File architecture overview

Summary

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

How do I imbed a picture?

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

Record Structure

Table B contains (at least) the base data of all the records in the file. These base records contain pointers to any extensions which exist (whether they are elsewhere in Table B or in Table X).

Table X when enabled, holds only any and all extension records. Table B is then used to store only base records, thus, maximizing the possible number of records can be stored regardless of the number of extension records.

Table E (when enabled) contains Large Object data for the file. The pointer to the starting point of the LOB is contained in Table B or X.

Indexing

The Indexing structures necessary for direct retrieval of records are contained in Tables C and D. There are two types of indexes:

Index Type	Description
Ordered Index	Is composed of the Ordered Index B-tree, contained in Table D, for ORDERED fields along with either IMMEDiate pointers to the record in Table B, or a secondary index (of list or bitmap pages) located in Table D.
Hashed Index	Is composed of Table C, which indexes KEY and NUMERIC RANGE RANGE fields, along with either a single direct pointer (per segment) to a base record in Table B or or a secondary index (of list or bitmap pages) located in Table D.

Table C

Table D

Procedures