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When a file is processed using the mapped format processing option, a map is used to describe the arrangement of the records. The map describes the records in terms of structures or components. Maps are managed by Mapping Services and exist as separate entities within your product. When carrying out a retrieval using mapped format processing, the contents of the file record are usually placed into an MDO, and a map is then used to interpret the contents of the MDO.
It is also possible to read and write NCL tokens using mapped format processing. This is a special case of mapped format processing and a special system map called $NCL is used for this purpose. When NCL variables are written to a file using mapped format processing, and the $NCL map, they appear on the file as sub vectors. The sub vectors consist of a header followed by data. There is one sub vector for each token written to the file. The sub vector headers consist of a 2 byte length field followed by a 2 byte key where the key is X'0000'. (For example, a variable containing the value X'C1C1' would appear on a file record as: X'00060000C1C1', where 0006 is the length, 0000 is the key, and C1C1 is the data.)
The process of writing variables to a mapped format file using the $NCL map is reversible. This means that if a set of variables is written to a record under these conditions, and the contents of this record are then read back into the variables at some other stage, the original values contained in the variables will be the restored.
Note: This allows tokens containing non-printable characters to be handled, that is, data transparent.
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