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The standard defines a data descriptive file to contain a data descriptive 
record and its companion data records, both of which must be present. It is 
expected that standard ISO File Labels (which are not part of the 
specification) will precede these files. 
THE GENERAL DATA INTERCHANGE LANGUAGE 
The General Data Interchange Language (GDIL) has been defined by Billingsley 
(1986) to provide a mechanism for the transmission and receipt of digital 
data vith a minimum of human intervention. It is defined to be media- and 
application-independent (i.e., at the ISO Level 6). 
For each record in a file, several components are defined. These are the 
CORE, EXTENSION, ERROR CORRECTING CODE and DATA fields. These components 
are separated by the ASCII Record Separator (RS). Only the CORE is 
mandatory. It contains information about the record identification and 
structure. The optional EXTENSION contains information about data fields, 
structures and relationships. The DATA fields may be in ASCII or binary 
form, while all other fields are ASCII only. 
Fach CORE or EXTENSION consists of a number of segments. Each segment 
consists of a two byte decimal size field which tells the number of 
characters which follow in the segment (including the separator), a data 
field and an ASCII Line Feed (LF). The CORE data field may be in the form 
of the name of a list of TAGs, followed by the ASCII Unit Separator (US) and 
the list of TAGs which can be used to select the required segments from 
specification-defined or user-defined lists. The specification-defined 
segments identify such topics as the Core Data Start Position, Record 
Length, Authority, and format definitions. The final segment is terminated 
with (RS) instead of (LF). 
A DATAGS EXTENSION segment contains a parenthesized list of the data fields 
and subfields which occur later in the data section. The set of allowable 
data labels for an application are those specified by the application 
documentation, and are expected to represent data keywords so that data 
fields may be located by name. The parenthetical structure permits the 
definition of the modules to as many levels as desired so that the data may 
be accessed in a manner that preserves their relationship. 
The In Line, Long (ILL) EXTENSION segment contains an ordered set of entries 
vhich define the structures of corresponding data fields. It allows the 
definition of data field structures either as completely located and defined 
fields, as  implicitly-located defined fields or as non-defined delimited 
fields. It contains an optional number to indicate the number of 
occurrences of identical data entries, a user-selected Label, length, 
position and format control for each corresponding data field. 
The intent of the GDIL is to provide a language which allows the transfer 
data to be parsed to determine the structure and location of data elements. 
It follows a format descriptive approach which may be used to describe any 
of the format structures defined by the various disciplines, allowing their 
data to be transferred between systems with a minimum of human intervention 
or the generation of special logging programs. 
189