All Model 204 files have the same page size,6184 bytes. Internally, these pages are organized into tables: the File Control Table; Table A; Table B; Table C; Table D; Table E; and Table X with any pages in the file, not allocated to one of these considered Free Space.
The components of a Model 204 File
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File Control Table
The File Control Table Contains file parameter settings, data set or file definition names of all data sets in the file, the status of the file, and other control information. The FCT is 8 pages.
The Internal File Directory
Table A contains three structures:
A dictionary of the fieldgroup / field names and their attributes.
Some attributes (notably 'CODED') require lists of values to be maintained. These lists are stored either in the FEW-VALUED or MANY-VALUED structures.
Table A usually is small in relation to the rest of the file. The field name section in particular should be as small as possible to aid efficient access, especially if your site uses field name variables
Data
Indexing