Field value pairs (File architecture): Difference between revisions

Revision as of 12:58, 16 May 2013

Overview

The basic unit of storage inside a Model 204 record is the field value pair (or, more literally, "field = value pair"). A record or a fieldgroup contains a series of fieldgroups and field value pairs.

A record can contain field value pairs for fields with the OCCURS attribute, which are described ??.

A record or a fieldgroup can contain non-OCCURS field value pairs, whose physical structure is described below, for each storage data type.

STRING fields

The field value pair for STRING fields contains three parts:

the field code from Table A; 2 bytes.
the length of the value for this pair (0 to 255); 2 bytes for FILEORG X'100' files or 1 byte for all others.
the value; variable length 0-255 bytes.

So, if you have a NAME field containing 'SMITH' it will be held in either 8 or 9 bytes (depending on the FILEORG), while a NAME of 'FRANKENSTEIN' will be held as either 15 or 16.

BINARY or CODED fields

Binary fields are stored (and work) differently depending on whether the file has the Optimized Field Extraction (FILEORG x'80') bit set. Note that FILEORG X'100' files automatically also turn on x'80'.

For non Optimized Field Extraction Files the field value pair contains two parts:

the field code from Table A; 2 bytes.
the value which is always held as 4 bytes.

For Optimized Field Extraction Files the field value pair contains three parts:

the field code from Table A; 2 bytes.
the length of the value for this pair (1 to 4); 2 bytes for FILEORG X'100' files or 1 byte for all others.
the value (variable length, 1-4 bytes).

This effectively makes the BINARY field storage similar to string fields, and improves the speed with which the data in the record can be read. The possible tradeoff on performance versus space is discussed in File Design.

Float Fields

Float fields consist of two parts:

the field code from Table A. This is 3 bytes for Enhanced Data Handling Files (FILEORG X'100' files) and 2 bytes for all others.
the value which is always the LENgth from the field definition (4, 8 or 16 bytes).

@@ Line 9: / Line 9: @@
 A record or a fieldgroup can contain non-<var>OCCURS</var> field value pairs, whose physical structure is described below, for each storage data type.
-== String Fields ==
+== STRING fields ==
-<p>The field value pair for string fields contains three parts: </p>
+<p>The field value pair for <var>STRING</var> fields contains three parts: </p>
-* the field code from [[Table A (File Architecture)|Table A]]. This is 3 bytes for Enhanced Data Handling Files ([[FILEORG parameter|FILEORG]] x'100' files) and 2 bytes for all others.
+* the field code from [[Table A (File Architecture)|Table A]]; 2 bytes.
-* 1 byte containing the length of the value for this pair (0 to 255).
+* the length of the value for this pair (0 to 255); 2 bytes for [[FILEORG parameter|FILEORG]] X'100' files or 1 byte for all others.
-* the value. Held in the number of bytes above.
+* the value; variable length 0-255 bytes.
 <p>So, if you have a NAME field containing 'SMITH' it will be held in either 8 or 9 bytes (depending on the FILEORG), while a NAME of 'FRANKENSTEIN' will be held as either 15 or 16.</p>
-== Binary Fields ==
+== BINARY or CODED fields ==
-<p>[[#Field Design (File Management)#BINARY attribute|Binary]] fields are stored (and work) differently depending on whether the file has the Optimized Field Extraction ([[FILEORG parameter|FILEORG]] x'80') bit set.</p>
+<p>[[#Field Design (File Management)#BINARY attribute|Binary]] fields are stored (and work) differently depending on whether the file has the Optimized Field Extraction ([[FILEORG parameter|FILEORG]] x'80') bit set.  Note that [[FILEORG parameter|FILEORG]] X'100' files automatically also turn on x'80'. </p>
 <p>For non Optimized Field Extraction Files the field value pair contains two parts:</p>
-* the field code from [[Table A (File Architecture)|Table A]]. This is 2 bytes. (Enhanced Data Handling Files ([[FILEORG parameter|FILEORG]] x'100' files automatically also turn on x'80'.)
+* the field code from [[Table A (File Architecture)|Table A]]; 2 bytes.
 * the value which is '''always''' held as 4 bytes.
 <p>For Optimized Field Extraction Files the field value pair contains three parts:</p>
-* the field code from [[Table A (File Architecture)|Table A]]. This is 3 bytes for Optimized Field Extraction Files ([[FILEORG parameter|FILEORG]] x'100' files) and 2 bytes for all others.
+* the field code from [[Table A (File Architecture)|Table A]]; 2 bytes.
-* one byte containing the length of the value (1 to 4 bytes).
+* the length of the value for this pair (1 to 4); 2 bytes for [[FILEORG parameter|FILEORG]] X'100' files or 1 byte for all others.
-* the value. Its length is as above.
+* the value (variable length, 1-4 bytes).
 <p>This effectively makes the BINARY field storage similar to string fields, and improves the speed with which the data in the record can be read. The possible tradeoff on performance versus space is discussed in [[#File Design (File Management) Optimized Field Extraction Files|File Design]].</p>
@@ Line 42: / Line 41: @@
 <p>[[#Field Design (File Management)#FLOAT attribute|Float]] fields consist of two parts:</p>
-* the field code from [[Table A (File Architecture)|Table A]]. This is 3 bytes for Enhanced Data Handling Files ([[FILEORG parameter|FILEORG]] x'100' files) and 2 bytes for all others.
+* the field code from [[Table A (File Architecture)|Table A]]. This is 3 bytes for Enhanced Data Handling Files ([[FILEORG parameter|FILEORG]] X'100' files) and 2 bytes for all others.
 * the value which is '''always''' the [[#Field Design (File Management)#LENGTH attribute|LENgth]] from the field definition (4, 8 or 16 bytes).